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rfc:rfc1583

Network Working Group J. Moy Request for Comments: 1583 Proteon, Inc. Obsoletes: 1247 March 1994 Category: Standards Track

                           OSPF Version 2

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is
  unlimited.

Abstract

  This memo documents version 2 of the OSPF protocol.  OSPF is a
  link-state routing protocol.  It is designed to be run internal to a
  single Autonomous System.  Each OSPF router maintains an identical
  database describing the Autonomous System's topology.  From this
  database, a routing table is calculated by constructing a shortest-
  path tree.
  OSPF recalculates routes quickly in the face of topological changes,
  utilizing a minimum of routing protocol traffic.  OSPF provides
  support for equal-cost multipath.  Separate routes can be calculated
  for each IP Type of Service.  An area routing capability is
  provided, enabling an additional level of routing protection and a
  reduction in routing protocol traffic.  In addition, all OSPF
  routing protocol exchanges are authenticated.
  OSPF Version 2 was originally documented in RFC 1247. The
  differences between RFC 1247 and this memo are explained in Appendix
  E. The differences consist of bug fixes and clarifications, and are
  backward-compatible in nature. Implementations of RFC 1247 and of
  this memo will interoperate.
  Please send comments to ospf@gated.cornell.edu.

Moy [Page 1] RFC 1583 OSPF Version 2 March 1994

Table of Contents

  1       Introduction ........................................... 5
  1.1     Protocol Overview ...................................... 5
  1.2     Definitions of commonly used terms ..................... 6
  1.3     Brief history of link-state routing technology ......... 9
  1.4     Organization of this document .......................... 9
  2       The Topological Database .............................. 10
  2.1     The shortest-path tree ................................ 13
  2.2     Use of external routing information ................... 16
  2.3     Equal-cost multipath .................................. 20
  2.4     TOS-based routing ..................................... 20
  3       Splitting the AS into Areas ........................... 21
  3.1     The backbone of the Autonomous System ................. 22
  3.2     Inter-area routing .................................... 22
  3.3     Classification of routers ............................. 23
  3.4     A sample area configuration ........................... 24
  3.5     IP subnetting support ................................. 30
  3.6     Supporting stub areas ................................. 31
  3.7     Partitions of areas ................................... 32
  4       Functional Summary .................................... 34
  4.1     Inter-area routing .................................... 35
  4.2     AS external routes .................................... 35
  4.3     Routing protocol packets .............................. 35
  4.4     Basic implementation requirements ..................... 38
  4.5     Optional OSPF capabilities ............................ 39
  5       Protocol data structures .............................. 41
  6       The Area Data Structure ............................... 42
  7       Bringing Up Adjacencies ............................... 45
  7.1     The Hello Protocol .................................... 45
  7.2     The Synchronization of Databases ...................... 46
  7.3     The Designated Router ................................. 47
  7.4     The Backup Designated Router .......................... 48
  7.5     The graph of adjacencies .............................. 49
  8       Protocol Packet Processing ............................ 50
  8.1     Sending protocol packets .............................. 51
  8.2     Receiving protocol packets ............................ 53
  9       The Interface Data Structure .......................... 55
  9.1     Interface states ...................................... 58
  9.2     Events causing interface state changes ................ 61
  9.3     The Interface state machine ........................... 62
  9.4     Electing the Designated Router ........................ 65
  9.5     Sending Hello packets ................................. 67
  9.5.1   Sending Hello packets on non-broadcast networks ....... 68
  10      The Neighbor Data Structure ........................... 69
  10.1    Neighbor states ....................................... 72
  10.2    Events causing neighbor state changes ................. 75
  10.3    The Neighbor state machine ............................ 77

Moy [Page 2] RFC 1583 OSPF Version 2 March 1994

  10.4    Whether to become adjacent ............................ 83
  10.5    Receiving Hello Packets ............................... 83
  10.6    Receiving Database Description Packets ................ 86
  10.7    Receiving Link State Request Packets .................. 89
  10.8    Sending Database Description Packets .................. 89
  10.9    Sending Link State Request Packets .................... 90
  10.10   An Example ............................................ 91
  11      The Routing Table Structure ........................... 93
  11.1    Routing table lookup .................................. 96
  11.2    Sample routing table, without areas ................... 97
  11.3    Sample routing table, with areas ...................... 98
  12      Link State Advertisements ............................ 100
  12.1    The Link State Advertisement Header .................. 101
  12.1.1  LS age ............................................... 102
  12.1.2  Options .............................................. 102
  12.1.3  LS type .............................................. 103
  12.1.4  Link State ID ........................................ 103
  12.1.5  Advertising Router ................................... 105
  12.1.6  LS sequence number ................................... 105
  12.1.7  LS checksum .......................................... 106
  12.2    The link state database .............................. 107
  12.3    Representation of TOS ................................ 108
  12.4    Originating link state advertisements ................ 109
  12.4.1  Router links ......................................... 112
  12.4.2  Network links ........................................ 118
  12.4.3  Summary links ........................................ 120
  12.4.4  Originating summary links into stub areas ............ 123
  12.4.5  AS external links .................................... 124
  13      The Flooding Procedure ............................... 126
  13.1    Determining which link state is newer ................ 130
  13.2    Installing link state advertisements in the database . 130
  13.3    Next step in the flooding procedure .................. 131
  13.4    Receiving self-originated link state ................. 134
  13.5    Sending Link State Acknowledgment packets ............ 135
  13.6    Retransmitting link state advertisements ............. 136
  13.7    Receiving link state acknowledgments ................. 138
  14      Aging The Link State Database ........................ 139
  14.1    Premature aging of advertisements .................... 139
  15      Virtual Links ........................................ 140
  16      Calculation Of The Routing Table ..................... 142
  16.1    Calculating the shortest-path tree for an area ....... 143
  16.1.1  The next hop calculation ............................. 149
  16.2    Calculating the inter-area routes .................... 150
  16.3    Examining transit areas' summary links ............... 152
  16.4    Calculating AS external routes ....................... 154
  16.5    Incremental updates -- summary link advertisements ... 156
  16.6    Incremental updates -- AS external link advertisements 157
  16.7    Events generated as a result of routing table changes  157

Moy [Page 3] RFC 1583 OSPF Version 2 March 1994

  16.8    Equal-cost multipath ................................. 158
  16.9    Building the non-zero-TOS portion of the routing table 158
          Footnotes ............................................ 161
          References ........................................... 164
  A       OSPF data formats .................................... 166
  A.1     Encapsulation of OSPF packets ........................ 166
  A.2     The Options field .................................... 168
  A.3     OSPF Packet Formats .................................. 170
  A.3.1   The OSPF packet header ............................... 171
  A.3.2   The Hello packet ..................................... 173
  A.3.3   The Database Description packet ...................... 175
  A.3.4   The Link State Request packet ........................ 177
  A.3.5   The Link State Update packet ......................... 179
  A.3.6   The Link State Acknowledgment packet ................. 181
  A.4     Link state advertisement formats ..................... 183
  A.4.1   The Link State Advertisement header .................. 184
  A.4.2   Router links advertisements .......................... 186
  A.4.3   Network links advertisements ......................... 190
  A.4.4   Summary link advertisements .......................... 192
  A.4.5   AS external link advertisements ...................... 194
  B       Architectural Constants .............................. 196
  C       Configurable Constants ............................... 198
  C.1     Global parameters .................................... 198
  C.2     Area parameters ...................................... 198
  C.3     Router interface parameters .......................... 200
  C.4     Virtual link parameters .............................. 202
  C.5     Non-broadcast, multi-access network parameters ....... 203
  C.6     Host route parameters ................................ 203
  D       Authentication ....................................... 205
  D.1     AuType 0 -- No authentication ........................ 205
  D.2     AuType 1 -- Simple password .......................... 205
  E       Differences from RFC 1247 ............................ 207
  E.1     A fix for a problem with OSPF Virtual links .......... 207
  E.2     Supporting supernetting and subnet 0 ................. 208
  E.3     Obsoleting LSInfinity in router links advertisements . 209
  E.4     TOS encoding updated ................................. 209
  E.5     Summarizing routes into transit areas ................ 210
  E.6     Summarizing routes into stub areas ................... 210
  E.7     Flushing anomalous network links advertisements ...... 210
  E.8     Required Statistics appendix deleted ................. 211
  E.9     Other changes ........................................ 211
  F.      An algorithm for assigning Link State IDs ............ 213
          Security Considerations .............................. 216
          Author's Address ..................................... 216

Moy [Page 4] RFC 1583 OSPF Version 2 March 1994

1. Introduction

  This document is a specification of the Open Shortest Path First
  (OSPF) TCP/IP internet routing protocol.  OSPF is classified as an
  Interior Gateway Protocol (IGP).  This means that it distributes
  routing information between routers belonging to a single Autonomous
  System.  The OSPF protocol is based on link-state or SPF technology.
  This is a departure from the Bellman-Ford base used by traditional
  TCP/IP internet routing protocols.
  The OSPF protocol was developed by the OSPF working group of the
  Internet Engineering Task Force.  It has been designed expressly for
  the TCP/IP internet environment, including explicit support for IP
  subnetting, TOS-based routing and the tagging of externally-derived
  routing information.  OSPF also provides for the authentication of
  routing updates, and utilizes IP multicast when sending/receiving
  the updates.  In addition, much work has been done to produce a
  protocol that responds quickly to topology changes, yet involves
  small amounts of routing protocol traffic.
  The author would like to thank Fred Baker, Jeffrey Burgan, Rob
  Coltun, Dino Farinacci, Vince Fuller, Phanindra Jujjavarapu, Milo
  Medin, Kannan Varadhan and the rest of the OSPF working group for
  the ideas and support they have given to this project.
  1.1.  Protocol overview
      OSPF routes IP packets based solely on the destination IP
      address and IP Type of Service found in the IP packet header.
      IP packets are routed "as is" -- they are not encapsulated in
      any further protocol headers as they transit the Autonomous
      System.  OSPF is a dynamic routing protocol.  It quickly detects
      topological changes in the AS (such as router interface
      failures) and calculates new loop-free routes after a period of
      convergence.  This period of convergence is short and involves a
      minimum of routing traffic.
      In a link-state routing protocol, each router maintains a
      database describing the Autonomous System's topology.  Each
      participating router has an identical database.  Each individual
      piece of this database is a particular router's local state
      (e.g., the router's usable interfaces and reachable neighbors).
      The router distributes its local state throughout the Autonomous
      System by flooding.
      All routers run the exact same algorithm, in parallel.  From the
      topological database, each router constructs a tree of shortest
      paths with itself as root.  This shortest-path tree gives the

Moy [Page 5] RFC 1583 OSPF Version 2 March 1994

      route to each destination in the Autonomous System.  Externally
      derived routing information appears on the tree as leaves.
      OSPF calculates separate routes for each Type of Service (TOS).
      When several equal-cost routes to a destination exist, traffic
      is distributed equally among them.  The cost of a route is
      described by a single dimensionless metric.
      OSPF allows sets of networks to be grouped together.  Such a
      grouping is called an area.  The topology of an area is hidden
      from the rest of the Autonomous System.  This information hiding
      enables a significant reduction in routing traffic.  Also,
      routing within the area is determined only by the area's own
      topology, lending the area protection from bad routing data.  An
      area is a generalization of an IP subnetted network.
      OSPF enables the flexible configuration of IP subnets.  Each
      route distributed by OSPF has a destination and mask.  Two
      different subnets of the same IP network number may have
      different sizes (i.e., different masks).  This is commonly
      referred to as variable length subnetting.  A packet is routed
      to the best (i.e., longest or most specific) match.  Host routes
      are considered to be subnets whose masks are "all ones"
      (0xffffffff).
      All OSPF protocol exchanges are authenticated.  This means that
      only trusted routers can participate in the Autonomous System's
      routing.  A variety of authentication schemes can be used; a
      single authentication scheme is configured for each area.  This
      enables some areas to use much stricter authentication than
      others.
      Externally derived routing data (e.g., routes learned from the
      Exterior Gateway Protocol (EGP)) is passed transparently
      throughout the Autonomous System.  This externally derived data
      is kept separate from the OSPF protocol's link state data.  Each
      external route can also be tagged by the advertising router,
      enabling the passing of additional information between routers
      on the boundaries of the Autonomous System.
  1.2.  Definitions of commonly used terms
      This section provides definitions for terms that have a specific
      meaning to the OSPF protocol and that are used throughout the
      text.  The reader unfamiliar with the Internet Protocol Suite is
      referred to [RS-85-153] for an introduction to IP.

Moy [Page 6] RFC 1583 OSPF Version 2 March 1994

      Router
          A level three Internet Protocol packet switch.  Formerly
          called a gateway in much of the IP literature.
      Autonomous System
          A group of routers exchanging routing information via a
          common routing protocol.  Abbreviated as AS.
      Interior Gateway Protocol
          The routing protocol spoken by the routers belonging to an
          Autonomous system.  Abbreviated as IGP.  Each Autonomous
          System has a single IGP.  Separate Autonomous Systems may be
          running different IGPs.
      Router ID
          A 32-bit number assigned to each router running the OSPF
          protocol.  This number uniquely identifies the router within
          an Autonomous System.
      Network
          In this memo, an IP network/subnet/supernet.  It is possible
          for one physical network to be assigned multiple IP
          network/subnet numbers.  We consider these to be separate
          networks.  Point-to-point physical networks are an exception
          - they are considered a single network no matter how many
          (if any at all) IP network/subnet numbers are assigned to
          them.
      Network mask
          A 32-bit number indicating the range of IP addresses
          residing on a single IP network/subnet/supernet.  This
          specification displays network masks as hexadecimal numbers.
          For example, the network mask for a class C IP network is
          displayed as 0xffffff00.  Such a mask is often displayed
          elsewhere in the literature as 255.255.255.0.
      Multi-access networks
          Those physical networks that support the attachment of
          multiple (more than two) routers.  Each pair of routers on
          such a network is assumed to be able to communicate directly
          (e.g., multi-drop networks are excluded).
      Interface
          The connection between a router and one of its attached
          networks.  An interface has state information associated
          with it, which is obtained from the underlying lower level
          protocols and the routing protocol itself.  An interface to
          a network has associated with it a single IP address and

Moy [Page 7] RFC 1583 OSPF Version 2 March 1994

          mask (unless the network is an unnumbered point-to-point
          network).  An interface is sometimes also referred to as a
          link.
      Neighboring routers
          Two routers that have interfaces to a common network.  On
          multi-access networks, neighbors are dynamically discovered
          by OSPF's Hello Protocol.
      Adjacency
          A relationship formed between selected neighboring routers
          for the purpose of exchanging routing information.  Not
          every pair of neighboring routers become adjacent.
      Link state advertisement
          Describes the local state of a router or network.  This
          includes the state of the router's interfaces and
          adjacencies.  Each link state advertisement is flooded
          throughout the routing domain.  The collected link state
          advertisements of all routers and networks forms the
          protocol's topological database.
      Hello Protocol
          The part of the OSPF protocol used to establish and maintain
          neighbor relationships.  On multi-access networks the Hello
          Protocol can also dynamically discover neighboring routers.
      Designated Router
          Each multi-access network that has at least two attached
          routers has a Designated Router.  The Designated Router
          generates a link state advertisement for the multi-access
          network and has other special responsibilities in the
          running of the protocol.  The Designated Router is elected
          by the Hello Protocol.
          The Designated Router concept enables a reduction in the
          number of adjacencies required on a multi-access network.
          This in turn reduces the amount of routing protocol traffic
          and the size of the topological database.
      Lower-level protocols
          The underlying network access protocols that provide
          services to the Internet Protocol and in turn the OSPF
          protocol.  Examples of these are the X.25 packet and frame
          levels for X.25 PDNs, and the ethernet data link layer for
          ethernets.

Moy [Page 8] RFC 1583 OSPF Version 2 March 1994

  1.3.  Brief history of link-state routing technology
      OSPF is a link state routing protocol.  Such protocols are also
      referred to in the literature as SPF-based or distributed-
      database protocols.  This section gives a brief description of
      the developments in link-state technology that have influenced
      the OSPF protocol.
      The first link-state routing protocol was developed for use in
      the ARPANET packet switching network.  This protocol is
      described in [McQuillan].  It has formed the starting point for
      all other link-state protocols.  The homogeneous Arpanet
      environment, i.e., single-vendor packet switches connected by
      synchronous serial lines, simplified the design and
      implementation of the original protocol.
      Modifications to this protocol were proposed in [Perlman].
      These modifications dealt with increasing the fault tolerance of
      the routing protocol through, among other things, adding a
      checksum to the link state advertisements (thereby detecting
      database corruption).  The paper also included means for
      reducing the routing traffic overhead in a link-state protocol.
      This was accomplished by introducing mechanisms which enabled
      the interval between link state advertisement originations to be
      increased by an order of magnitude.
      A link-state algorithm has also been proposed for use as an ISO
      IS-IS routing protocol.  This protocol is described in [DEC].
      The protocol includes methods for data and routing traffic
      reduction when operating over broadcast networks.  This is
      accomplished by election of a Designated Router for each
      broadcast network, which then originates a link state
      advertisement for the network.
      The OSPF subcommittee of the IETF has extended this work in
      developing the OSPF protocol.  The Designated Router concept has
      been greatly enhanced to further reduce the amount of routing
      traffic required.  Multicast capabilities are utilized for
      additional routing bandwidth reduction.  An area routing scheme
      has been developed enabling information
      hiding/protection/reduction.  Finally, the algorithm has been
      modified for efficient operation in TCP/IP internets.
  1.4.  Organization of this document
      The first three sections of this specification give a general
      overview of the protocol's capabilities and functions.  Sections

Moy [Page 9] RFC 1583 OSPF Version 2 March 1994

      4-16 explain the protocol's mechanisms in detail.  Packet
      formats, protocol constants and configuration items are
      specified in the appendices.
      Labels such as HelloInterval encountered in the text refer to
      protocol constants.  They may or may not be configurable.  The
      architectural constants are explained in Appendix B.  The
      configurable constants are explained in Appendix C.
      The detailed specification of the protocol is presented in terms
      of data structures.  This is done in order to make the
      explanation more precise.  Implementations of the protocol are
      required to support the functionality described, but need not
      use the precise data structures that appear in this memo.

2. The Topological Database

  The Autonomous System's topological database describes a directed
  graph.  The vertices of the graph consist of routers and networks.
  A graph edge connects two routers when they are attached via a
  physical point-to-point network.  An edge connecting a router to a
  network indicates that the router has an interface on the network.
  The vertices of the graph can be further typed according to
  function.  Only some of these types carry transit data traffic; that
  is, traffic that is neither locally originated nor locally destined.
  Vertices that can carry transit traffic are indicated on the graph
  by having both incoming and outgoing edges.
                   Vertex type   Vertex name    Transit?
                   _____________________________________
                   1             Router         yes
                   2             Network        yes
                   3             Stub network   no
                        Table 1: OSPF vertex types.
  OSPF supports the following types of physical networks:
  Point-to-point networks
      A network that joins a single pair of routers.  A 56Kb serial
      line is an example of a point-to-point network.

Moy [Page 10] RFC 1583 OSPF Version 2 March 1994

  Broadcast networks
      Networks supporting many (more than two) attached routers,
      together with the capability to address a single physical
      message to all of the attached routers (broadcast).  Neighboring
      routers are discovered dynamically on these nets using OSPF's
      Hello Protocol.  The Hello Protocol itself takes advantage of
      the broadcast capability.  The protocol makes further use of
      multicast capabilities, if they exist.  An ethernet is an
      example of a broadcast network.
  Non-broadcast networks
      Networks supporting many (more than two) routers, but having no
      broadcast capability.  Neighboring routers are also discovered
      on these nets using OSPF's Hello Protocol.  However, due to the
      lack of broadcast capability, some configuration information is
      necessary for the correct operation of the Hello Protocol.  On
      these networks, OSPF protocol packets that are normally
      multicast need to be sent to each neighboring router, in turn.
      An X.25 Public Data Network (PDN) is an example of a non-
      broadcast network.
  The neighborhood of each network node in the graph depends on
  whether the network has multi-access capabilities (either broadcast
  or non-broadcast) and, if so, the number of routers having an
  interface to the network.  The three cases are depicted in Figure 1.
  Rectangles indicate routers.  Circles and oblongs indicate multi-
  access networks.  Router names are prefixed with the letters RT and
  network names with the letter N.  Router interface names are
  prefixed by the letter I.  Lines between routers indicate point-to-
  point networks.  The left side of the figure shows a network with
  its connected routers, with the resulting graph shown on the right.
  Two routers joined by a point-to-point network are represented in
  the directed graph as being directly connected by a pair of edges,
  one in each direction.  Interfaces to physical point-to-point
  networks need not be assigned IP addresses.  Such a point-to-point
  network is called unnumbered.  The graphical representation of
  point-to-point networks is designed so that unnumbered networks can
  be supported naturally.  When interface addresses exist, they are
  modelled as stub routes.  Note that each router would then have a
  stub connection to the other router's interface address (see Figure
  1).
  When multiple routers are attached to a multi-access network, the
  directed graph shows all routers bidirectionally connected to the
  network vertex (again, see Figure 1).  If only a single router is
  attached to a multi-access network, the network will appear in the

Moy [Page 11] RFC 1583 OSPF Version 2 March 1994

  • *FROM * |RT1|RT2| +—+Ia +—+ * ———— |RT1|——|RT2| T RT1| | X | +—+ Ib+—+ O RT2| X | | * Ia| | X | * Ib| X | | Physical point-to-point networks FROM +—+ +—+ |RT3| |RT4| |RT3|RT4|RT5|RT6|N2 | +—+ +—+ * ———————— | N2 | * RT3| | | | | X | +———————-+ T RT4| | | | | X | | | O RT5| | | | | X | +—+ +—+ * RT6| | | | | X | |RT5| |RT6| * N2| X | X | X | X | | +—+ +—+ Multi-access networks FROM +—+ * |RT7| * |RT7| N3| +—+ T ———— | O RT7| | | +———————-+ * N3| X | | N3 * Stub multi-access networks Figure 1: Network map components Networks and routers are represented by vertices. An edge connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X. Moy [Page 12] RFC 1583 OSPF Version 2 March 1994 directed graph as a stub connection. Each network (stub or transit) in the graph has an IP address and associated network mask. The mask indicates the number of nodes on the network. Hosts attached directly to routers (referred to as host routes) appear on the graph as stub networks. The network mask for a host route is always 0xffffffff, which indicates the presence of a single node. Figure 2 shows a sample map of an Autonomous System. The rectangle labelled H1 indicates a host, which has a SLIP connection to Router RT12. Router RT12 is therefore advertising a host route. Lines between routers indicate physical point-to-point networks. The only point-to-point network that has been assigned interface addresses is the one joining Routers RT6 and RT10. Routers RT5 and RT7 have EGP connections to other Autonomous Systems. A set of EGP-learned routes have been displayed for both of these routers. A cost is associated with the output side of each router interface. This cost is configurable by the system administrator. The lower the cost, the more likely the interface is to be used to forward data traffic. Costs are also associated with the externally derived routing data (e.g., the EGP-learned routes). The directed graph resulting from the map in Figure 2 is depicted in Figure 3. Arcs are labelled with the cost of the corresponding router output interface. Arcs having no labelled cost have a cost of 0. Note that arcs leading from networks to routers always have cost 0; they are significant nonetheless. Note also that the externally derived routing data appears on the graph as stubs. The topological database (or what has been referred to above as the directed graph) is pieced together from link state advertisements generated by the routers. The neighborhood of each transit vertex is represented in a single, separate link state advertisement. Figure 4 shows graphically the link state representation of the two kinds of transit vertices: routers and multi-access networks. Router RT12 has an interface to two broadcast networks and a SLIP line to a host. Network N6 is a broadcast network with three attached routers. The cost of all links from Network N6 to its attached routers is 0. Note that the link state advertisement for Network N6 is actually generated by one of the attached routers: the router that has been elected Designated Router for the network. 2.1. The shortest-path tree When no OSPF areas are configured, each router in the Autonomous System has an identical topological database, leading to an Moy [Page 13] RFC 1583 OSPF Version 2 March 1994 + | 3+—+ N12 N14 N1|–|RT1|\ 1 \ N13 / | +—+ \ 8\ |8/8 + \ \|/ / \ 1+—+8 8+—+6 * N3 *—|RT4|——|RT5|——–+ \/ +—+ +—+ | + / | |7 | | 3+—+ / | | | N2|–|RT2|/1 |1 |6 | | +—+ +—+8 6+—+ | + |RT3|————–|RT6| | +—+ +—+ | |2 Ia|7 | | | | +———+ | | N4 | | | | | | N11 | | +———+ | | | | | N12 |3 | |6 2/ +—+ | +—+/ |RT9| | |RT7|—N15 +—+ | +—+ 9 |1 + | |1 _| | Ib|5 |_ / \ 1+—-+2 | 3+—-+1 / \ * N9 *——|RT11|—-|—|RT10|—* N6 * \/ +—-+ | +—-+ \/ | | | |1 + |1 +–+ 10+—-+ N8 +—+ |H1|—–|RT12| |RT8| +–+SLIP +—-+ +—+ |2 |4 | | +———+ +——–+ N10 N7 Figure 2: A sample Autonomous System Moy [Page 14] RFC 1583 OSPF Version 2 March 1994 FROM |RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT| |1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9| —– ——————————————— RT1| | | | | | | | | | | | |0 | | | | RT2| | | | | | | | | | | | |0 | | | | RT3| | | | | |6 | | | | | | |0 | | | | RT4| | | | |8 | | | | | | | |0 | | | | RT5| | | |8 | |6 |6 | | | | | | | | | | RT6| | |8 | |7 | | | | |5 | | | | | | | RT7| | | | |6 | | | | | | | | |0 | | | * RT8| | | | | | | | | | | | | |0 | | | * RT9| | | | | | | | | | | | | | | |0 | T RT10| | | | | |7 | | | | | | | |0 |0 | | O RT11| | | | | | | | | | | | | | |0 |0 | * RT12| | | | | | | | | | | | | | | |0 | * N1|3 | | | | | | | | | | | | | | | | N2| |3 | | | | | | | | | | | | | | | N3|1 |1 |1 |1 | | | | | | | | | | | | | N4| | |2 | | | | | | | | | | | | | | N6| | | | | | |1 |1 | |1 | | | | | | | N7| | | | | | | |4 | | | | | | | | | N8| | | | | | | | | |3 |2 | | | | | | N9| | | | | | | | |1 | |1 |1 | | | | | N10| | | | | | | | | | | |2 | | | | | N11| | | | | | | | |3 | | | | | | | | N12| | | | |8 | |2 | | | | | | | | | | N13| | | | |8 | | | | | | | | | | | | N14| | | | |8 | | | | | | | | | | | | N15| | | | | | |9 | | | | | | | | | | H1| | | | | | | | | | | |10| | | | | Figure 3: The resulting directed graph Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X. Moy [Page 15] RFC 1583 OSPF Version 2 March 1994 FROM FROM |RT12|N9|N10|H1| |RT9|RT11|RT12|N9| * ——————– * ———————- * RT12| | | | | * RT9| | | |0 | T N9|1 | | | | T RT11| | | |0 | O N10|2 | | | | O RT12| | | |0 | * H1|10 | | | | * N9| | | | | * * RT12's router links N9's network links advertisement advertisement Figure 4: Individual link state components Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X. identical graphical representation. A router generates its routing table from this graph by calculating a tree of shortest paths with the router itself as root. Obviously, the shortest- path tree depends on the router doing the calculation. The shortest-path tree for Router RT6 in our example is depicted in Figure 5. The tree gives the entire route to any destination network or host. However, only the next hop to the destination is used in the forwarding process. Note also that the best route to any router has also been calculated. For the processing of external data, we note the next hop and distance to any router advertising external routes. The resulting routing table for Router RT6 is pictured in Table 2. Note that there is a separate route for each end of a numbered serial line (in this case, the serial line between Routers RT6 and RT10). Routes to networks belonging to other AS'es (such as N12) appear as dashed lines on the shortest path tree in Figure 5. Use of this externally derived routing information is considered in the next section. 2.2. Use of external routing information After the tree is created the external routing information is examined. This external routing information may originate from another routing protocol such as EGP, or be statically Moy [Page 16] RFC 1583 OSPF Version 2 March 1994 RT6(origin) RT5 o————o———–o Ib /|\ 6 |\ 7 8/8|8\ | \ / | \ | \ o | o | \7 N12 o N14 | \ N13 2 | \ N4 o—–o RT3 \ / \ 5 1/ RT10 o——-o Ia / |\ RT4 o—–o N3 3| \1 /| | \ N6 RT7 / | N8 o o———o / | | | /| RT2 o o RT1 | | 2/ |9 / | | |RT8 / | /3 |3 RT11 o o o o / | | | N12 N15 N2 o o N1 1| |4 | | N9 o o N7 /| / | N11 RT9 / |RT12 o——–o——-o o——–o H1 3 | 10 |2 | o N10 Figure 5: The SPF tree for Router RT6 Edges that are not marked with a cost have a cost of of zero (these are network-to-router links). Routes to networks N12-N15 are external information that is considered in Section 2.2 Moy [Page 17] RFC 1583 OSPF Version 2 March 1994 Destination Next Hop Distance N1 RT3 10 N2 RT3 10 N3 RT3 7 N4 RT3 8 Ib * 7 Ia RT10 12 N6 RT10 8 N7 RT10 12 N8 RT10 10 N9 RT10 11 N10 RT10 13 N11 RT10 14 H1 RT10 21 RT5 RT5 6 RT7 RT10 8 Table 2: The portion of Router RT6's routing table listing local destinations. configured (static routes). Default routes can also be included as part of the Autonomous System's external routing information. External routing information is flooded unaltered throughout the AS. In our example, all the routers in the Autonomous System know that Router RT7 has two external routes, with metrics 2 and 9. OSPF supports two types of external metrics. Type 1 external metrics are equivalent to the link state metric. Type 2 external metrics are greater than the cost of any path internal to the AS. Use of Type 2 external metrics assumes that routing between AS'es is the major cost of routing a packet, and eliminates the need for conversion of external costs to internal link state metrics. As an example of Type 1 external metric processing, suppose that the Routers RT7 and RT5 in Figure 2 are advertising Type 1 external metrics. For each external route, the distance from Router RT6 is calculated as the sum of the external route's cost and the distance from Router RT6 to the advertising router. For every external destination, the router advertising the shortest route is discovered, and the next hop to the advertising router becomes the next hop to the destination. Moy [Page 18] RFC 1583 OSPF Version 2 March 1994 Both Router RT5 and RT7 are advertising an external route to destination Network N12. Router RT7 is preferred since it is advertising N12 at a distance of 10 (8+2) to Router RT6, which is better than Router RT5's 14 (6+8). Table 3 shows the entries that are added to the routing table when external routes are examined: Destination Next Hop Distance N12 RT10 10 N13 RT5 14 N14 RT5 14 N15 RT10 17 Table 3: The portion of Router RT6's routing table listing external destinations. Processing of Type 2 external metrics is simpler. The AS boundary router advertising the smallest external metric is chosen, regardless of the internal distance to the AS boundary router. Suppose in our example both Router RT5 and Router RT7 were advertising Type 2 external routes. Then all traffic destined for Network N12 would be forwarded to Router RT7, since 2 < 8. When several equal-cost Type 2 routes exist, the internal distance to the advertising routers is used to break the tie. Both Type 1 and Type 2 external metrics can be present in the AS at the same time. In that event, Type 1 external metrics always take precedence. This section has assumed that packets destined for external destinations are always routed through the advertising AS boundary router. This is not always desirable. For example, suppose in Figure 2 there is an additional router attached to Network N6, called Router RTX. Suppose further that RTX does not participate in OSPF routing, but does exchange EGP information with the AS boundary router RT7. Then, Router RT7 would end up advertising OSPF external routes for all destinations that should be routed to RTX. An extra hop will sometimes be introduced if packets for these destinations need always be routed first to Router RT7 (the advertising router). To deal with this situation, the OSPF protocol allows an AS Moy [Page 19] RFC 1583 OSPF Version 2 March 1994 boundary router to specify a "forwarding address" in its external advertisements. In the above example, Router RT7 would specify RTX's IP address as the "forwarding address" for all those destinations whose packets should be routed directly to RTX. The "forwarding address" has one other application. It enables routers in the Autonomous System's interior to function as "route servers". For example, in Figure 2 the router RT6 could become a route server, gaining external routing information through a combination of static configuration and external routing protocols. RT6 would then start advertising itself as an AS boundary router, and would originate a collection of OSPF external advertisements. In each external advertisement, Router RT6 would specify the correct Autonomous System exit point to use for the destination through appropriate setting of the advertisement's "forwarding address" field. 2.3. Equal-cost multipath The above discussion has been simplified by considering only a single route to any destination. In reality, if multiple equal-cost routes to a destination exist, they are all discovered and used. This requires no conceptual changes to the algorithm, and its discussion is postponed until we consider the tree-building process in more detail. With equal cost multipath, a router potentially has several available next hops towards any given destination. 2.4. TOS-based routing OSPF can calculate a separate set of routes for each IP Type of Service. This means that, for any destination, there can potentially be multiple routing table entries, one for each IP TOS. The IP TOS values are represented in OSPF exactly as they appear in the IP packet header. Up to this point, all examples shown have assumed that routes do not vary on TOS. In order to differentiate routes based on TOS, separate interface costs can be configured for each TOS. For example, in Figure 2 there could be multiple costs (one for each TOS) listed for each interface. A cost for TOS 0 must always be specified. When interface costs vary based on TOS, a separate shortest path Moy [Page 20] RFC 1583 OSPF Version 2 March 1994 tree is calculated for each TOS (see Section 2.1). In addition, external costs can vary based on TOS. For example, in Figure 2 Router RT7 could advertise a separate type 1 external metric for each TOS. Then, when calculating the TOS X distance to Network N15 the cost of the shortest TOS X path to RT7 would be added to the TOS X cost advertised by RT7 for Network N15 (see Section 2.2). All OSPF implementations must be capable of calculating routes based on TOS. However, OSPF routers can be configured to route all packets on the TOS 0 path (see Appendix C), eliminating the need to calculate non-zero TOS paths. This can be used to conserve routing table space and processing resources in the router. These TOS-0-only routers can be mixed with routers that do route based on TOS. TOS-0-only routers will be avoided as much as possible when forwarding traffic requesting a non-zero TOS. It may be the case that no path exists for some non-zero TOS, even if the router is calculating non-zero TOS paths. In that case, packets requesting that non-zero TOS are routed along the TOS 0 path (see Section 11.1). 3. Splitting the AS into Areas OSPF allows collections of contiguous networks and hosts to be grouped together. Such a group, together with the routers having interfaces to any one of the included networks, is called an area. Each area runs a separate copy of the basic link-state routing algorithm. This means that each area has its own topological database and corresponding graph, as explained in the previous section. The topology of an area is invisible from the outside of the area. Conversely, routers internal to a given area know nothing of the detailed topology external to the area. This isolation of knowledge enables the protocol to effect a marked reduction in routing traffic as compared to treating the entire Autonomous System as a single link-state domain. With the introduction of areas, it is no longer true that all routers in the AS have an identical topological database. A router actually has a separate topological database for each area it is connected to. (Routers connected to multiple areas are called area border routers). Two routers belonging to the same area have, for that area, identical area topological databases. Moy [Page 21] RFC 1583 OSPF Version 2 March 1994 Routing in the Autonomous System takes place on two levels, depending on whether the source and destination of a packet reside in the same area (intra-area routing is used) or different areas (inter-area routing is used). In intra-area routing, the packet is routed solely on information obtained within the area; no routing information obtained from outside the area can be used. This protects intra-area routing from the injection of bad routing information. We discuss inter-area routing in Section 3.2. 3.1. The backbone of the Autonomous System The backbone consists of those networks not contained in any area, their attached routers, and those routers that belong to multiple areas. The backbone must be contiguous. It is possible to define areas in such a way that the backbone is no longer contiguous. In this case the system administrator must restore backbone connectivity by configuring virtual links. Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to-point network. On the graph of the backbone, two such routers are joined by arcs whose costs are the intra-area distances between the two routers. The routing protocol traffic that flows along the virtual link uses intra- area routing only. The backbone is responsible for distributing routing information between areas. The backbone itself has all of the properties of an area. The topology of the backbone is invisible to each of the areas, while the backbone itself knows nothing of the topology of the areas. 3.2. Inter-area routing When routing a packet between two areas the backbone is used. The path that the packet will travel can be broken up into three contiguous pieces: an intra-area path from the source to an area border router, a backbone path between the source and destination areas, and then another intra-area path to the destination. The algorithm finds the set of such paths that have the smallest cost. Looking at this another way, inter-area routing can be pictured Moy [Page 22] RFC 1583 OSPF Version 2 March 1994 as forcing a star configuration on the Autonomous System, with the backbone as hub and each of the areas as spokes. The topology of the backbone dictates the backbone paths used between areas. The topology of the backbone can be enhanced by adding virtual links. This gives the system administrator some control over the routes taken by inter-area traffic. The correct area border router to use as the packet exits the source area is chosen in exactly the same way routers advertising external routes are chosen. Each area border router in an area summarizes for the area its cost to all networks external to the area. After the SPF tree is calculated for the area, routes to all other networks are calculated by examining the summaries of the area border routers. 3.3. Classification of routers Before the introduction of areas, the only OSPF routers having a specialized function were those advertising external routing information, such as Router RT5 in Figure 2. When the AS is split into OSPF areas, the routers are further divided according to function into the following four overlapping categories: Internal routers A router with all directly connected networks belonging to the same area. Routers with only backbone interfaces also belong to this category. These routers run a single copy of the basic routing algorithm. Area border routers A router that attaches to multiple areas. Area border routers run multiple copies of the basic algorithm, one copy for each attached area and an additional copy for the backbone. Area border routers condense the topological information of their attached areas for distribution to the backbone. The backbone in turn distributes the information to the other areas. Backbone routers A router that has an interface to the backbone. This includes all routers that interface to more than one area (i.e., area border routers). However, backbone routers do not have to be area border routers. Routers with all interfaces connected to the backbone are considered to be internal routers. Moy [Page 23] RFC 1583 OSPF Version 2 March 1994 AS boundary routers A router that exchanges routing information with routers belonging to other Autonomous Systems. Such a router has AS external routes that are advertised throughout the Autonomous System. The path to each AS boundary router is known by every router in the AS. This classification is completely independent of the previous classifications: AS boundary routers may be internal or area border routers, and may or may not participate in the backbone. 3.4. A sample area configuration Figure 6 shows a sample area configuration. The first area consists of networks N1-N4, along with their attached routers RT1-RT4. The second area consists of networks N6-N8, along with their attached routers RT7, RT8, RT10 and RT11. The third area consists of networks N9-N11 and Host H1, along with their attached routers RT9, RT11 and RT12. The third area has been configured so that networks N9-N11 and Host H1 will all be grouped into a single route, when advertised external to the area (see Section 3.5 for more details). In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are internal routers. Routers RT3, RT4, RT7, RT10 and RT11 are area border routers. Finally, as before, Routers RT5 and RT7 are AS boundary routers. Figure 7 shows the resulting topological database for the Area 1. The figure completely describes that area's intra-area routing. It also shows the complete view of the internet for the two internal routers RT1 and RT2. It is the job of the area border routers, RT3 and RT4, to advertise into Area 1 the distances to all destinations external to the area. These are indicated in Figure 7 by the dashed stub routes. Also, RT3 and RT4 must advertise into Area 1 the location of the AS boundary routers RT5 and RT7. Finally, external advertisements from RT5 and RT7 are flooded throughout the entire AS, and in particular throughout Area 1. These advertisements are included in Area 1's database, and yield routes to Networks N12-N15. Routers RT3 and RT4 must also summarize Area 1's topology for distribution to the backbone. Their backbone advertisements are shown in Table 4. These summaries show which networks are contained in Area 1 (i.e., Networks N1-N4), and the distance to these networks from the routers RT3 and RT4 respectively. Moy [Page 24] RFC 1583 OSPF Version 2 March 1994 ……………………… . + . . | 3+—+ . N12 N14 . N1|–|RT1|\ 1 . \ N13 / . | +—+ \ . 8\ |8/8 . + \ . \|/ . / \ 1+—+8 8+—+6 . * N3 *—|RT4|——|RT5|——–+ . \/ +—+ +—+ | . + / \ . |7 | . | 3+—+ / \ . | | . N2|–|RT2|/1 1\ . |6 | . | +—+ +—+8 6+—+ | . + |RT3|——|RT6| | . +—+ +—+ | . 2/ . Ia|7 | . / . | | . +———+ . | | .Area 1 N4 . | | ……………………… | | …………………….. | | . N11 . | | . +———+ . | | . | . | | N12 . |3 . Ib|5 |6 2/ . +—+ . +—-+ +—+/ . |RT9| . ………|RT10|…..|RT7|—N15. . +—+ . . +—-+ +—+ 9 . . |1 . . + /3 1\ |1 . . _| . . | / \ |_ . . / \ 1+—-+2 |/ \ / \ . . * N9 *——|RT11|—-| * N6 * . . \/ +—-+ | \/ . . | . . | | . . |1 . . + |1 . . +–+ 10+—-+ . . N8 +—+ . . |H1|—–|RT12| . . |RT8| . . +–+SLIP +—-+ . . +—+ . . |2 . . |4 . . | . . | . . +———+ . . +——–+ . . N10 . . N7 . . . .Area 2 . .Area 3 . ………………………….. …………………….. Figure 6: A sample OSPF area configuration Moy [Page 25] RFC 1583 OSPF Version 2 March 1994 Network RT3 adv. RT4 adv. _ N1 4 4 N2 4 4 N3 1 1 N4 2 3 Table 4: Networks advertised to the backbone by Routers RT3 and RT4. The topological database for the backbone is shown in Figure 8. The set of routers pictured are the backbone routers. Router RT11 is a backbone router because it belongs to two areas. In order to make the backbone connected, a virtual link has been configured between Routers R10 and R11. Again, Routers RT3, RT4, RT7, RT10 and RT11 are area border routers. As Routers RT3 and RT4 did above, they have condensed the routing information of their attached areas for distribution via the backbone; these are the dashed stubs that appear in Figure 8. Remember that the third area has been configured to condense Networks N9-N11 and Host H1 into a single route. This yields a single dashed line for networks N9-N11 and Host H1 in Figure 8. Routers RT5 and RT7 are AS boundary routers; their externally derived information also appears on the graph in Figure 8 as stubs. The backbone enables the exchange of summary information between area border routers. Every area border router hears the area summaries from all other area border routers. It then forms a picture of the distance to all networks outside of its area by examining the collected advertisements, and adding in the backbone distance to each advertising router. Again using Routers RT3 and RT4 as an example, the procedure goes as follows: They first calculate the SPF tree for the backbone. This gives the distances to all other area border routers. Also noted are the distances to networks (Ia and Ib) and AS boundary routers (RT5 and RT7) that belong to the backbone. This calculation is shown in Table 5. Next, by looking at the area summaries from these area border routers, RT3 and RT4 can determine the distance to all networks outside their area. These distances are then advertised internally to the area by RT3 and RT4. The advertisements that Router RT3 and RT4 will make into Area 1 are shown in Table 6. Moy [Page 26] RFC 1583 OSPF Version 2 March 1994 FROM |RT|RT|RT|RT|RT|RT| |1 |2 |3 |4 |5 |7 |N3| —– ——————- RT1| | | | | | |0 | RT2| | | | | | |0 | RT3| | | | | | |0 | * RT4| | | | | | |0 | * RT5| | |14|8 | | | | T RT7| | |20|14| | | | O N1|3 | | | | | | | * N2| |3 | | | | | | * N3|1 |1 |1 |1 | | | | N4| | |2 | | | | | Ia,Ib| | |15|22| | | | N6| | |16|15| | | | N7| | |20|19| | | | N8| | |18|18| | | | N9-N11,H1| | |19|16| | | | N12| | | | |8 |2 | | N13| | | | |8 | | | N14| | | | |8 | | | N15| | | | | |9 | | Figure 7: Area 1's Database. Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X. Moy [Page 27] RFC 1583 OSPF Version 2 March 1994 FROM |RT|RT|RT|RT|RT|RT|RT |3 |4 |5 |6 |7 |10|11| ———————— RT3| | | |6 | | | | RT4| | |8 | | | | | RT5| |8 | |6 |6 | | | RT6|8 | |7 | | |5 | | RT7| | |6 | | | | | * RT10| | | |7 | | |2 | * RT11| | | | | |3 | | T N1|4 |4 | | | | | | O N2|4 |4 | | | | | | * N3|1 |1 | | | | | | * N4|2 |3 | | | | | | Ia| | | | | |5 | | Ib| | | |7 | | | | N6| | | | |1 |1 |3 | N7| | | | |5 |5 |7 | N8| | | | |4 |3 |2 | N9-N11,H1| | | | | | |1 | N12| | |8 | |2 | | | N13| | |8 | | | | | N14| | |8 | | | | | N15| | | | |9 | | | Figure 8: The backbone's database. Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X. Moy [Page 28] RFC 1583 OSPF Version 2 March 1994 Area border dist from dist from router RT3 RT4 to RT3 * 21 to RT4 22 * to RT7 20 14 to RT10 15 22 to RT11 18 25 to Ia 20 27 to Ib 15 22 to RT5 14 8 to RT7 20 14 Table 5: Backbone distances calculated by Routers RT3 and RT4. Note that Table 6 assumes that an area range has been configured for the backbone which groups Ia and Ib into a single advertisement. The information imported into Area 1 by Routers RT3 and RT4 enables an internal router, such as RT1, to choose an area border router intelligently. Router RT1 would use RT4 for traffic to Network N6, RT3 for traffic to Network N10, and would load share between the two for traffic to Network N8. Destination RT3 adv. RT4 adv. _ Ia,Ib 15 22 N6 16 15 N7 20 19 N8 18 18 N9-N11,H1 19 26 _ RT5 14 8 RT7 20 14 Table 6: Destinations advertised into Area 1 by Routers RT3 and RT4. Moy [Page 29] RFC 1583 OSPF Version 2 March 1994 Router RT1 can also determine in this manner the shortest path to the AS boundary routers RT5 and RT7. Then, by looking at RT5 and RT7's external advertisements, Router RT1 can decide between RT5 or RT7 when sending to a destination in another Autonomous System (one of the networks N12-N15). Note that a failure of the line between Routers RT6 and RT10 will cause the backbone to become disconnected. Configuring a virtual link between Routers RT7 and RT10 will give the backbone more connectivity and more resistance to such failures. Also, a virtual link between RT7 and RT10 would allow a much shorter path between the third area (containing N9) and the router RT7, which is advertising a good route to external network N12. 3.5. IP subnetting support OSPF attaches an IP address mask to each advertised route. The mask indicates the range of addresses being described by the particular route. For example, a summary advertisement for the destination 128.185.0.0 with a mask of 0xffff0000 actually is describing a single route to the collection of destinations 128.185.0.0 - 128.185.255.255. Similarly, host routes are always advertised with a mask of 0xffffffff, indicating the presence of only a single destination. Including the mask with each advertised destination enables the implementation of what is commonly referred to as variable- length subnetting. This means that a single IP class A, B, or C network number can be broken up into many subnets of various sizes. For example, the network 128.185.0.0 could be broken up into 62 variable-sized subnets: 15 subnets of size 4K, 15 subnets of size 256, and 32 subnets of size 8. Table 7 shows some of the resulting network addresses together with their masks: Network address IP address mask Subnet size _ 128.185.16.0 0xfffff000 4K 128.185.1.0 0xffffff00 256 128.185.0.8 0xfffffff8 8 Table 7: Some sample subnet sizes. Moy [Page 30] RFC 1583 OSPF Version 2 March 1994 There are many possible ways of dividing up a class A, B, and C network into variable sized subnets. The precise procedure for doing so is beyond the scope of this specification. This specification however establishes the following guideline: When an IP packet is forwarded, it is always forwarded to the network that is the best match for the packet's destination. Here best match is synonymous with the longest or most specific match. For example, the default route with destination of 0.0.0.0 and mask 0x00000000 is always a match for every IP destination. Yet it is always less specific than any other match. Subnet masks must be assigned so that the best match for any IP destination is unambiguous. The OSPF area concept is modelled after an IP subnetted network. OSPF areas have been loosely defined to be a collection of networks. In actuality, an OSPF area is specified to be a list of address ranges (see Section C.2 for more details). Each address range is defined as an [address,mask] pair. Many separate networks may then be contained in a single address range, just as a subnetted network is composed of many separate subnets. Area border routers then summarize the area contents (for distribution to the backbone) by advertising a single route for each address range. The cost of the route is the minimum cost to any of the networks falling in the specified range. For example, an IP subnetted network can be configured as a single OSPF area. In that case, the area would be defined as a single address range: a class A, B, or C network number along with its natural IP mask. Inside the area, any number of variable sized subnets could be defined. External to the area, a single route for the entire subnetted network would be distributed, hiding even the fact that the network is subnetted at all. The cost of this route is the minimum of the set of costs to the component subnets. 3.6. Supporting stub areas In some Autonomous Systems, the majority of the topological database may consist of AS external advertisements. An OSPF AS external advertisement is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as "stub areas". AS external advertisements are not flooded into/throughout stub areas; routing to AS external destinations in these areas is based on a (per-area) default only. This reduces the topological database size, and therefore the memory requirements, for a stub area's internal routers. Moy [Page 31] RFC 1583 OSPF Version 2 March 1994 In order to take advantage of the OSPF stub area support, default routing must be used in the stub area. This is accomplished as follows. One or more of the stub area's area border routers must advertise a default route into the stub area via summary link advertisements. These summary defaults are flooded throughout the stub area, but no further. (For this reason these defaults pertain only to the particular stub area). These summary default routes will match any destination that is not explicitly reachable by an intra-area or inter-area path (i.e., AS external destinations). An area can be configured as stub when there is a single exit point from the area, or when the choice of exit point need not be made on a per-external-destination basis. For example, Area 3 in Figure 6 could be configured as a stub area, because all external traffic must travel though its single area border router RT11. If Area 3 were configured as a stub, Router RT11 would advertise a default route for distribution inside Area 3 (in a summary link advertisement), instead of flooding the AS external advertisements for Networks N12-N15 into/throughout the area. The OSPF protocol ensures that all routers belonging to an area agree on whether the area has been configured as a stub. This guarantees that no confusion will arise in the flooding of AS external advertisements. There are a couple of restrictions on the use of stub areas. Virtual links cannot be configured through stub areas. In addition, AS boundary routers cannot be placed internal to stub areas. 3.7. Partitions of areas OSPF does not actively attempt to repair area partitions. When an area becomes partitioned, each component simply becomes a separate area. The backbone then performs routing between the new areas. Some destinations reachable via intra-area routing before the partition will now require inter-area routing. In the previous section, an area was described as a list of address ranges. Any particular address range must still be completely contained in a single component of the area partition. This has to do with the way the area contents are summarized to the backbone. Also, the backbone itself must not partition. If it does, parts of the Autonomous System will become unreachable. Backbone partitions can be repaired by Moy [Page 32] RFC 1583 OSPF Version 2 March 1994 configuring virtual links (see Section 15). Another way to think about area partitions is to look at the Autonomous System graph that was introduced in Section 2. Area IDs can be viewed as colors for the graph's edges.[1] Each edge of the graph connects to a network, or is itself a point-to- point network. In either case, the edge is colored with the network's Area ID. A group of edges, all having the same color, and interconnected by vertices, represents an area. If the topology of the Autonomous System is intact, the graph will have several regions of color, each color being a distinct Area ID. When the AS topology changes, one of the areas may become partitioned. The graph of the AS will then have multiple regions of the same color (Area ID). The routing in the Autonomous System will continue to function as long as these regions of same color are connected by the single backbone region. Moy [Page 33] RFC 1583 OSPF Version 2 March 1994 4. Functional Summary A separate copy of OSPF's basic routing algorithm runs in each area. Routers having interfaces to multiple areas run multiple copies of the algorithm. A brief summary of the routing algorithm follows. When a router starts, it first initializes the routing protocol data structures. The router then waits for indications from the lower- level protocols that its interfaces are functional. A router then uses the OSPF's Hello Protocol to acquire neighbors. The router sends Hello packets to its neighbors, and in turn receives their Hello packets. On broadcast and point-to-point networks, the router dynamically detects its neighboring routers by sending its Hello packets to the multicast address AllSPFRouters. On non-broadcast networks, some configuration information is necessary in order to discover neighbors. On all multi-access networks (broadcast or non-broadcast), the Hello Protocol also elects a Designated router for the network. The router will attempt to form adjacencies with some of its newly acquired neighbors. Topological databases are synchronized between pairs of adjacent routers. On multi-access networks, the Designated Router determines which routers should become adjacent. Adjacencies control the distribution of routing protocol packets. Routing protocol packets are sent and received only on adjacencies. In particular, distribution of topological database updates proceeds along adjacencies. A router periodically advertises its state, which is also called link state. Link state is also advertised when a router's state changes. A router's adjacencies are reflected in the contents of its link state advertisements. This relationship between adjacencies and link state allows the protocol to detect dead routers in a timely fashion. Link state advertisements are flooded throughout the area. The flooding algorithm is reliable, ensuring that all routers in an area have exactly the same topological database. This database consists of the collection of link state advertisements received from each router belonging to the area. From this database each router calculates a shortest-path tree, with itself as root. This shortest-path tree in turn yields a routing table for the protocol. Moy [Page 34] RFC 1583 OSPF Version 2 March 1994 4.1. Inter-area routing The previous section described the operation of the protocol within a single area. For intra-area routing, no other routing information is pertinent. In order to be able to route to destinations outside of the area, the area border routers inject additional routing information into the area. This additional information is a distillation of the rest of the Autonomous System's topology. This distillation is accomplished as follows: Each area border router is by definition connected to the backbone. Each area border router summarizes the topology of its attached areas for transmission on the backbone, and hence to all other area border routers. An area border router then has complete topological information concerning the backbone, and the area summaries from each of the other area border routers. From this information, the router calculates paths to all destinations not contained in its attached areas. The router then advertises these paths into its attached areas. This enables the area's internal routers to pick the best exit router when forwarding traffic to destinations in other areas. 4.2. AS external routes Routers that have information regarding other Autonomous Systems can flood this information throughout the AS. This external routing information is distributed verbatim to every participating router. There is one exception: external routing information is not flooded into "stub" areas (see Section 3.6). To utilize external routing information, the path to all routers advertising external information must be known throughout the AS (excepting the stub areas). For that reason, the locations of these AS boundary routers are summarized by the (non-stub) area border routers. 4.3. Routing protocol packets The OSPF protocol runs directly over IP, using IP protocol 89. OSPF does not provide any explicit fragmentation/reassembly support. When fragmentation is necessary, IP fragmentation/reassembly is used. OSPF protocol packets have been designed so that large protocol packets can generally be split into several smaller protocol packets. This practice is recommended; IP fragmentation should be avoided whenever Moy [Page 35] RFC 1583 OSPF Version 2 March 1994 possible. Routing protocol packets should always be sent with the IP TOS field set to 0. If at all possible, routing protocol packets should be given preference over regular IP data traffic, both when being sent and received. As an aid to accomplishing this, OSPF protocol packets should have their IP precedence field set to the value Internetwork Control (see [RFC 791]). All OSPF protocol packets share a common protocol header that is described in Appendix A. The OSPF packet types are listed below in Table 8. Their formats are also described in Appendix A. Type Packet name Protocol function 1 Hello Discover/maintain neighbors 2 Database Description Summarize database contents 3 Link State Request Database download 4 Link State Update Database update 5 Link State Ack Flooding acknowledgment Table 8: OSPF packet types. OSPF's Hello protocol uses Hello packets to discover and maintain neighbor relationships. The Database Description and Link State Request packets are used in the forming of adjacencies. OSPF's reliable update mechanism is implemented by the Link State Update and Link State Acknowledgment packets. Each Link State Update packet carries a set of new link state advertisements one hop further away from their point of origination. A single Link State Update packet may contain the link state advertisements of several routers. Each advertisement is tagged with the ID of the originating router and a checksum of its link state contents. The five different types of OSPF link state advertisements are listed below in Table 9. As mentioned above, OSPF routing packets (with the exception of Hellos) are sent only over adjacencies. Note that this means that all OSPF protocol packets travel a single IP hop, except those that are sent over virtual adjacencies. The IP source address of an OSPF protocol packet is one end of a router adjacency, and the IP destination address is either the other Moy [Page 36] RFC 1583 OSPF Version 2 March 1994 LS Advertisement Advertisement description type name _ 1 Router links Originated by all routers. advertisements This advertisement describes the collected states of the router's interfaces to an area. Flooded throughout a single area only. _ 2 Network links Originated for multi-access advertisements networks by the Designated Router. This advertisement contains the list of routers connected to the network. Flooded throughout a single area only. _ 3,4 Summary link Originated by area border advertisements routers, and flooded through- out the advertisement's associated area. Each summary link advertisement describes a route to a destination out- side the area, yet still inside the AS (i.e., an inter-area route). Type 3 advertisements describe routes to networks. Type 4 advertisements describe routes to AS boundary routers. _ 5 AS external link Originated by AS boundary advertisements routers, and flooded through- out the AS. Each AS external link advertisement describes a route to a destination in another Autonomous System. Default routes for the AS can also be described by AS external link advertisements. Table 9: OSPF link state advertisements. Moy [Page 37] RFC 1583 OSPF Version 2 March 1994 end of the adjacency or an IP multicast address. 4.4. Basic implementation requirements An implementation of OSPF requires the following pieces of system support: Timers Two different kind of timers are required. The first kind, called single shot timers, fire once and cause a protocol event to be processed. The second kind, called interval timers, fire at continuous intervals. These are used for the sending of packets at regular intervals. A good example of this is the regular broadcast of Hello packets (on broadcast networks). The granularity of both kinds of timers is one second. Interval timers should be implemented to avoid drift. In some router implementations, packet processing can affect timer execution. When multiple routers are attached to a single network, all doing broadcasts, this can lead to the synchronization of routing packets (which should be avoided). If timers cannot be implemented to avoid drift, small random amounts should be added to/subtracted from the timer interval at each firing. IP multicast Certain OSPF packets take the form of IP multicast datagrams. Support for receiving and sending IP multicast datagrams, along with the appropriate lower-level protocol support, is required. The IP multicast datagrams used by OSPF never travel more than one hop. For this reason, the ability to forward IP multicast datagrams is not required. For information on IP multicast, see [RFC 1112]. Variable-length subnet support The router's IP protocol support must include the ability to divide a single IP class A, B, or C network number into many subnets of various sizes. This is commonly called variable-length subnetting; see Section 3.5 for details. IP supernetting support The router's IP protocol support must include the ability to aggregate contiguous collections of IP class A, B, and C networks into larger quantities called supernets. Supernetting has been proposed as one way to improve the Moy [Page 38] RFC 1583 OSPF Version 2 March 1994 scaling of IP routing in the worldwide Internet. For more information on IP supernetting, see [RFC 1519]. Lower-level protocol support The lower level protocols referred to here are the network access protocols, such as the Ethernet data link layer. Indications must be passed from these protocols to OSPF as the network interface goes up and down. For example, on an ethernet it would be valuable to know when the ethernet transceiver cable becomes unplugged. Non-broadcast lower-level protocol support Remember that non-broadcast networks are multi-access networks such as a X.25 PDN. On these networks, the Hello Protocol can be aided by providing an indication to OSPF when an attempt is made to send a packet to a dead or non- existent router. For example, on an X.25 PDN a dead neighboring router may be indicated by the reception of a X.25 clear with an appropriate cause and diagnostic, and this information would be passed to OSPF. List manipulation primitives Much of the OSPF functionality is described in terms of its operation on lists of link state advertisements. For example, the collection of advertisements that will be retransmitted to an adjacent router until acknowledged are described as a list. Any particular advertisement may be on many such lists. An OSPF implementation needs to be able to manipulate these lists, adding and deleting constituent advertisements as necessary. Tasking support Certain procedures described in this specification invoke other procedures. At times, these other procedures should be executed in-line, that is, before the current procedure is finished. This is indicated in the text by instructions to execute a procedure. At other times, the other procedures are to be executed only when the current procedure has finished. This is indicated by instructions to schedule a task. 4.5. Optional OSPF capabilities The OSPF protocol defines several optional capabilities. A router indicates the optional capabilities that it supports in its OSPF Hello packets, Database Description packets and in its link state advertisements. This enables routers supporting a Moy [Page 39] RFC 1583 OSPF Version 2 March 1994 mix of optional capabilities to coexist in a single Autonomous System. Some capabilities must be supported by all routers attached to a specific area. In this case, a router will not accept a neighbor's Hello Packet unless there is a match in reported capabilities (i.e., a capability mismatch prevents a neighbor relationship from forming). An example of this is the ExternalRoutingCapability (see below). Other capabilities can be negotiated during the Database Exchange process. This is accomplished by specifying the optional capabilities in Database Description packets. A capability mismatch with a neighbor in this case will result in only a subset of link state advertisements being exchanged between the two neighbors. The routing table build process can also be affected by the presence/absence of optional capabilities. For example, since the optional capabilities are reported in link state advertisements, routers incapable of certain functions can be avoided when building the shortest path tree. An example of this is the TOS routing capability (see below). The current OSPF optional capabilities are listed below. See Section A.2 for more information. ExternalRoutingCapability Entire OSPF areas can be configured as "stubs" (see Section 3.6). AS external advertisements will not be flooded into stub areas. This capability is represented by the E-bit in the OSPF options field (see Section A.2). In order to ensure consistent configuration of stub areas, all routers interfacing to such an area must have the E-bit clear in their Hello packets (see Sections 9.5 and 10.5). TOS capability All OSPF implementations must be able to calculate separate routes based on IP Type of Service. However, to save routing table space and processing resources, an OSPF router can be configured to ignore TOS when forwarding packets. In this case, the router calculates routes for TOS 0 only. This capability is represented by the T-bit in the OSPF options field (see Section A.2). TOS-capable routers will attempt to avoid non-TOS-capable routers when calculating non-zero TOS paths. Moy [Page 40] RFC 1583 OSPF Version 2 March 1994 5. Protocol Data Structures The OSPF protocol is described in this specification in terms of its operation on various protocol data structures. The following list comprises the top-level OSPF data structures. Any initialization that needs to be done is noted. OSPF areas, interfaces and neighbors also have associated data structures that are described later in this specification. Router ID A 32-bit number that uniquely identifies this router in the AS. One possible implementation strategy would be to use the smallest IP interface address belonging to the router. If a router's OSPF Router ID is changed, the router's OSPF software should be restarted before the new Router ID takes effect. Before restarting in order to change its Router ID, the router should flush its self-originated link state advertisements from the routing domain (see Section 14.1), or they will persist for up to MaxAge minutes. Area structures Each one of the areas to which the router is connected has its own data structure. This data structure describes the working of the basic algorithm. Remember that each area runs a separate copy of the basic algorithm. Backbone (area) structure The basic algorithm operates on the backbone as if it were an area. For this reason the backbone is represented as an area structure. Virtual links configured The virtual links configured with this router as one endpoint. In order to have configured virtual links, the router itself must be an area border router. Virtual links are identified by the Router ID of the other endpoint – which is another area border router. These two endpoint routers must be attached to a common area, called the virtual link's Transit area. Virtual links are part of the backbone, and behave as if they were unnumbered point-to-point networks between the two routers. A virtual link uses the intra-area routing of its Transit area to forward packets. Virtual links are brought up and down through the building of the shortest-path trees for the Transit area. List of external routes These are routes to destinations external to the Autonomous System, that have been gained either through direct experience Moy [Page 41] RFC 1583 OSPF Version 2 March 1994 with another routing protocol (such as EGP), or through configuration information, or through a combination of the two (e.g., dynamic external information to be advertised by OSPF with configured metric). Any router having these external routes is called an AS boundary router. These routes are advertised by the router into the OSPF routing domain via AS external link advertisements. List of AS external link advertisements Part of the topological database. These have originated from the AS boundary routers. They comprise routes to destinations external to the Autonomous System. Note that, if the router is itself an AS boundary router, some of these AS external link advertisements have been self-originated. The routing table Derived from the topological database. Each destination that the router can forward to is represented by a cost and a set of paths. A path is described by its type and next hop. For more information, see Section 11. TOS capability This item indicates whether the router will calculate separate routes based on TOS. This is a configurable parameter. For more information, see Sections 4.5 and 16.9. Figure 9 shows the collection of data structures present in a typical router. The router pictured is RT10, from the map in Figure 6. Note that Router RT10 has a virtual link configured to Router RT11, with Area 2 as the link's Transit area. This is indicated by the dashed line in Figure 9. When the virtual link becomes active, through the building of the shortest path tree for Area 2, it becomes an interface to the backbone (see the two backbone interfaces depicted in Figure 9). 6. The Area Data Structure The area data structure contains all the information used to run the basic routing algorithm. Each area maintains its own topological database. A network belongs to a single area, and a router interface connects to a single area. Each router adjacency also belongs to a single area. The OSPF backbone has all the properties of an area. For that reason it is also represented by an area data structure. Note that some items in the structure apply differently to the backbone than to non-backbone areas. Moy [Page 42] RFC 1583 OSPF Version 2 March 1994 +—-+ |RT10|——+ +—-+ \+————-+ / \ |Routing Table| / \ +————-+ / \ +——+ / \ +——–+ |Area 2|—+ +—|Backbone| +——+*+ +——–+

/ \ * / \

          /          \           *      /            \
     +---------+  +---------+    +------------+       +------------+
     |Interface|  |Interface|    |Virtual Link|       |Interface Ib|
     |  to N6  |  |  to N8  |    |   to RT11  |       +------------+
     +---------+  +---------+    +------------+             |
         /  \           |               |                   |
        /    \          |               |                   |
 +--------+ +--------+  |        +-------------+      +------------+
 |Neighbor| |Neighbor|  |        |Neighbor RT11|      |Neighbor RT6|
 |  RT8   | |  RT7   |  |        +-------------+      +------------+
 +--------+ +--------+  |
                        |
                   +-------------+
                   |Neighbor RT11|
                   +-------------+
              Figure 9: Router RT10's Data structures
  The area topological (or link state) database consists of the
  collection of router links, network links and summary link
  advertisements that have originated from the area's routers.  This
  information is flooded throughout a single area only.  The list of
  AS external link advertisements (see Section 5) is also considered
  to be part of each area's topological database.
  Area ID
      A 32-bit number identifying the area.  0.0.0.0 is reserved for
      the Area ID of the backbone.  If assigning subnetted networks as
      separate areas, the IP network number could be used as the Area
      ID.
  List of component address ranges
      The address ranges that define the area.  Each address range is

Moy [Page 43] RFC 1583 OSPF Version 2 March 1994

      specified by an [address,mask] pair and a status indication of
      either Advertise or DoNotAdvertise (see Section 12.4.3). Each
      network is then assigned to an area depending on the address
      range that it falls into (specified address ranges are not
      allowed to overlap).  As an example, if an IP subnetted network
      is to be its own separate OSPF area, the area is defined to
      consist of a single address range - an IP network number with
      its natural (class A, B or C) mask.
  Associated router interfaces
      This router's interfaces connecting to the area.  A router
      interface belongs to one and only one area (or the backbone).
      For the backbone structure this list includes all the virtual
      links.  A virtual link is identified by the Router ID of its
      other endpoint; its cost is the cost of the shortest intra-area
      path through the Transit area that exists between the two
      routers.
  List of router links advertisements
      A router links advertisement is generated by each router in the
      area.  It describes the state of the router's interfaces to the
      area.
  List of network links advertisements
      One network links advertisement is generated for each transit
      multi-access network in the area.  A network links advertisement
      describes the set of routers currently connected to the network.
  List of summary link advertisements
      Summary link advertisements originate from the area's area
      border routers.  They describe routes to destinations internal
      to the Autonomous System, yet external to the area.
  Shortest-path tree
      The shortest-path tree for the area, with this router itself as
      root.  Derived from the collected router links and network links
      advertisements by the Dijkstra algorithm (see Section 16.1).
  AuType
      The type of authentication used for this area.  Authentication
      types are defined in Appendix D.  All OSPF packet exchanges are
      authenticated.  Different authentication schemes may be used in
      different areas.
  TransitCapability
      Set to TRUE if and only if there are one or more active virtual
      links using the area as a Transit area. Equivalently, this
      parameter indicates whether the area can carry data traffic that

Moy [Page 44] RFC 1583 OSPF Version 2 March 1994

      neither originates nor terminates in the area itself. This
      parameter is calculated when the area's shortest-path tree is
      built (see Section 16.1, and is used as an input to a subsequent
      step of the routing table build process (see Section 16.3).
  ExternalRoutingCapability
      Whether AS external advertisements will be flooded
      into/throughout the area.  This is a configurable parameter.  If
      AS external advertisements are excluded from the area, the area
      is called a "stub".  Internal to stub areas, routing to AS
      external destinations will be based solely on a default summary
      route.  The backbone cannot be configured as a stub area.  Also,
      virtual links cannot be configured through stub areas.  For more
      information, see Section 3.6.
  StubDefaultCost
      If the area has been configured as a stub area, and the router
      itself is an area border router, then the StubDefaultCost
      indicates the cost of the default summary link that the router
      should advertise into the area.  There can be a separate cost
      configured for each IP TOS.  See Section 12.4.3 for more
      information.
  Unless otherwise specified, the remaining sections of this document
  refer to the operation of the protocol in a single area.

7. Bringing Up Adjacencies

  OSPF creates adjacencies between neighboring routers for the purpose
  of exchanging routing information.  Not every two neighboring
  routers will become adjacent.  This section covers the generalities
  involved in creating adjacencies.  For further details consult
  Section 10.
  7.1.  The Hello Protocol
      The Hello Protocol is responsible for establishing and
      maintaining neighbor relationships.  It also ensures that
      communication between neighbors is bidirectional.  Hello packets
      are sent periodically out all router interfaces.  Bidirectional
      communication is indicated when the router sees itself listed in
      the neighbor's Hello Packet.
      On multi-access networks, the Hello Protocol elects a Designated
      Router for the network.  Among other things, the Designated

Moy [Page 45] RFC 1583 OSPF Version 2 March 1994

      Router controls what adjacencies will be formed over the network
      (see below).
      The Hello Protocol works differently on broadcast networks, as
      compared to non-broadcast networks.  On broadcast networks, each
      router advertises itself by periodically multicasting Hello
      Packets.  This allows neighbors to be discovered dynamically.
      These Hello Packets contain the router's view of the Designated
      Router's identity, and the list of routers whose Hello Packets
      have been seen recently.
      On non-broadcast networks some configuration information is
      necessary for the operation of the Hello Protocol.  Each router
      that may potentially become Designated Router has a list of all
      other routers attached to the network.  A router, having
      Designated Router potential, sends Hello Packets to all other
      potential Designated Routers when its interface to the non-
      broadcast network first becomes operational.  This is an attempt
      to find the Designated Router for the network.  If the router
      itself is elected Designated Router, it begins sending Hello
      Packets to all other routers attached to the network.
      After a neighbor has been discovered, bidirectional
      communication ensured, and (if on a multi-access network) a
      Designated Router elected, a decision is made regarding whether
      or not an adjacency should be formed with the neighbor (see
      Section 10.4).  An attempt is always made to establish
      adjacencies over point-to-point networks and virtual links.  The
      first step in bringing up an adjacency is to synchronize the
      neighbors' topological databases.  This is covered in the next
      section.
  7.2.  The Synchronization of Databases
      In a link-state routing algorithm, it is very important for all
      routers' topological databases to stay synchronized.  OSPF
      simplifies this by requiring only adjacent routers to remain
      synchronized.  The synchronization process begins as soon as the
      routers attempt to bring up the adjacency.  Each router
      describes its database by sending a sequence of Database
      Description packets to its neighbor.  Each Database Description
      Packet describes a set of link state advertisements belonging to
      the router's database.  When the neighbor sees a link state
      advertisement that is more recent than its own database copy, it
      makes a note that this newer advertisement should be requested.
      This sending and receiving of Database Description packets is

Moy [Page 46] RFC 1583 OSPF Version 2 March 1994

      called the "Database Exchange Process".  During this process,
      the two routers form a master/slave relationship.  Each Database
      Description Packet has a sequence number.  Database Description
      Packets sent by the master (polls) are acknowledged by the slave
      through echoing of the sequence number.  Both polls and their
      responses contain summaries of link state data.  The master is
      the only one allowed to retransmit Database Description Packets.
      It does so only at fixed intervals, the length of which is the
      configured constant RxmtInterval.
      Each Database Description contains an indication that there are
      more packets to follow --- the M-bit.  The Database Exchange
      Process is over when a router has received and sent Database
      Description Packets with the M-bit off.
      During and after the Database Exchange Process, each router has
      a list of those link state advertisements for which the neighbor
      has more up-to-date instances.  These advertisements are
      requested in Link State Request Packets.  Link State Request
      packets that are not satisfied are retransmitted at fixed
      intervals of time RxmtInterval.  When the Database Description
      Process has completed and all Link State Requests have been
      satisfied, the databases are deemed synchronized and the routers
      are marked fully adjacent.  At this time the adjacency is fully
      functional and is advertised in the two routers' link state
      advertisements.
      The adjacency is used by the flooding procedure as soon as the
      Database Exchange Process begins.  This simplifies database
      synchronization, and guarantees that it finishes in a
      predictable period of time.
  7.3.  The Designated Router
      Every multi-access network has a Designated Router.  The
      Designated Router performs two main functions for the routing
      protocol:
      o   The Designated Router originates a network links
          advertisement on behalf of the network.  This advertisement
          lists the set of routers (including the Designated Router
          itself) currently attached to the network.  The Link State
          ID for this advertisement (see Section 12.1.4) is the IP
          interface address of the Designated Router.  The IP network
          number can then be obtained by using the subnet/network
          mask.

Moy [Page 47] RFC 1583 OSPF Version 2 March 1994

      o   The Designated Router becomes adjacent to all other routers
          on the network.  Since the link state databases are
          synchronized across adjacencies (through adjacency bring-up
          and then the flooding procedure), the Designated Router
          plays a central part in the synchronization process.
      The Designated Router is elected by the Hello Protocol.  A
      router's Hello Packet contains its Router Priority, which is
      configurable on a per-interface basis.  In general, when a
      router's interface to a network first becomes functional, it
      checks to see whether there is currently a Designated Router for
      the network.  If there is, it accepts that Designated Router,
      regardless of its Router Priority.  (This makes it harder to
      predict the identity of the Designated Router, but ensures that
      the Designated Router changes less often.  See below.)
      Otherwise, the router itself becomes Designated Router if it has
      the highest Router Priority on the network.  A more detailed
      (and more accurate) description of Designated Router election is
      presented in Section 9.4.
      The Designated Router is the endpoint of many adjacencies.  In
      order to optimize the flooding procedure on broadcast networks,
      the Designated Router multicasts its Link State Update Packets
      to the address AllSPFRouters, rather than sending separate
      packets over each adjacency.
      Section 2 of this document discusses the directed graph
      representation of an area.  Router nodes are labelled with their
      Router ID.  Multi-access network nodes are actually labelled
      with the IP address of their Designated Router.  It follows that
      when the Designated Router changes, it appears as if the network
      node on the graph is replaced by an entirely new node.  This
      will cause the network and all its attached routers to originate
      new link state advertisements.  Until the topological databases
      again converge, some temporary loss of connectivity may result.
      This may result in ICMP unreachable messages being sent in
      response to data traffic.  For that reason, the Designated
      Router should change only infrequently.  Router Priorities
      should be configured so that the most dependable router on a
      network eventually becomes Designated Router.
  7.4.  The Backup Designated Router
      In order to make the transition to a new Designated Router
      smoother, there is a Backup Designated Router for each multi-
      access network.  The Backup Designated Router is also adjacent

Moy [Page 48] RFC 1583 OSPF Version 2 March 1994

      to all routers on the network, and becomes Designated Router
      when the previous Designated Router fails.  If there were no
      Backup Designated Router, when a new Designated Router became
      necessary, new adjacencies would have to be formed between the
      new Designated Router and all other routers attached to the
      network.  Part of the adjacency forming process is the
      synchronizing of topological databases, which can potentially
      take quite a long time.  During this time, the network would not
      be available for transit data traffic.  The Backup Designated
      obviates the need to form these adjacencies, since they already
      exist.  This means the period of disruption in transit traffic
      lasts only as long as it takes to flood the new link state
      advertisements (which announce the new Designated Router).
      The Backup Designated Router does not generate a network links
      advertisement for the network.  (If it did, the transition to a
      new Designated Router would be even faster.  However, this is a
      tradeoff between database size and speed of convergence when the
      Designated Router disappears.)
      The Backup Designated Router is also elected by the Hello
      Protocol.  Each Hello Packet has a field that specifies the
      Backup Designated Router for the network.
      In some steps of the flooding procedure, the Backup Designated
      Router plays a passive role, letting the Designated Router do
      more of the work.  This cuts down on the amount of local routing
      traffic.  See Section 13.3 for more information.
  7.5.  The graph of adjacencies
      An adjacency is bound to the network that the two routers have
      in common.  If two routers have multiple networks in common,
      they may have multiple adjacencies between them.
      One can picture the collection of adjacencies on a network as
      forming an undirected graph.  The vertices consist of routers,
      with an edge joining two routers if they are adjacent.  The
      graph of adjacencies describes the flow of routing protocol
      packets, and in particular Link State Update Packets, through
      the Autonomous System.
      Two graphs are possible, depending on whether the common network
      is multi-access.  On physical point-to-point networks (and
      virtual links), the two routers joined by the network will be
      adjacent after their databases have been synchronized.  On
      multi-access networks, both the Designated Router and the Backup

Moy [Page 49] RFC 1583 OSPF Version 2 March 1994

      Designated Router are adjacent to all other routers attached to
      the network, and these account for all adjacencies.
      These graphs are shown in Figure 10.  It is assumed that Router
      RT7 has become the Designated Router, and Router RT3 the Backup
      Designated Router, for the Network N2.  The Backup Designated
      Router performs a lesser function during the flooding procedure
      than the Designated Router (see Section 13.3).  This is the
      reason for the dashed lines connecting the Backup Designated
      Router RT3.

8. Protocol Packet Processing

  This section discusses the general processing of OSPF routing
  protocol packets.  It is very important that the router topological
  databases remain synchronized.  For this reason, routing protocol
  packets should get preferential treatment over ordinary data
  packets, both in sending and receiving.
  Routing protocol packets are sent along adjacencies only (with the
        +---+            +---+
        |RT1|------------|RT2|            o---------------o
        +---+    N1      +---+           RT1             RT2
                                               RT7
                                                o---------+
          +---+   +---+   +---+                /|\        |
          |RT7|   |RT3|   |RT4|               / | \       |
          +---+   +---+   +---+              /  |  \      |
            |       |       |               /   |   \     |
       +-----------------------+        RT5o RT6o    oRT4 |
                |       |     N2            *   *   *     |
              +---+   +---+                  *  *  *      |
              |RT5|   |RT6|                   * * *       |
              +---+   +---+                    ***        |
                                                o---------+
                                               RT3
                Figure 10: The graph of adjacencies

Moy [Page 50] RFC 1583 OSPF Version 2 March 1994

  exception of Hello packets, which are used to discover the
  adjacencies).  This means that all routing protocol packets travel a
  single IP hop, except those sent over virtual links.
  All routing protocol packets begin with a standard header.  The
  sections below give the details on how to fill in and verify this
  standard header.  Then, for each packet type, the section is listed
  that gives more details on that particular packet type's processing.
  8.1.  Sending protocol packets
      When a router sends a routing protocol packet, it fills in the
      fields of the standard OSPF packet header as follows.  For more
      details on the header format consult Section A.3.1:
      Version #
          Set to 2, the version number of the protocol as documented
          in this specification.
      Packet type
          The type of OSPF packet, such as Link state Update or Hello
          Packet.
      Packet length
          The length of the entire OSPF packet in bytes, including the
          standard OSPF packet header.
      Router ID
          The identity of the router itself (who is originating the
          packet).
      Area ID
          The OSPF area that the packet is being sent into.
      Checksum
          The standard IP 16-bit one's complement checksum of the
          entire OSPF packet, excluding the 64-bit authentication
          field.  This checksum should be calculated before handing
          the packet to the appropriate authentication procedure.
      AuType and Authentication
          Each OSPF packet exchange is authenticated.  Authentication
          types are assigned by the protocol and documented in
          Appendix D.  A different authentication scheme can be used
          for each OSPF area.  The 64-bit authentication field is set
          by the appropriate authentication procedure (determined by
          AuType).  This procedure should be the last called when

Moy [Page 51] RFC 1583 OSPF Version 2 March 1994

          forming the packet to be sent.  The setting of the
          authentication field is determined by the packet contents
          and the authentication key (which is configurable on a per-
          interface basis).
      The IP destination address for the packet is selected as
      follows.  On physical point-to-point networks, the IP
      destination is always set to the address AllSPFRouters.  On all
      other network types (including virtual links), the majority of
      OSPF packets are sent as unicasts, i.e., sent directly to the
      other end of the adjacency.  In this case, the IP destination is
      just the Neighbor IP address associated with the other end of
      the adjacency (see Section 10).  The only packets not sent as
      unicasts are on broadcast networks; on these networks Hello
      packets are sent to the multicast destination AllSPFRouters, the
      Designated Router and its Backup send both Link State Update
      Packets and Link State Acknowledgment Packets to the multicast
      address AllSPFRouters, while all other routers send both their
      Link State Update and Link State Acknowledgment Packets to the
      multicast address AllDRouters.
      Retransmissions of Link State Update packets are ALWAYS sent as
      unicasts.
      The IP source address should be set to the IP address of the
      sending interface.  Interfaces to unnumbered point-to-point
      networks have no associated IP address.  On these interfaces,
      the IP source should be set to any of the other IP addresses
      belonging to the router.  For this reason, there must be at
      least one IP address assigned to the router.[2] Note that, for
      most purposes, virtual links act precisely the same as
      unnumbered point-to-point networks.  However, each virtual link
      does have an IP interface address (discovered during the routing
      table build process) which is used as the IP source when sending
      packets over the virtual link.
      For more information on the format of specific OSPF packet
      types, consult the sections listed in Table 10.

Moy [Page 52] RFC 1583 OSPF Version 2 March 1994

           Type   Packet name            detailed section (transmit)
           _________________________________________________________
           1      Hello                  Section  9.5
           2      Database description   Section 10.8
           3      Link state request     Section 10.9
           4      Link state update      Section 13.3
           5      Link state ack         Section 13.5
          Table 10: Sections describing OSPF protocol packet transmission.
  8.2.  Receiving protocol packets
      Whenever a protocol packet is received by the router it is
      marked with the interface it was received on.  For routers that
      have virtual links configured, it may not be immediately obvious
      which interface to associate the packet with.  For example,
      consider the Router RT11 depicted in Figure 6.  If RT11 receives
      an OSPF protocol packet on its interface to Network N8, it may
      want to associate the packet with the interface to Area 2, or
      with the virtual link to Router RT10 (which is part of the
      backbone).  In the following, we assume that the packet is
      initially associated with the non-virtual  link.[3]
      In order for the packet to be accepted at the IP level, it must
      pass a number of tests, even before the packet is passed to OSPF
      for processing:
      o   The IP checksum must be correct.
      o   The packet's IP destination address must be the IP address
          of the receiving interface, or one of the IP multicast
          addresses AllSPFRouters or AllDRouters.
      o   The IP protocol specified must be OSPF (89).
      o   Locally originated packets should not be passed on to OSPF.
          That is, the source IP address should be examined to make
          sure this is not a multicast packet that the router itself
          generated.
      Next, the OSPF packet header is verified.  The fields specified
      in the header must match those configured for the receiving

Moy [Page 53] RFC 1583 OSPF Version 2 March 1994

      interface.  If they do not, the packet should be discarded:
      o   The version number field must specify protocol version 2.
      o   The 16-bit one's complement checksum of the OSPF packet's
          contents must be verified.  Remember that the 64-bit
          authentication field must be excluded from the checksum
          calculation.
      o   The Area ID found in the OSPF header must be verified.  If
          both of the following cases fail, the packet should be
          discarded.  The Area ID specified in the header must either:
          (1) Match the Area ID of the receiving interface.  In this
              case, the packet has been sent over a single hop.
              Therefore, the packet's IP source address must be on the
              same network as the receiving interface.  This can be
              determined by comparing the packet's IP source address
              to the interface's IP address, after masking both
              addresses with the interface mask.  This comparison
              should not be performed on point-to-point networks. On
              point-to-point networks, the interface addresses of each
              end of the link are assigned independently, if they are
              assigned at all.
          (2) Indicate the backbone.  In this case, the packet has
              been sent over a virtual link.  The receiving router
              must be an area border router, and the Router ID
              specified in the packet (the source router) must be the
              other end of a configured virtual link.  The receiving
              interface must also attach to the virtual link's
              configured Transit area.  If all of these checks
              succeed, the packet is accepted and is from now on
              associated with the virtual link (and the backbone
              area).
      o   Packets whose IP destination is AllDRouters should only be
          accepted if the state of the receiving interface is DR or
          Backup (see Section 9.1).
      o   The AuType specified in the packet must match the AuType
          specified for the associated area.
      Next, the packet must be authenticated.  This depends on the
      AuType specified (see Appendix D).  The authentication procedure
      may use an Authentication key, which can be configured on a

Moy [Page 54] RFC 1583 OSPF Version 2 March 1994

      per-interface basis.  If the authentication fails, the packet
      should be discarded.
      If the packet type is Hello, it should then be further processed
      by the Hello Protocol (see Section 10.5).  All other packet
      types are sent/received only on adjacencies.  This means that
      the packet must have been sent by one of the router's active
      neighbors.  If the receiving interface is a multi-access network
      (either broadcast or non-broadcast) the sender is identified by
      the IP source address found in the packet's IP header.  If the
      receiving interface is a point-to-point link or a virtual link,
      the sender is identified by the Router ID (source router) found
      in the packet's OSPF header.  The data structure associated with
      the receiving interface contains the list of active neighbors.
      Packets not matching any active neighbor are discarded.
      At this point all received protocol packets are associated with
      an active neighbor.  For the further input processing of
      specific packet types, consult the sections listed in Table 11.
            Type   Packet name            detailed section (receive)
            ________________________________________________________
            1      Hello                  Section 10.5
            2      Database description   Section 10.6
            3      Link state request     Section 10.7
            4      Link state update      Section 13
            5      Link state ack         Section 13.7
          Table 11: Sections describing OSPF protocol packet reception.

9. The Interface Data Structure

  An OSPF interface is the connection between a router and a network.
  There is a single OSPF interface structure for each attached
  network; each interface structure has at most one IP interface
  address (see below).  The support for multiple addresses on a single
  network is a matter for future consideration.
  An OSPF interface can be considered to belong to the area that
  contains the attached network.  All routing protocol packets
  originated by the router over this interface are labelled with the
  interface's Area ID.  One or more router adjacencies may develop
  over an interface.  A router's link state advertisements reflect the

Moy [Page 55] RFC 1583 OSPF Version 2 March 1994

  state of its interfaces and their associated adjacencies.
  The following data items are associated with an interface.  Note
  that a number of these items are actually configuration for the
  attached network; those items must be the same for all routers
  connected to the network.
  Type
      The kind of network to which the interface attaches.  Its value
      is either broadcast, non-broadcast yet still multi-access,
      point-to-point or virtual link.
  State
      The functional level of an interface.  State determines whether
      or not full adjacencies are allowed to form over the interface.
      State is also reflected in the router's link state
      advertisements.
  IP interface address
      The IP address associated with the interface.  This appears as
      the IP source address in all routing protocol packets originated
      over this interface.  Interfaces to unnumbered point-to-point
      networks do not have an associated IP address.
  IP interface mask
      Also referred to as the subnet mask, this indicates the portion
      of the IP interface address that identifies the attached
      network.  Masking the IP interface address with the IP interface
      mask yields the IP network number of the attached network.  On
      point-to-point networks and virtual links, the IP interface mask
      is not defined. On these networks, the link itself is not
      assigned an IP network number, and so the addresses of each side
      of the link are assigned independently, if they are assigned at
      all.
  Area ID
      The Area ID of the area to which the attached network belongs.
      All routing protocol packets originating from the interface are
      labelled with this Area ID.
  HelloInterval
      The length of time, in seconds, between the Hello packets that
      the router sends on the interface.  Advertised in Hello packets
      sent out this interface.
  RouterDeadInterval
      The number of seconds before the router's neighbors will declare

Moy [Page 56] RFC 1583 OSPF Version 2 March 1994

      it down, when they stop hearing the router's Hello Packets.
      Advertised in Hello packets sent out this interface.
  InfTransDelay
      The estimated number of seconds it takes to transmit a Link
      State Update Packet over this interface.  Link state
      advertisements contained in the Link State Update packet will
      have their age incremented by this amount before transmission.
      This value should take into account transmission and propagation
      delays; it must be greater than zero.
  Router Priority
      An 8-bit unsigned integer.  When two routers attached to a
      network both attempt to become Designated Router, the one with
      the highest Router Priority takes precedence.  A router whose
      Router Priority is set to 0 is ineligible to become Designated
      Router on the attached network.  Advertised in Hello packets
      sent out this interface.
  Hello Timer
      An interval timer that causes the interface to send a Hello
      packet.  This timer fires every HelloInterval seconds.  Note
      that on non-broadcast networks a separate Hello packet is sent
      to each qualified neighbor.
  Wait Timer
      A single shot timer that causes the interface to exit the
      Waiting state, and as a consequence select a Designated Router
      on the network.  The length of the timer is RouterDeadInterval
      seconds.
  List of neighboring routers
      The other routers attached to this network.  On multi-access
      networks, this list is formed by the Hello Protocol.
      Adjacencies will be formed to some of these neighbors.  The set
      of adjacent neighbors can be determined by an examination of all
      of the neighbors' states.
  Designated Router
      The Designated Router selected for the attached network.  The
      Designated Router is selected on all multi-access networks by
      the Hello Protocol.  Two pieces of identification are kept for
      the Designated Router: its Router ID and its IP interface
      address on the network.  The Designated Router advertises link
      state for the network; this network link state advertisement is
      labelled with the Designated Router's IP address.  The
      Designated Router is initialized to 0.0.0.0, which indicates the
      lack of a Designated Router.

Moy [Page 57] RFC 1583 OSPF Version 2 March 1994

  Backup Designated Router
      The Backup Designated Router is also selected on all multi-
      access networks by the Hello Protocol.  All routers on the
      attached network become adjacent to both the Designated Router
      and the Backup Designated Router.  The Backup Designated Router
      becomes Designated Router when the current Designated Router
      fails.  The Backup Designated Router is initialized to 0.0.0.0,
      indicating the lack of a Backup Designated Router.
  Interface output cost(s)
      The cost of sending a data packet on the interface, expressed in
      the link state metric.  This is advertised as the link cost for
      this interface in the router links advertisement.  There may be
      a separate cost for each IP Type of Service.  The cost of an
      interface must be greater than zero.
  RxmtInterval
      The number of seconds between link state advertisement
      retransmissions, for adjacencies belonging to this interface.
      Also used when retransmitting Database Description and Link
      State Request Packets.
  Authentication key
      This configured data allows the authentication procedure to
      generate and/or verify the Authentication field in the OSPF
      header.  The Authentication key can be configured on a per-
      interface basis.  For example, if the AuType indicates simple
      password, the Authentication key would be a 64-bit password.
      This key would be inserted directly into the OSPF header when
      originating routing protocol packets, and there could be a
      separate password for each network.
  9.1.  Interface states
      The various states that router interfaces may attain is
      documented in this section.  The states are listed in order of
      progressing functionality.  For example, the inoperative state
      is listed first, followed by a list of intermediate states
      before the final, fully functional state is achieved.  The
      specification makes use of this ordering by sometimes making
      references such as "those interfaces in state greater than X".
      Figure 11 shows the graph of interface state changes.  The arcs
      of the graph are labelled with the event causing the state
      change.  These events are documented in Section 9.2.  The
      interface state machine is described in more detail in Section
      9.3.

Moy [Page 58] RFC 1583 OSPF Version 2 March 1994

                                +----+   UnloopInd   +--------+
                                |Down|<--------------|Loopback|
                                +----+               +--------+
                                   |
                                   |InterfaceUp
                        +-------+  |               +--------------+
                        |Waiting|<-+-------------->|Point-to-point|
                        +-------+                  +--------------+
                            |
                   WaitTimer|BackupSeen
                            |
                            |
                            |   NeighborChange
        +------+           +-+<---------------- +-------+
        |Backup|<----------|?|----------------->|DROther|
        +------+---------->+-+<-----+           +-------+
                  Neighbor  |       |
                  Change    |       |Neighbor
                            |       |Change
                            |     +--+
                            +---->|DR|
                                  +--+
                    Figure 11: Interface State changes
               In addition to the state transitions pictured,
               Event InterfaceDown always forces Down State, and
               Event LoopInd always forces Loopback State
      Down
          This is the initial interface state.  In this state, the
          lower-level protocols have indicated that the interface is
          unusable.  No protocol traffic at all will be sent or
          received on such a interface.  In this state, interface
          parameters should be set to their initial values.  All
          interface timers should be disabled, and there should be no
          adjacencies associated with the interface.
      Loopback
          In this state, the router's interface to the network is
          looped back.  The interface may be looped back in hardware
          or software.  The interface will be unavailable for regular
          data traffic.  However, it may still be desirable to gain
          information on the quality of this interface, either through
          sending ICMP pings to the interface or through something
          like a bit error test.  For this reason, IP packets may

Moy [Page 59] RFC 1583 OSPF Version 2 March 1994

          still be addressed to an interface in Loopback state.  To
          facilitate this, such interfaces are advertised in router
          links advertisements as single host routes, whose
          destination is the IP interface address.[4]
      Waiting
          In this state, the router is trying to determine the
          identity of the (Backup) Designated Router for the network.
          To do this, the router monitors the Hello Packets it
          receives.  The router is not allowed to elect a Backup
          Designated Router nor a Designated Router until it
          transitions out of Waiting state.  This prevents unnecessary
          changes of (Backup) Designated Router.
      Point-to-point
          In this state, the interface is operational, and connects
          either to a physical point-to-point network or to a virtual
          link.  Upon entering this state, the router attempts to form
          an adjacency with the neighboring router.  Hello Packets are
          sent to the neighbor every HelloInterval seconds.
      DR Other
          The interface is to a multi-access network on which another
          router has been selected to be the Designated Router.  In
          this state, the router itself has not been selected Backup
          Designated Router either.  The router forms adjacencies to
          both the Designated Router and the Backup Designated Router
          (if they exist).
      Backup
          In this state, the router itself is the Backup Designated
          Router on the attached network.  It will be promoted to
          Designated Router when the present Designated Router fails.
          The router establishes adjacencies to all other routers
          attached to the network.  The Backup Designated Router
          performs slightly different functions during the Flooding
          Procedure, as compared to the Designated Router (see Section
          13.3).  See Section 7.4 for more details on the functions
          performed by the Backup Designated Router.
      DR  In this state, this router itself is the Designated Router
          on the attached network.  Adjacencies are established to all
          other routers attached to the network.  The router must also
          originate a network links advertisement for the network
          node.  The advertisement will contain links to all routers
          (including the Designated Router itself) attached to the
          network.  See Section 7.3 for more details on the functions
          performed by the Designated Router.

Moy [Page 60] RFC 1583 OSPF Version 2 March 1994

  9.2.  Events causing interface state changes
      State changes can be effected by a number of events.  These
      events are pictured as the labelled arcs in Figure 11.  The
      label definitions are listed below.  For a detailed explanation
      of the effect of these events on OSPF protocol operation,
      consult Section 9.3.
      InterfaceUp
          Lower-level protocols have indicated that the network
          interface is operational.  This enables the interface to
          transition out of Down state.  On virtual links, the
          interface operational indication is actually a result of the
          shortest path calculation (see Section 16.7).
      WaitTimer
          The Wait Timer has fired, indicating the end of the waiting
          period that is required before electing a (Backup)
          Designated Router.
      BackupSeen
          The router has detected the existence or non-existence of a
          Backup Designated Router for the network.  This is done in
          one of two ways.  First, an Hello Packet may be received
          from a neighbor claiming to be itself the Backup Designated
          Router.  Alternatively, an Hello Packet may be received from
          a neighbor claiming to be itself the Designated Router, and
          indicating that there is no Backup Designated Router.  In
          either case there must be bidirectional communication with
          the neighbor, i.e., the router must also appear in the
          neighbor's Hello Packet.  This event signals an end to the
          Waiting state.
      NeighborChange
          There has been a change in the set of bidirectional
          neighbors associated with the interface.  The (Backup)
          Designated Router needs to be recalculated.  The following
          neighbor changes lead to the NeighborChange event.  For an
          explanation of neighbor states, see Section 10.1.
          o   Bidirectional communication has been established to a
              neighbor.  In other words, the state of the neighbor has
              transitioned to 2-Way or higher.
          o   There is no longer bidirectional communication with a
              neighbor.  In other words, the state of the neighbor has
              transitioned to Init or lower.

Moy [Page 61] RFC 1583 OSPF Version 2 March 1994

          o   One of the bidirectional neighbors is newly declaring
              itself as either Designated Router or Backup Designated
              Router.  This is detected through examination of that
              neighbor's Hello Packets.
          o   One of the bidirectional neighbors is no longer
              declaring itself as Designated Router, or is no longer
              declaring itself as Backup Designated Router.  This is
              again detected through examination of that neighbor's
              Hello Packets.
          o   The advertised Router Priority for a bidirectional
              neighbor has changed.  This is again detected through
              examination of that neighbor's Hello Packets.
      LoopInd
          An indication has been received that the interface is now
          looped back to itself.  This indication can be received
          either from network management or from the lower level
          protocols.
      UnloopInd
          An indication has been received that the interface is no
          longer looped back.  As with the LoopInd event, this
          indication can be received either from network management or
          from the lower level protocols.
      InterfaceDown
          Lower-level protocols indicate that this interface is no
          longer functional.  No matter what the current interface
          state is, the new interface state will be Down.
  9.3.  The Interface state machine
      A detailed description of the interface state changes follows.
      Each state change is invoked by an event (Section 9.2).  This
      event may produce different effects, depending on the current
      state of the interface.  For this reason, the state machine
      below is organized by current interface state and received
      event.  Each entry in the state machine describes the resulting
      new interface state and the required set of additional actions.
      When an interface's state changes, it may be necessary to
      originate a new router links advertisement.  See Section 12.4
      for more details.
      Some of the required actions below involve generating events for

Moy [Page 62] RFC 1583 OSPF Version 2 March 1994

      the neighbor state machine.  For example, when an interface
      becomes inoperative, all neighbor connections associated with
      the interface must be destroyed.  For more information on the
      neighbor state machine, see Section 10.3.
       State(s):  Down
          Event:  InterfaceUp
      New state:  Depends upon action routine
         Action:  Start the interval Hello Timer, enabling the
                  periodic sending of Hello packets out the interface.
                  If the attached network is a physical point-to-point
                  network or virtual link, the interface state
                  transitions to Point-to-Point.  Else, if the router
                  is not eligible to become Designated Router the
                  interface state transitions to DR Other.
                  Otherwise, the attached network is multi-access and
                  the router is eligible to become Designated Router.
                  In this case, in an attempt to discover the attached
                  network's Designated Router the interface state is
                  set to Waiting and the single shot Wait Timer is
                  started.  If in addition the attached network is
                  non-broadcast, examine the configured list of
                  neighbors for this interface and generate the
                  neighbor event Start for each neighbor that is also
                  eligible to become Designated Router.
       State(s):  Waiting
          Event:  BackupSeen
      New state:  Depends upon action routine.
         Action:  Calculate the attached network's Backup Designated
                  Router and Designated Router, as shown in Section
                  9.4.  As a result of this calculation, the new state
                  of the interface will be either DR Other, Backup or
                  DR.
       State(s):  Waiting

Moy [Page 63] RFC 1583 OSPF Version 2 March 1994

          Event:  WaitTimer
      New state:  Depends upon action routine.
         Action:  Calculate the attached network's Backup Designated
                  Router and Designated Router, as shown in Section
                  9.4.  As a result of this calculation, the new state
                  of the interface will be either DR Other, Backup or
                  DR.
       State(s):  DR Other, Backup or DR
          Event:  NeighborChange
      New state:  Depends upon action routine.
         Action:  Recalculate the attached network's Backup Designated
                  Router and Designated Router, as shown in Section
                  9.4.  As a result of this calculation, the new state
                  of the interface will be either DR Other, Backup or
                  DR.
       State(s):  Any State
          Event:  InterfaceDown
      New state:  Down
         Action:  All interface variables are reset, and interface
                  timers disabled.  Also, all neighbor connections
                  associated with the interface are destroyed.  This
                  is done by generating the event KillNbr on all
                  associated neighbors (see Section 10.2).
       State(s):  Any State
          Event:  LoopInd
      New state:  Loopback
         Action:  Since this interface is no longer connected to the
                  attached network the actions associated with the
                  above InterfaceDown event are executed.

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       State(s):  Loopback
          Event:  UnloopInd
      New state:  Down
         Action:  No actions are necessary.  For example, the
                  interface variables have already been reset upon
                  entering the Loopback state.  Note that reception of
                  an InterfaceUp event is necessary before the
                  interface again becomes fully functional.
  9.4.  Electing the Designated Router
      This section describes the algorithm used for calculating a
      network's Designated Router and Backup Designated Router.  This
      algorithm is invoked by the Interface state machine.  The
      initial time a router runs the election algorithm for a network,
      the network's Designated Router and Backup Designated Router are
      initialized to 0.0.0.0.  This indicates the lack of both a
      Designated Router and a Backup Designated Router.
      The Designated Router election algorithm proceeds as follows:
      Call the router doing the calculation Router X.  The list of
      neighbors attached to the network and having established
      bidirectional communication with Router X is examined.  This
      list is precisely the collection of Router X's neighbors (on
      this network) whose state is greater than or equal to 2-Way (see
      Section 10.1).  Router X itself is also considered to be on the
      list.  Discard all routers from the list that are ineligible to
      become Designated Router.  (Routers having Router Priority of 0
      are ineligible to become Designated Router.)  The following
      steps are then executed, considering only those routers that
      remain on the list:
      (1) Note the current values for the network's Designated Router
          and Backup Designated Router.  This is used later for
          comparison purposes.
      (2) Calculate the new Backup Designated Router for the network
          as follows.  Only those routers on the list that have not
          declared themselves to be Designated Router are eligible to
          become Backup Designated Router.  If one or more of these
          routers have declared themselves Backup Designated Router
          (i.e., they are currently listing themselves as Backup
          Designated Router, but not as Designated Router, in their

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          Hello Packets) the one having highest Router Priority is
          declared to be Backup Designated Router.  In case of a tie,
          the one having the highest Router ID is chosen.  If no
          routers have declared themselves Backup Designated Router,
          choose the router having highest Router Priority, (again
          excluding those routers who have declared themselves
          Designated Router), and again use the Router ID to break
          ties.
      (3) Calculate the new Designated Router for the network as
          follows.  If one or more of the routers have declared
          themselves Designated Router (i.e., they are currently
          listing themselves as Designated Router in their Hello
          Packets) the one having highest Router Priority is declared
          to be Designated Router.  In case of a tie, the one having
          the highest Router ID is chosen.  If no routers have
          declared themselves Designated Router, assign the Designated
          Router to be the same as the newly elected Backup Designated
          Router.
      (4) If Router X is now newly the Designated Router or newly the
          Backup Designated Router, or is now no longer the Designated
          Router or no longer the Backup Designated Router, repeat
          steps 2 and 3, and then proceed to step 5.  For example, if
          Router X is now the Designated Router, when step 2 is
          repeated X will no longer be eligible for Backup Designated
          Router election.  Among other things, this will ensure that
          no router will declare itself both Backup Designated Router
          and Designated Router.[5]
      (5) As a result of these calculations, the router itself may now
          be Designated Router or Backup Designated Router.  See
          Sections 7.3 and 7.4 for the additional duties this would
          entail.  The router's interface state should be set
          accordingly.  If the router itself is now Designated Router,
          the new interface state is DR.  If the router itself is now
          Backup Designated Router, the new interface state is Backup.
          Otherwise, the new interface state is DR Other.
      (6) If the attached network is non-broadcast, and the router
          itself has just become either Designated Router or Backup
          Designated Router, it must start sending Hello Packets to
          those neighbors that are not eligible to become Designated
          Router (see Section 9.5.1).  This is done by invoking the
          neighbor event Start for each neighbor having a Router
          Priority of 0.

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      (7) If the above calculations have caused the identity of either
          the Designated Router or Backup Designated Router to change,
          the set of adjacencies associated with this interface will
          need to be modified.  Some adjacencies may need to be
          formed, and others may need to be broken.  To accomplish
          this, invoke the event AdjOK?  on all neighbors whose state
          is at least 2-Way.  This will cause their eligibility for
          adjacency to be reexamined (see Sections 10.3 and 10.4).
      The reason behind the election algorithm's complexity is the
      desire for an orderly transition from Backup Designated Router
      to Designated Router, when the current Designated Router fails.
      This orderly transition is ensured through the introduction of
      hysteresis: no new Backup Designated Router can be chosen until
      the old Backup accepts its new Designated Router
      responsibilities.
      The above procedure may elect the same router to be both
      Designated Router and Backup Designated Router, although that
      router will never be the calculating router (Router X) itself.
      The elected Designated Router may not be the router having the
      highest Router Priority, nor will the Backup Designated Router
      necessarily have the second highest Router Priority.  If Router
      X is not itself eligible to become Designated Router, it is
      possible that neither a Backup Designated Router nor a
      Designated Router will be selected in the above procedure.  Note
      also that if Router X is the only attached router that is
      eligible to become Designated Router, it will select itself as
      Designated Router and there will be no Backup Designated Router
      for the network.
  9.5.  Sending Hello packets
      Hello packets are sent out each functioning router interface.
      They are used to discover and maintain neighbor
      relationships.[6] On multi-access networks, Hello Packets are
      also used to elect the Designated Router and Backup Designated
      Router, and in that way determine what adjacencies should be
      formed.
      The format of an Hello packet is detailed in Section A.3.2.  The
      Hello Packet contains the router's Router Priority (used in
      choosing the Designated Router), and the interval between Hello
      Packets sent out the interface (HelloInterval).  The Hello
      Packet also indicates how often a neighbor must be heard from to
      remain active (RouterDeadInterval).  Both HelloInterval and

Moy [Page 67] RFC 1583 OSPF Version 2 March 1994

      RouterDeadInterval must be the same for all routers attached to
      a common network.  The Hello packet also contains the IP address
      mask of the attached network (Network Mask).  On unnumbered
      point-to-point networks and on virtual links this field should
      be set to 0.0.0.0.
      The Hello packet's Options field describes the router's optional
      OSPF capabilities.  There are currently two optional
      capabilities defined (see Sections 4.5 and A.2).  The T-bit of
      the Options field should be set if the router is capable of
      calculating separate routes for each IP TOS.  The E-bit should
      be set if and only if the attached area is capable of processing
      AS external advertisements (i.e., it is not a stub area).  If
      the E-bit is set incorrectly the neighboring routers will refuse
      to accept the Hello Packet (see Section 10.5).  The rest of the
      Hello Packet's Options field should be set to zero.
      In order to ensure two-way communication between adjacent
      routers, the Hello packet contains the list of all routers from
      which Hello Packets have been seen recently.  The Hello packet
      also contains the router's current choice for Designated Router
      and Backup Designated Router.  A value of 0.0.0.0 in these
      fields means that one has not yet been selected.
      On broadcast networks and physical point-to-point networks,
      Hello packets are sent every HelloInterval seconds to the IP
      multicast address AllSPFRouters.  On virtual links, Hello
      packets are sent as unicasts (addressed directly to the other
      end of the virtual link) every HelloInterval seconds.  On non-
      broadcast networks, the sending of Hello packets is more
      complicated.  This will be covered in the next section.
      9.5.1.  Sending Hello packets on non-broadcast networks
          Static configuration information is necessary in order for
          the Hello Protocol to function on non-broadcast networks
          (see Section C.5).  Every attached router which is eligible
          to become Designated Router has a configured list of all of
          its neighbors on the network.  Each listed neighbor is
          labelled with its Designated Router eligibility.
          The interface state must be at least Waiting for any Hello
          Packets to be sent.  Hello Packets are then sent directly
          (as unicasts) to some subset of a router's neighbors.
          Sometimes an Hello Packet is sent periodically on a timer;
          at other times it is sent as a response to a received Hello
          Packet.  A router's hello-sending behavior varies depending

Moy [Page 68] RFC 1583 OSPF Version 2 March 1994

          on whether the router itself is eligible to become
          Designated Router.
          If the router is eligible to become Designated Router, it
          must periodically send Hello Packets to all neighbors that
          are also eligible.  In addition, if the router is itself the
          Designated Router or Backup Designated Router, it must also
          send periodic Hello Packets to all other neighbors.  This
          means that any two eligible routers are always exchanging
          Hello Packets, which is necessary for the correct operation
          of the Designated Router election algorithm.  To minimize
          the number of Hello Packets sent, the number of eligible
          routers on a non-broadcast network should be kept small.
          If the router is not eligible to become Designated Router,
          it must periodically send Hello Packets to both the
          Designated Router and the Backup Designated Router (if they
          exist).  It must also send an Hello Packet in reply to an
          Hello Packet received from any eligible neighbor (other than
          the current Designated Router and Backup Designated Router).
          This is needed to establish an initial bidirectional
          relationship with any potential Designated Router.
          When sending Hello packets periodically to any neighbor, the
          interval between Hello Packets is determined by the
          neighbor's state.  If the neighbor is in state Down, Hello
          Packets are sent every PollInterval seconds.  Otherwise,
          Hello Packets are sent every HelloInterval seconds.

10. The Neighbor Data Structure

  An OSPF router converses with its neighboring routers.  Each
  separate conversation is described by a "neighbor data structure".
  Each conversation is bound to a particular OSPF router interface,
  and is identified either by the neighboring router's OSPF Router ID
  or by its Neighbor IP address (see below).  Thus if the OSPF router
  and another router have multiple attached networks in common,
  multiple conversations ensue, each described by a unique neighbor
  data structure.  Each separate conversation is loosely referred to
  in the text as being a separate "neighbor".
  The neighbor data structure contains all information pertinent to
  the forming or formed adjacency between the two neighbors.
  (However, remember that not all neighbors become adjacent.)  An
  adjacency can be viewed as a highly developed conversation between
  two routers.

Moy [Page 69] RFC 1583 OSPF Version 2 March 1994

  State
      The functional level of the neighbor conversation.  This is
      described in more detail in Section 10.1.
  Inactivity Timer
      A single shot timer whose firing indicates that no Hello Packet
      has been seen from this neighbor recently.  The length of the
      timer is RouterDeadInterval seconds.
  Master/Slave
      When the two neighbors are exchanging databases, they form a
      master/slave relationship.  The master sends the first Database
      Description Packet, and is the only part that is allowed to
      retransmit.  The slave can only respond to the master's Database
      Description Packets.  The master/slave relationship is
      negotiated in state ExStart.
  DD Sequence Number
      A 32-bit number identifying individual Database Description
      packets.  When the neighbor state ExStart is entered, the DD
      sequence number should be set to a value not previously seen by
      the neighboring router.  One possible scheme is to use the
      machine's time of day counter.  The DD sequence number is then
      incremented by the master with each new Database Description
      packet sent.  The slave's DD sequence number indicates the last
      packet received from the master.  Only one packet is allowed
      outstanding at a time.
  Neighbor ID
      The OSPF Router ID of the neighboring router.  The Neighbor ID
      is learned when Hello packets are received from the neighbor, or
      is configured if this is a virtual adjacency (see Section C.4).
  Neighbor Priority
      The Router Priority of the neighboring router.  Contained in the
      neighbor's Hello packets, this item is used when selecting the
      Designated Router for the attached network.
  Neighbor IP address
      The IP address of the neighboring router's interface to the
      attached network.  Used as the Destination IP address when
      protocol packets are sent as unicasts along this adjacency.
      Also used in router links advertisements as the Link ID for the
      attached network if the neighboring router is selected to be
      Designated Router (see Section 12.4.1).  The Neighbor IP address
      is learned when Hello packets are received from the neighbor.
      For virtual links, the Neighbor IP address is learned during the
      routing table build process (see Section 15).

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  Neighbor Options
      The optional OSPF capabilities supported by the neighbor.
      Learned during the Database Exchange process (see Section 10.6).
      The neighbor's optional OSPF capabilities are also listed in its
      Hello packets.  This enables received Hello Packets to be
      rejected (i.e., neighbor relationships will not even start to
      form) if there is a mismatch in certain crucial OSPF
      capabilities (see Section 10.5).  The optional OSPF capabilities
      are documented in Section 4.5.
  Neighbor's Designated Router
      The neighbor's idea of the Designated Router.  If this is the
      neighbor itself, this is important in the local calculation of
      the Designated Router.  Defined only on multi-access networks.
  Neighbor's Backup Designated Router
      The neighbor's idea of the Backup Designated Router.  If this is
      the neighbor itself, this is important in the local calculation
      of the Backup Designated Router.  Defined only on multi-access
      networks.
  The next set of variables are lists of link state advertisements.
  These lists describe subsets of the area topological database.
  There can be five distinct types of link state advertisements in an
  area topological database: router links, network links, and Type 3
  and 4 summary links (all stored in the area data structure), and AS
  external links (stored in the global data structure).
  Link state retransmission list
      The list of link state advertisements that have been flooded but
      not acknowledged on this adjacency.  These will be retransmitted
      at intervals until they are acknowledged, or until the adjacency
      is destroyed.
  Database summary list
      The complete list of link state advertisements that make up the
      area topological database, at the moment the neighbor goes into
      Database Exchange state.  This list is sent to the neighbor in
      Database Description packets.
  Link state request list
      The list of link state advertisements that need to be received
      from this neighbor in order to synchronize the two neighbors'
      topological databases.  This list is created as Database
      Description packets are received, and is then sent to the
      neighbor in Link State Request packets.  The list is depleted as

Moy [Page 71] RFC 1583 OSPF Version 2 March 1994

      appropriate Link State Update packets are received.
  10.1.  Neighbor states
      The state of a neighbor (really, the state of a conversation
      being held with a neighboring router) is documented in the
      following sections.  The states are listed in order of
      progressing functionality.  For example, the inoperative state
      is listed first, followed by a list of intermediate states
      before the final, fully functional state is achieved.  The
      specification makes use of this ordering by sometimes making
      references such as "those neighbors/adjacencies in state greater
      than X".  Figures 12 and 13 show the graph of neighbor state
      changes.  The arcs of the graphs are labelled with the event
      causing the state change.  The neighbor events are documented in
      Section 10.2.
      The graph in Figure 12 shows the state changes effected by the
      Hello Protocol.  The Hello Protocol is responsible for neighbor
                                 +----+
                                 |Down|
                                 +----+
                                   |                               | Start
                                   |        +-------+
                           Hello   |   +---->|Attempt|
                          Received |         +-------+
                                   |             |
                           +----+<-+             |HelloReceived
                           |Init|<---------------+
                           +----+<--------+
                              |           |
                              |2-Way      |1-Way
                              |Received   |Received
                              |           |
            +-------+         |        +-----+
            |ExStart|<--------+------->|2-Way|
            +-------+                  +-----+
            Figure 12: Neighbor state changes (Hello Protocol)
                In addition to the state transitions pictured,
                Event KillNbr always forces Down State,
                Event InactivityTimer always forces Down State,
                Event LLDown always forces Down State

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      acquisition and maintenance, and for ensuring two way
      communication between neighbors.
      The graph in Figure 13 shows the forming of an adjacency.  Not
      every two neighboring routers become adjacent (see Section
      10.4).  The adjacency starts to form when the neighbor is in
      state ExStart.  After the two routers discover their
      master/slave status, the state transitions to Exchange.  At this
      point the neighbor starts to be used in the flooding procedure,
      and the two neighboring routers begin synchronizing their
      databases.  When this synchronization is finished, the neighbor
      is in state Full and we say that the two routers are fully
      adjacent.  At this point the adjacency is listed in link state
      advertisements.
      For a more detailed description of neighbor state changes,
      together with the additional actions involved in each change,
      see Section 10.3.
                                +-------+
                                |ExStart|
                                +-------+
                                  |
                   NegotiationDone|
                                  +->+--------+
                                     |Exchange|
                                  +--+--------+
                                  |
                          Exchange|
                            Done  |
                  +----+          |      +-------+
                  |Full|<---------+----->|Loading|
                  +----+<-+              +-------+
                          |  LoadingDone     |
                          +------------------+
          Figure 13: Neighbor state changes (Database Exchange)
              In addition to the state transitions pictured,
              Event SeqNumberMismatch forces ExStart state,
              Event BadLSReq forces ExStart state,
              Event 1-Way forces Init state,
              Event KillNbr always forces Down State,
              Event InactivityTimer always forces Down State,
              Event LLDown always forces Down State,
              Event AdjOK? leads to adjacency forming/breaking

Moy [Page 73] RFC 1583 OSPF Version 2 March 1994

      Down
          This is the initial state of a neighbor conversation.  It
          indicates that there has been no recent information received
          from the neighbor.  On non-broadcast networks, Hello packets
          may still be sent to "Down" neighbors, although at a reduced
          frequency (see Section 9.5.1).
      Attempt
          This state is only valid for neighbors attached to non-
          broadcast networks.  It indicates that no recent information
          has been received from the neighbor, but that a more
          concerted effort should be made to contact the neighbor.
          This is done by sending the neighbor Hello packets at
          intervals of HelloInterval (see Section 9.5.1).
      Init
          In this state, an Hello packet has recently been seen from
          the neighbor.  However, bidirectional communication has not
          yet been established with the neighbor (i.e., the router
          itself did not appear in the neighbor's Hello packet).  All
          neighbors in this state (or higher) are listed in the Hello
          packets sent from the associated interface.
      2-Way
          In this state, communication between the two routers is
          bidirectional.  This has been assured by the operation of
          the Hello Protocol.  This is the most advanced state short
          of beginning adjacency establishment.  The (Backup)
          Designated Router is selected from the set of neighbors in
          state 2-Way or greater.
      ExStart
          This is the first step in creating an adjacency between the
          two neighboring routers.  The goal of this step is to decide
          which router is the master, and to decide upon the initial
          DD sequence number.  Neighbor conversations in this state or
          greater are called adjacencies.
      Exchange
          In this state the router is describing its entire link state
          database by sending Database Description packets to the
          neighbor.  Each Database Description Packet has a DD
          sequence number, and is explicitly acknowledged.  Only one
          Database Description Packet is allowed outstanding at any
          one time.  In this state, Link State Request Packets may
          also be sent asking for the neighbor's more recent
          advertisements.  All adjacencies in Exchange state or
          greater are used by the flooding procedure.  In fact, these

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          adjacencies are fully capable of transmitting and receiving
          all types of OSPF routing protocol packets.
      Loading
          In this state, Link State Request packets are sent to the
          neighbor asking for the more recent advertisements that have
          been discovered (but not yet received) in the Exchange
          state.
      Full
          In this state, the neighboring routers are fully adjacent.
          These adjacencies will now appear in router links and
          network links advertisements.
  10.2.  Events causing neighbor state changes
      State changes can be effected by a number of events.  These
      events are shown in the labels of the arcs in Figures 12 and 13.
      The label definitions are as follows:
      HelloReceived
          A Hello packet has been received from a neighbor.
      Start
          This is an indication that Hello Packets should now be sent
          to the neighbor at intervals of HelloInterval seconds.  This
          event is generated only for neighbors associated with non-
          broadcast networks.
      2-WayReceived
          Bidirectional communication has been realized between the
          two neighboring routers.  This is indicated by this router
          seeing itself in the other's Hello packet.
      NegotiationDone
          The Master/Slave relationship has been negotiated, and DD
          sequence numbers have been exchanged.  This signals the
          start of the sending/receiving of Database Description
          packets.  For more information on the generation of this
          event, consult Section 10.8.
      ExchangeDone
          Both routers have successfully transmitted a full sequence
          of Database Description packets.  Each router now knows what
          parts of its link state database are out of date.  For more
          information on the generation of this event, consult Section

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          10.8.
      BadLSReq
          A Link State Request has been received for a link state
          advertisement not contained in the database.  This indicates
          an error in the Database Exchange process.
      Loading Done
          Link State Updates have been received for all out-of-date
          portions of the database.  This is indicated by the Link
          state request list becoming empty after the Database
          Exchange process has completed.
      AdjOK?
          A decision must be made (again) as to whether an adjacency
          should be established/maintained with the neighbor.  This
          event will start some adjacencies forming, and destroy
          others.
      The following events cause well developed neighbors to revert to
      lesser states.  Unlike the above events, these events may occur
      when the neighbor conversation is in any of a number of states.
      SeqNumberMismatch
          A Database Description packet has been received that either
          a) has an unexpected DD sequence number, b) unexpectedly has
          the Init bit set or c) has an Options field differing from
          the last Options field received in a Database Description
          packet.  Any of these conditions indicate that some error
          has occurred during adjacency establishment.
      1-Way
          An Hello packet has been received from the neighbor, in
          which this router is not mentioned.  This indicates that
          communication with the neighbor is not bidirectional.
      KillNbr
          This  is  an  indication that  all  communication  with  the
          neighbor  is now  impossible,  forcing  the  neighbor  to
          revert  to  Down  state.
      InactivityTimer
          The inactivity Timer has fired.  This means that no Hello
          packets have been seen recently from the neighbor.  The
          neighbor reverts to Down state.

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      LLDown
          This is an indication from the lower level protocols that
          the neighbor is now unreachable.  For example, on an X.25
          network this could be indicated by an X.25 clear indication
          with appropriate cause and diagnostic fields.  This event
          forces the neighbor into Down state.
  10.3.  The Neighbor state machine
      A detailed description of the neighbor state changes follows.
      Each state change is invoked by an event (Section 10.2).  This
      event may produce different effects, depending on the current
      state of the neighbor.  For this reason, the state machine below
      is organized by current neighbor state and received event.  Each
      entry in the state machine describes the resulting new neighbor
      state and the required set of additional actions.
      When a neighbor's state changes, it may be necessary to rerun
      the Designated Router election algorithm.  This is determined by
      whether the interface NeighborChange event is generated (see
      Section 9.2).  Also, if the Interface is in DR state (the router
      is itself Designated Router), changes in neighbor state may
      cause a new network links advertisement to be originated (see
      Section 12.4).
      When the neighbor state machine needs to invoke the interface
      state machine, it should be done as a scheduled task (see
      Section 4.4).  This simplifies things, by ensuring that neither
      state machine will be executed recursively.
       State(s):  Down
          Event:  Start
      New state:  Attempt
         Action:  Send an Hello Packet to the neighbor (this neighbor
                  is always associated with a non-broadcast network)
                  and start the Inactivity Timer for the neighbor.
                  The timer's later firing would indicate that
                  communication with the neighbor was not attained.
       State(s):  Attempt

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          Event:  HelloReceived
      New state:  Init
         Action:  Restart the Inactivity Timer for the neighbor, since
                  the neighbor has now been heard from.
       State(s):  Down
          Event:  HelloReceived
      New state:  Init
         Action:  Start the Inactivity Timer for the neighbor.  The
                  timer's later firing would indicate that the
                  neighbor is dead.
       State(s):  Init or greater
          Event:  HelloReceived
      New state:  No state change.
         Action:  Restart the Inactivity Timer for the neighbor, since
                  the neighbor has again been heard from.
       State(s):  Init
          Event:  2-WayReceived
      New state:  Depends upon action routine.
         Action:  Determine whether an adjacency should be established
                  with the neighbor (see Section 10.4).  If not, the
                  new neighbor state is 2-Way.
                  Otherwise (an adjacency should be established) the
                  neighbor state transitions to ExStart.  Upon
                  entering this state, the router increments the DD
                  sequence number for this neighbor.  If this is the
                  first time that an adjacency has been attempted, the
                  DD sequence number should be assigned some unique
                  value (like the time of day clock).  It then
                  declares itself master (sets the master/slave bit to
                  master), and starts sending Database Description

Moy [Page 78] RFC 1583 OSPF Version 2 March 1994

                  Packets, with the initialize (I), more (M) and
                  master (MS) bits set.  This Database Description
                  Packet should be otherwise empty.  This Database
                  Description Packet should be retransmitted at
                  intervals of RxmtInterval until the next state is
                  entered (see Section 10.8).
       State(s):  ExStart
          Event:  NegotiationDone
      New state:  Exchange
         Action:  The router must list the contents of its entire area
                  link state database in the neighbor Database summary
                  list.  The area link state database consists of the
                  router links, network links and summary links
                  contained in the area structure, along with the AS
                  external links contained in the global structure.
                  AS external link advertisements are omitted from a
                  virtual neighbor's Database summary list.  AS
                  external advertisements are omitted from the
                  Database summary list if the area has been
                  configured as a stub (see Section 3.6).
                  Advertisements whose age is equal to MaxAge are
                  instead added to the neighbor's Link state
                  retransmission list.  A summary of the Database
                  summary list will be sent to the neighbor in
                  Database Description packets.  Each Database
                  Description Packet has a DD sequence number, and is
                  explicitly acknowledged.  Only one Database
                  Description Packet is allowed outstanding at any one
                  time.  For more detail on the sending and receiving
                  of Database Description packets, see Sections 10.8
                  and 10.6.
       State(s):  Exchange
          Event:  ExchangeDone
      New state:  Depends upon action routine.
         Action:  If the neighbor Link state request list is empty,
                  the new neighbor state is Full.  No other action is
                  required.  This is an adjacency's final state.

Moy [Page 79] RFC 1583 OSPF Version 2 March 1994

                  Otherwise, the new neighbor state is Loading.  Start
                  (or continue) sending Link State Request packets to
                  the neighbor (see Section 10.9).  These are requests
                  for the neighbor's more recent advertisements (which
                  were discovered but not yet received in the Exchange
                  state).  These advertisements are listed in the Link
                  state request list associated with the neighbor.
       State(s):  Loading
          Event:  Loading Done
      New state:  Full
         Action:  No action required.  This is an adjacency's final
                  state.
       State(s):  2-Way
          Event:  AdjOK?
      New state:  Depends upon action routine.
         Action:  Determine whether an adjacency should be formed with
                  the neighboring router (see Section 10.4).  If not,
                  the neighbor state remains at 2-Way.  Otherwise,
                  transition the neighbor state to ExStart and perform
                  the actions associated with the above state machine
                  entry for state Init and event 2-WayReceived.
       State(s):  ExStart or greater
          Event:  AdjOK?
      New state:  Depends upon action routine.
         Action:  Determine whether the neighboring router should
                  still be adjacent.  If yes, there is no state change
                  and no further action is necessary.
                  Otherwise, the (possibly partially formed) adjacency
                  must be destroyed.  The neighbor state transitions
                  to 2-Way.  The Link state retransmission list,
                  Database summary list and Link state request list
                  are cleared of link state advertisements.

Moy [Page 80] RFC 1583 OSPF Version 2 March 1994

       State(s):  Exchange or greater
          Event:  SeqNumberMismatch
      New state:  ExStart
         Action:  The (possibly partially formed) adjacency is torn
                  down, and then an attempt is made at
                  reestablishment.  The neighbor state first
                  transitions to ExStart.  The Link state
                  retransmission list, Database summary list and Link
                  state request list are cleared of link state
                  advertisements.  Then the router increments the DD
                  sequence number for this neighbor, declares itself
                  master (sets the master/slave bit to master), and
                  starts sending Database Description Packets, with
                  the initialize (I), more (M) and master (MS) bits
                  set.  This Database Description Packet should be
                  otherwise empty (see Section 10.8).
       State(s):  Exchange or greater
          Event:  BadLSReq
      New state:  ExStart
         Action:  The action for event BadLSReq is exactly the same as
                  for the neighbor event SeqNumberMismatch.  The
                  (possibly partially formed) adjacency is torn down,
                  and then an attempt is made at reestablishment.  For
                  more information, see the neighbor state machine
                  entry that is invoked when event SeqNumberMismatch
                  is generated in state Exchange or greater.
       State(s):  Any state
          Event:  KillNbr
      New state:  Down
         Action:  The Link state retransmission list, Database summary
                  list and Link state request list are cleared of link
                  state advertisements.  Also, the Inactivity Timer is
                  disabled.

Moy [Page 81] RFC 1583 OSPF Version 2 March 1994

       State(s):  Any state
          Event:  LLDown
      New state:  Down
         Action:  The Link state retransmission list, Database summary
                  list and Link state request list are cleared of link
                  state advertisements.  Also, the Inactivity Timer is
                  disabled.
       State(s):  Any state
          Event:  InactivityTimer
      New state:  Down
         Action:  The Link state retransmission list, Database summary
                  list and Link state request list are cleared of link
                  state advertisements.
       State(s):  2-Way or greater
          Event:  1-WayReceived
      New state:  Init
         Action:  The Link state retransmission list, Database summary
                  list and Link state request list are cleared of link
                  state advertisements.
       State(s):  2-Way or greater
          Event:  2-WayReceived
      New state:  No state change.
         Action:  No action required.
       State(s):  Init
          Event:  1-WayReceived

Moy [Page 82] RFC 1583 OSPF Version 2 March 1994

      New state:  No state change.
         Action:  No action required.
  10.4.  Whether to become adjacent
      Adjacencies are established with some subset of the router's
      neighbors.  Routers connected by point-to-point networks and
      virtual links always become adjacent.  On multi-access networks,
      all routers become adjacent to both the Designated Router and
      the Backup Designated Router.
      The adjacency-forming decision occurs in two places in the
      neighbor state machine.  First, when bidirectional communication
      is initially established with the neighbor, and secondly, when
      the identity of the attached network's (Backup) Designated
      Router changes.  If the decision is made to not attempt an
      adjacency, the state of the neighbor communication stops at 2-
      Way.
      An adjacency should be established with a bidirectional neighbor
      when at least one of the following conditions holds:
      o   The underlying network type is point-to-point
      o   The underlying network type is virtual link
      o   The router itself is the Designated Router
      o   The router itself is the Backup Designated Router
      o   The neighboring router is the Designated Router
      o   The neighboring router is the Backup Designated Router
  10.5.  Receiving Hello Packets
      This section explains the detailed processing of a received
      Hello Packet.  (See Section A.3.2 for the format of Hello
      packets.)  The generic input processing of OSPF packets will
      have checked the validity of the IP header and the OSPF packet
      header.  Next, the values of the Network Mask, HelloInterval,
      and RouterDeadInterval fields in the received Hello packet must
      be checked against the values configured for the receiving
      interface.  Any mismatch causes processing to stop and the

Moy [Page 83] RFC 1583 OSPF Version 2 March 1994

      packet to be dropped.  In other words, the above fields are
      really describing the attached network's configuration. However,
      there is one exception to the above rule: on point-to-point
      networks and on virtual links, the Network Mask in the received
      Hello Packet should be ignored.
      The receiving interface attaches to a single OSPF area (this
      could be the backbone).  The setting of the E-bit found in the
      Hello Packet's Options field must match this area's
      ExternalRoutingCapability.  If AS external advertisements are
      not flooded into/throughout the area (i.e, the area is a "stub")
      the E-bit must be clear in received Hello Packets, otherwise the
      E-bit must be set.  A mismatch causes processing to stop and the
      packet to be dropped.  The setting of the rest of the bits in
      the Hello Packet's Options field should be ignored.
      At this point, an attempt is made to match the source of the
      Hello Packet to one of the receiving interface's neighbors.  If
      the receiving interface is a multi-access network (either
      broadcast or non-broadcast) the source is identified by the IP
      source address found in the Hello's IP header.  If the receiving
      interface is a point-to-point link or a virtual link, the source
      is identified by the Router ID found in the Hello's OSPF packet
      header.  The interface's current list of neighbors is contained
      in the interface's data structure.  If a matching neighbor
      structure cannot be found, (i.e., this is the first time the
      neighbor has been detected), one is created.  The initial state
      of a newly created neighbor is set to Down.
      When receiving an Hello Packet from a neighbor on a multi-access
      network (broadcast or non-broadcast), set the neighbor
      structure's Neighbor ID equal to the Router ID found in the
      packet's OSPF header.  When receiving an Hello on a point-to-
      point network (but not on a virtual link) set the neighbor
      structure's Neighbor IP address to the packet's IP source
      address.
      Now the rest of the Hello Packet is examined, generating events
      to be given to the neighbor and interface state machines.  These
      state machines are specified either to be executed or scheduled
      (see Section 4.4).  For example, by specifying below that the
      neighbor state machine be executed in line, several neighbor
      state transitions may be effected by a single received Hello:
      o   Each Hello Packet causes the neighbor state machine to be
          executed with the event HelloReceived.

Moy [Page 84] RFC 1583 OSPF Version 2 March 1994

      o   Then the list of neighbors contained in the Hello Packet is
          examined.  If the router itself appears in this list, the
          neighbor state machine should be executed with the event 2-
          WayReceived.  Otherwise, the neighbor state machine should
          be executed with the event 1-WayReceived, and the processing
          of the packet stops.
      o   Next, the Hello Packet's Router Priority field is examined.
          If this field is different than the one previously received
          from the neighbor, the receiving interface's state machine
          is scheduled with the event NeighborChange.  In any case,
          the Router Priority field in the neighbor data structure
          should be updated accordingly.
      o   Next the Designated Router field in the Hello Packet is
          examined.  If the neighbor is both declaring itself to be
          Designated Router (Designated Router field = Neighbor IP
          address) and the Backup Designated Router field in the
          packet is equal to 0.0.0.0 and the receiving interface is in
          state Waiting, the receiving interface's state machine is
          scheduled with the event BackupSeen.  Otherwise, if the
          neighbor is declaring itself to be Designated Router and it
          had not previously, or the neighbor is not declaring itself
          Designated Router where it had previously, the receiving
          interface's state machine is scheduled with the event
          NeighborChange.  In any case, the Neighbors' Designated
          Router item in the neighbor structure is updated
          accordingly.
      o   Finally, the Backup Designated Router field in the Hello
          Packet is examined.  If the neighbor is declaring itself to
          be Backup Designated Router (Backup Designated Router field
          = Neighbor IP address) and the receiving interface is in
          state Waiting, the receiving interface's state machine is
          scheduled with the event BackupSeen.  Otherwise, if the
          neighbor is declaring itself to be Backup Designated Router
          and it had not previously, or the neighbor is not declaring
          itself Backup Designated Router where it had previously, the
          receiving interface's state machine is scheduled with the
          event NeighborChange.  In any case, the Neighbor's Backup
          Designated Router item in the neighbor structure is updated
          accordingly.
      On non-broadcast multi-access networks, receipt of an Hello
      Packet may also cause an Hello Packet to be sent back to the
      neighbor in response. See Section 9.5.1 for more details.

Moy [Page 85] RFC 1583 OSPF Version 2 March 1994

  10.6.  Receiving Database Description Packets
      This section explains the detailed processing of a received
      Database Description Packet.  The incoming Database Description
      Packet has already been associated with a neighbor and receiving
      interface by the generic input packet processing (Section 8.2).
      The further processing of the Database Description Packet
      depends on the neighbor state.  If the neighbor's state is Down
      or Attempt the packet should be ignored.  Otherwise, if the
      state is:
      Init
          The neighbor state machine should be executed with the event
          2-WayReceived.  This causes an immediate state change to
          either state 2-Way or state ExStart. If the new state is
          ExStart, the processing of the current packet should then
          continue in this new state by falling through to case
          ExStart below.
      2-Way
          The packet should be ignored.  Database Description Packets
          are used only for the purpose of bringing up adjacencies.[7]
      ExStart
          If the received packet matches one of the following cases,
          then the neighbor state machine should be executed with the
          event NegotiationDone (causing the state to transition to
          Exchange), the packet's Options field should be recorded in
          the neighbor structure's Neighbor Options field and the
          packet should be accepted as next in sequence and processed
          further (see below).  Otherwise, the packet should be
          ignored.
          o   The initialize(I), more (M) and master(MS) bits are set,
              the contents of the packet are empty, and the neighbor's
              Router ID is larger than the router's own.  In this case
              the router is now Slave.  Set the master/slave bit to
              slave, and set the DD sequence number to that specified
              by the master.
          o   The initialize(I) and master(MS) bits are off, the
              packet's DD sequence number equals the router's own DD
              sequence number (indicating acknowledgment) and the
              neighbor's Router ID is smaller than the router's own.
              In this case the router is Master.

Moy [Page 86] RFC 1583 OSPF Version 2 March 1994

      Exchange
          If the state of the MS-bit is inconsistent with the
          master/slave state of the connection, generate the neighbor
          event SeqNumberMismatch and stop processing the packet.
          Otherwise:
          o   If the initialize(I) bit is set, generate the neighbor
              event SeqNumberMismatch and stop processing the packet.
          o   If the packet's Options field indicates a different set
              of optional OSPF capabilities than were previously
              received from the neighbor (recorded in the Neighbor
              Options field of the neighbor structure), generate the
              neighbor event SeqNumberMismatch and stop processing the
              packet.
          o   If the router is master, and the packet's DD sequence
              number equals the router's own DD sequence number (this
              packet is the next in sequence) the packet should be
              accepted and its contents processed (below).
          o   If the router is master, and the packet's DD sequence
              number is one less than the router's DD sequence number,
              the packet is a duplicate.  Duplicates should be
              discarded by the master.
          o   If the router is slave, and the packet's DD sequence
              number is one more than the router's own DD sequence
              number (this packet is the next in sequence) the packet
              should be accepted and its contents processed (below).
          o   If the router is slave, and the packet's DD sequence
              number is equal to the router's DD sequence number, the
              packet is a duplicate.  The slave must respond to
              duplicates by repeating the last Database Description
              packet that it had sent.
          o   Else, generate the neighbor event SeqNumberMismatch and
              stop processing the packet.
      Loading or Full
          In this state, the router has sent and received an entire
          sequence of Database Description Packets.  The only packets
          received should be duplicates (see above).  In particular,
          the packet's Options field should match the set of optional
          OSPF capabilities previously indicated by the neighbor
          (stored in the neighbor structure's Neighbor Options field).
          Any other packets received, including the reception of a

Moy [Page 87] RFC 1583 OSPF Version 2 March 1994

          packet with the Initialize(I) bit set, should generate the
          neighbor event SeqNumberMismatch.[8] Duplicates should be
          discarded by the master.  The slave must respond to
          duplicates by repeating the last Database Description packet
          that it had sent.
      When the router accepts a received Database Description Packet
      as the next in sequence the packet contents are processed as
      follows.  For each link state advertisement listed, the
      advertisement's LS type is checked for validity.  If the LS type
      is unknown (e.g., not one of the LS types 1-5 defined by this
      specification), or if this is a AS external advertisement (LS
      type = 5) and the neighbor is associated with a stub area,
      generate the neighbor event SeqNumberMismatch and stop
      processing the packet.  Otherwise, the router looks up the
      advertisement in its database to see whether it also has an
      instance of the link state advertisement.  If it does not, or if
      the database copy is less recent (see Section 13.1), the link
      state advertisement is put on the Link state request list so
      that it can be requested (immediately or at some later time) in
      Link State Request Packets.
      When the router accepts a received Database Description Packet
      as the next in sequence, it also performs the following actions,
      depending on whether it is master or slave:
      Master
          Increments the DD sequence number.  If the router has
          already sent its entire sequence of Database Description
          Packets, and the just accepted packet has the more bit (M)
          set to 0, the neighbor event ExchangeDone is generated.
          Otherwise, it should send a new Database Description to the
          slave.
      Slave
          Sets the DD sequence number to the DD sequence number
          appearing in the received packet.  The slave must send a
          Database Description Packet in reply.  If the received
          packet has the more bit (M) set to 0, and the packet to be
          sent by the slave will also have the M-bit set to 0, the
          neighbor event ExchangeDone is generated.  Note that the
          slave always generates this event before the master.

Moy [Page 88] RFC 1583 OSPF Version 2 March 1994

  10.7.  Receiving Link State Request Packets
      This section explains the detailed processing of received Link
      State Request packets.  Received Link State Request Packets
      specify a list of link state advertisements that the neighbor
      wishes to receive.  Link State Request Packets should be
      accepted when the neighbor is in states Exchange, Loading, or
      Full.  In all other states Link State Request Packets should be
      ignored.
      Each link state advertisement specified in the Link State
      Request packet should be located in the router's database, and
      copied into Link State Update packets for transmission to the
      neighbor.  These link state advertisements should NOT be placed
      on the Link state retransmission list for the neighbor.  If a
      link state advertisement cannot be found in the database,
      something has gone wrong with the Database Exchange process, and
      neighbor event BadLSReq should be generated.
  10.8.  Sending Database Description Packets
      This section describes how Database Description Packets are sent
      to a neighbor.  The router's optional OSPF capabilities (see
      Section 4.5) are transmitted to the neighbor in the Options
      field of the Database Description packet.  The router should
      maintain the same set of optional capabilities throughout the
      Database Exchange and flooding procedures.  If for some reason
      the router's optional capabilities change, the Database Exchange
      procedure should be restarted by reverting to neighbor state
      ExStart.  There are currently two optional capabilities defined.
      The T-bit should be set if and only if the router is capable of
      calculating separate routes for each IP TOS.  The E-bit should
      be set if and only if the attached network belongs to a non-stub
      area.  The rest of the Options field should be set to zero.
      The sending of Database Description packets depends on the
      neighbor's state.  In state ExStart the router sends empty
      Database Description packets, with the initialize (I), more (M)
      and master (MS) bits set.  These packets are retransmitted every
      RxmtInterval seconds.
      In state Exchange the Database Description Packets actually
      contain summaries of the link state information contained in the
      router's database.  Each link state advertisement in the area's
      topological database (at the time the neighbor transitions into
      Exchange state) is listed in the neighbor Database summary list.
      When a new Database Description Packet is to be sent, the

Moy [Page 89] RFC 1583 OSPF Version 2 March 1994

      packet's DD sequence number is incremented, and the (new) top of
      the Database summary list is described by the packet.  Items are
      removed from the Database summary list when the previous packet
      is acknowledged.
      In state Exchange, the determination of when to send a Database
      Description packet depends on whether the router is master or
      slave:
      Master
          Database Description packets are sent when either a) the
          slave acknowledges the previous Database Description packet
          by echoing the DD sequence number or b) RxmtInterval seconds
          elapse without an acknowledgment, in which case the previous
          Database Description packet is retransmitted.
      Slave
          Database Description packets are sent only in response to
          Database Description packets received from the master.  If
          the Database Description packet received from the master is
          new, a new Database Description packet is sent, otherwise
          the previous Database Description packet is resent.
      In states Loading and Full the slave must resend its last
      Database Description packet in response to duplicate Database
      Description packets received from the master.  For this reason
      the slave must wait RouterDeadInterval seconds before freeing
      the last Database Description packet.  Reception of a Database
      Description packet from the master after this interval will
      generate a SeqNumberMismatch neighbor event.
  10.9.  Sending Link State Request Packets
      In neighbor states Exchange or Loading, the Link state request
      list contains a list of those link state advertisements that
      need to be obtained from the neighbor.  To request these
      advertisements, a router sends the neighbor the beginning of the
      Link state request list, packaged in a Link State Request
      packet.
      When the neighbor responds to these requests with the proper
      Link State Update packet(s), the Link state request list is
      truncated and a new Link State Request packet is sent.  This
      process continues until the Link state request list becomes
      empty.  Unsatisfied Link State Request packets are retransmitted

Moy [Page 90] RFC 1583 OSPF Version 2 March 1994

      at intervals of RxmtInterval.  There should be at most one Link
      State Request packet outstanding at any one time.
      When the Link state request list becomes empty, and the neighbor
      state is Loading (i.e., a complete sequence of Database
      Description packets has been sent to and received from the
      neighbor), the Loading Done neighbor event is generated.
  10.10.  An Example
      Figure 14 shows an example of an adjacency forming.  Routers RT1
      and RT2 are both connected to a broadcast network.  It is
      assumed that RT2 is the Designated Router for the network, and
      that RT2 has a higher Router ID than Router RT1.
      The neighbor state changes realized by each router are listed on
      the sides of the figure.
      At the beginning of Figure 14, Router RT1's interface to the
      network becomes operational.  It begins sending Hello Packets,
      although it doesn't know the identity of the Designated Router
      or of any other neighboring routers.  Router RT2 hears this
      hello (moving the neighbor to Init state), and in its next Hello
      Packet indicates that it is itself the Designated Router and
      that it has heard Hello Packets from RT1.  This in turn causes
      RT1 to go to state ExStart, as it starts to bring up the
      adjacency.
      RT1 begins by asserting itself as the master.  When it sees that
      RT2 is indeed the master (because of RT2's higher Router ID),
      RT1 transitions to slave state and adopts its neighbor's DD
      sequence number.  Database Description packets are then
      exchanged, with polls coming from the master (RT2) and responses
      from the slave (RT1).  This sequence of Database Description
      Packets ends when both the poll and associated response has the
      M-bit off.
      In this example, it is assumed that RT2 has a completely up to
      date database.  In that case, RT2 goes immediately into Full
      state.  RT1 will go into Full state after updating the necessary
      parts of its database.  This is done by sending Link State
      Request Packets, and receiving Link State Update Packets in
      response.  Note that, while RT1 has waited until a complete set
      of Database Description Packets has been received (from RT2)
      before sending any Link State Request Packets, this need not be
      the case.  RT1 could have interleaved the sending of Link State
      Request Packets with the reception of Database Description

Moy [Page 91] RFC 1583 OSPF Version 2 March 1994

          +---+                                         +---+
          |RT1|                                         |RT2|
          +---+                                         +---+
          Down                                          Down
                          Hello(DR=0,seen=0)
                     ------------------------------>
                       Hello (DR=RT2,seen=RT1,...)      Init
                     <------------------------------
          ExStart        D-D (Seq=x,I,M,Master)
                     ------------------------------>
                         D-D (Seq=y,I,M,Master)         ExStart
                     <------------------------------
          Exchange       D-D (Seq=y,M,Slave)
                     ------------------------------>
                         D-D (Seq=y+1,M,Master)         Exchange
                     <------------------------------
                         D-D (Seq=y+1,M,Slave)
                     ------------------------------>
                                   ...
                                   ...
                                   ...
                         D-D (Seq=y+n, Master)
                     <------------------------------
                         D-D (Seq=y+n, Slave)
           Loading   ------------------------------>
                               LS Request                Full
                     ------------------------------>
                               LS Update
                     <------------------------------
                               LS Request
                     ------------------------------>
                               LS Update
                     <------------------------------
           Full
                 Figure 14: An adjacency bring-up example

Moy [Page 92] RFC 1583 OSPF Version 2 March 1994

      Packets.

11. The Routing Table Structure

  The routing table data structure contains all the information
  necessary to forward an IP data packet toward its destination.  Each
  routing table entry describes the collection of best paths to a
  particular destination.  When forwarding an IP data packet, the
  routing table entry providing the best match for the packet's IP
  destination is located.  The matching routing table entry then
  provides the next hop towards the packet's destination.  OSPF also
  provides for the existence of a default route (Destination ID =
  DefaultDestination, Address Mask =  0x00000000).  When the default
  route exists, it matches all IP destinations (although any other
  matching entry is a better match).  Finding the routing table entry
  that best matches an IP destination is further described in Section
  11.1.
  There is a single routing table in each router.  Two sample routing
  tables are described in Sections 11.2 and 11.3.  The building of the
  routing table is discussed in Section 16.
  The rest of this section defines the fields found in a routing table
  entry.  The first set of fields describes the routing table entry's
  destination.
  Destination Type
      The destination can be one of three types.  Only the first type,
      Network, is actually used when forwarding IP data traffic.  The
      other destinations are used solely as intermediate steps in the
      routing table build process.
      Network
          A range of IP addresses, to which IP data traffic may be
          forwarded.  This includes IP networks (class A, B, or C), IP
          subnets, IP supernets and single IP hosts.  The default
          route also falls in this category.
      Area border router
          Routers that are connected to multiple OSPF areas.  Such
          routers originate summary link advertisements.  These
          routing table entries are used when calculating the inter-
          area routes (see Section 16.2).  These routing table entries
          may also be associated with configured virtual links.

Moy [Page 93] RFC 1583 OSPF Version 2 March 1994

      AS boundary router
          Routers that originate AS external link advertisements.
          These routing table entries are used when calculating the AS
          external routes (see Section 16.4).
  Destination ID
      The destination's identifier or name.  This depends on the
      Destination Type.  For networks, the identifier is their
      associated IP address.  For all other types, the identifier is
      the OSPF Router ID.[9]
  Address Mask
      Only defined for networks.  The network's IP address together
      with its address mask defines a range of IP addresses.  For IP
      subnets, the address mask is referred to as the subnet mask.
      For host routes, the mask is "all ones" (0xffffffff).
  Optional Capabilities
      When the destination is a router (either an area border router
      or an AS boundary router) this field indicates the optional OSPF
      capabilities supported by the destination router.  The two
      optional capabilities currently defined by this specification
      are the ability to route based on IP TOS and the ability to
      process AS external link advertisements.  For a further
      discussion of OSPF's optional capabilities, see Section 4.5.
  The set of paths to use for a destination may vary based on IP Type
  of Service and the OSPF area to which the paths belong.  This means
  that there may be multiple routing table entries for the same
  destination, depending on the values of the next two fields.
  Type of Service
      There can be a separate set of routes for each IP Type of
      Service.  The encoding of TOS in OSPF link state advertisements
      is described in Section 12.3.
  Area
      This field indicates the area whose link state information has
      led to the routing table entry's collection of paths.  This is
      called the entry's associated area.  For sets of AS external
      paths, this field is not defined.  For destinations of type
      "area border router", there may be separate sets of paths (and
      therefore separate routing table entries) associated with each
      of several areas.  This will happen when two area border routers
      share multiple areas in common.  For all other destination
      types, only the set of paths associated with the best area (the

Moy [Page 94] RFC 1583 OSPF Version 2 March 1994

      one providing the shortest route) is kept.
  The rest of the routing table entry describes the set of paths to
  the destination.  The following fields pertain to the set of paths
  as a whole.  In other words, each one of the paths contained in a
  routing table entry is of the same path-type and cost (see below).
  Path-type
      There are four possible types of paths used to route traffic to
      the destination, listed here in order of preference: intra-area,
      inter-area, type 1 external or type 2 external.  Intra-area
      paths indicate destinations belonging to one of the router's
      attached areas.  Inter-area paths are paths to destinations in
      other OSPF areas.  These are discovered through the examination
      of received summary link advertisements.  AS external paths are
      paths to destinations external to the AS.  These are detected
      through the examination of received AS external link
      advertisements.
  Cost
      The link state cost of the path to the destination.  For all
      paths except type 2 external paths this describes the entire
      path's cost.  For Type 2 external paths, this field describes
      the cost of the portion of the path internal to the AS.  This
      cost is calculated as the sum of the costs of the path's
      constituent links.
  Type 2 cost
      Only valid for type 2 external paths.  For these paths, this
      field indicates the cost of the path's external portion.  This
      cost has been advertised by an AS boundary router, and is the
      most significant part of the total path cost.  For example, a
      type 2 external path with type 2 cost of 5 is always preferred
      over a path with type 2 cost of 10, regardless of the cost of
      the two paths' internal components.
  Link State Origin
      Valid only for intra-area paths, this field indicates the link
      state advertisement (router links or network links) that
      directly references the destination.  For example, if the
      destination is a transit network, this is the transit network's
      network links advertisement.  If the destination is a stub
      network, this is the router links advertisement for the attached
      router.  The advertisement is discovered during the shortest-
      path tree calculation (see Section 16.1).  Multiple
      advertisements may reference the destination, however a tie-

Moy [Page 95] RFC 1583 OSPF Version 2 March 1994

      breaking scheme always reduces the choice to a single
      advertisement. The Link State Origin field is not used by the
      OSPF protocol, but it is used by the routing table calculation
      in OSPF's Multicast routing extensions (MOSPF).
  When multiple paths of equal path-type and cost exist to a
  destination (called elsewhere "equal-cost" paths), they are stored
  in a single routing table entry.  Each one of the "equal-cost" paths
  is distinguished by the following fields:
  Next hop
      The outgoing router interface to use when forwarding traffic to
      the destination.  On multi-access networks, the next hop also
      includes the IP address of the next router (if any) in the path
      towards the destination.  This next router will always be one of
      the adjacent neighbors.
  Advertising router
      Valid only for inter-area and AS external paths.  This field
      indicates the Router ID of the router advertising the summary
      link or AS external link that led to this path.
  11.1.  Routing table lookup
      When an IP data packet is received, an OSPF router finds the
      routing table entry that best matches the packet's destination.
      This routing table entry then provides the outgoing interface
      and next hop router to use in forwarding the packet. This
      section describes the process of finding the best matching
      routing table entry. The process consists of a number of steps,
      wherein the collection of routing table entries is progressively
      pruned. In the end, the single routing table entry remaining is
      the called best match.
      Note that the steps described below may fail to produce a best
      match routing table entry (i.e., all existing routing table
      entries are pruned for some reason or another). In this case,
      the packet's IP destination is considered unreachable. Instead
      of being forwarded, the packet should be dropped and an ICMP
      destination unreachable message should be returned to the
      packet's source.
      (1) Select the complete set of "matching" routing table entries
          from the routing table.  Each routing table entry describes
          a (set of) path(s) to a range of IP addresses. If the data

Moy [Page 96] RFC 1583 OSPF Version 2 March 1994

          packet's IP destination falls into an entry's range of IP
          addresses, the routing table entry is called a match. (It is
          quite likely that multiple entries will match the data
          packet.  For example, a default route will match all
          packets.)
      (2) Suppose that the packet's IP destination falls into one of
          the router's configured area address ranges (see Section
          3.5), and that the particular area address range is active.
          This means that there are one or more reachable (by intra-
          area paths) networks contained in the area address range.
          The packet's IP destination is then required to belong to
          one of these constituent networks. For this reason, only
          matching routing table entries with path-type of intra-area
          are considered (all others are pruned). If no such matching
          entries exist, the destination is unreachable (see above).
          Otherwise, skip to step 4.
      (3) Reduce the set of matching entries to those having the most
          preferential path-type (see Section 11). OSPF has a four
          level hierarchy of paths. Intra-area paths are the most
          preferred, followed in order by inter-area, type 1 external
          and type 2 external paths.
      (4) Select the remaining routing table entry that provides the
          longest (most specific) match. Another way of saying this is
          to choose the remaining entry that specifies the narrowest
          range of IP addresses.[10] For example, the entry for the
          address/mask pair of (128.185.1.0, 0xffffff00) is more
          specific than an entry for the pair (128.185.0.0,
          0xffff0000). The default route is the least specific match,
          since it matches all destinations.
      (5) At this point, there may still be multiple routing table
          entries remaining. Each routing entry will specify the same
          range of IP addresses, but a different IP Type of Service.
          Select the routing table entry whose TOS value matches the
          TOS found in the packet header. If there is no routing table
          entry for this TOS, select the routing table entry for TOS
          0. In other words, packets requesting TOS X are routed along
          the TOS 0 path if a TOS X path does not exist.
  11.2.  Sample routing table, without areas
      Consider the Autonomous System pictured in Figure 2.  No OSPF
      areas have been configured.  A single metric is shown per
      outbound interface, indicating that routes will not vary based

Moy [Page 97] RFC 1583 OSPF Version 2 March 1994

      on TOS.  The calculation of Router RT6's routing table proceeds
      as described in Section 2.1.  The resulting routing table is
      shown in Table 12.  Destination types are abbreviated: Network
      as "N", area border router as "BR" and AS boundary router as
      "ASBR".
      There are no instances of multiple equal-cost shortest paths in
      this example.  Also, since there are no areas, there are no
      inter-area paths.
      Routers RT5 and RT7 are AS boundary routers.  Intra-area routes
      have been calculated to Routers RT5 and RT7.  This allows
      external routes to be calculated to the destinations advertised
      by RT5 and RT7 (i.e., Networks N12, N13, N14 and N15).  It is
      assumed all AS external advertisements originated by RT5 and RT7
      are advertising type 1 external metrics.  This results in type 1
      external paths being calculated to destinations N12-N15.
  11.3.  Sample routing table, with areas
      Consider the previous example, this time split into OSPF areas.
      An OSPF area configuration is pictured in Figure 6.  Router
      RT4's routing table will be described for this area
      configuration.  Router RT4 has a connection to Area 1 and a
      backbone connection.  This causes Router RT4 to view the AS as
      the concatenation of the two graphs shown in Figures 7 and 8.
      The resulting routing table is displayed in Table 13.
      Again, Routers RT5 and RT7 are AS boundary routers.  Routers
      RT3, RT4, RT7, RT10 and RT11 are area border routers.  Note that
      there are two routing table entries (in this case having
      identical paths) for Router RT7, in its dual capacities as an
      area border router and an AS boundary router.  Note also that
      there are two routing entries for the area border router RT3,
      since it has two areas in common with RT4 (Area 1 and the
      backbone).
      Backbone paths have been calculated to all area border routers
      (BR).  These are used when determining the inter-area routes.
      Note that all of the inter-area routes are associated with the
      backbone; this is always the case when the calculating router is
      itself an area border router.  Routing information is condensed
      at area boundaries.  In this example, we assume that Area 3 has
      been defined so that networks N9-N11 and the host route to H1
      are all condensed to a single route when advertised into the
      backbone (by Router RT11).  Note that the cost of this route is

Moy [Page 98] RFC 1583 OSPF Version 2 March 1994

    Type   Dest   Area   Path  Type    Cost   Next     Adv.
                                              Hop(s)   Router(s)
    ____________________________________________________________
    N      N1     0      intra-area    10     RT3      *
    N      N2     0      intra-area    10     RT3      *
    N      N3     0      intra-area    7      RT3      *
    N      N4     0      intra-area    8      RT3      *
    N      Ib     0      intra-area    7      *        *
    N      Ia     0      intra-area    12     RT10     *
    N      N6     0      intra-area    8      RT10     *
    N      N7     0      intra-area    12     RT10     *
    N      N8     0      intra-area    10     RT10     *
    N      N9     0      intra-area    11     RT10     *
    N      N10    0      intra-area    13     RT10     *
    N      N11    0      intra-area    14     RT10     *
    N      H1     0      intra-area    21     RT10     *
    ASBR   RT5    0      intra-area    6      RT5      *
    ASBR   RT7    0      intra-area    8      RT10     *
    ____________________________________________________________
    N      N12    *      type 1 ext.   10     RT10     RT7
    N      N13    *      type 1 ext.   14     RT5      RT5
    N      N14    *      type 1 ext.   14     RT5      RT5
    N      N15    *      type 1 ext.   17     RT10     RT7
             Table 12: The routing table for Router RT6
                       (no configured areas).
      the minimum of the set of costs to its individual components.
      There is a virtual link configured between Routers RT10 and
      RT11.  Without this configured virtual link, RT11 would be
      unable to advertise a route for networks N9-N11 and Host H1 into
      the backbone, and there would not be an entry for these networks
      in Router RT4's routing table.
      In this example there are two equal-cost paths to Network N12.
      However, they both use the same next hop (Router RT5).
      Router RT4's routing table would improve (i.e., some of the
      paths in the routing table would become shorter) if an
      additional virtual link were configured between Router RT4 and
      Router RT3.  The new virtual link would itself be associated
      with the first entry for area border router RT3 in Table 13 (an

Moy [Page 99] RFC 1583 OSPF Version 2 March 1994

 Type   Dest        Area   Path  Type    Cost   Next      Adv.
                                                Hops(s)   Router(s)
 __________________________________________________________________
 N      N1          1      intra-area    4      RT1       *
 N      N2          1      intra-area    4      RT2       *
 N      N3          1      intra-area    1      *         *
 N      N4          1      intra-area    3      RT3       *
 BR     RT3         1      intra-area    1      *         *
 __________________________________________________________________
 N      Ib          0      intra-area    22     RT5       *
 N      Ia          0      intra-area    27     RT5       *
 BR     RT3         0      intra-area    21     RT5       *
 BR     RT7         0      intra-area    14     RT5       *
 BR     RT10        0      intra-area    22     RT5       *
 BR     RT11        0      intra-area    25     RT5       *
 ASBR   RT5         0      intra-area    8      *         *
 ASBR   RT7         0      intra-area    14     RT5       *
 __________________________________________________________________
 N      N6          0      inter-area    15     RT5       RT7
 N      N7          0      inter-area    19     RT5       RT7
 N      N8          0      inter-area    18     RT5       RT7
 N      N9-N11,H1   0      inter-area    26     RT5       RT11
 __________________________________________________________________
 N      N12         *      type 1 ext.   16     RT5       RT5,RT7
 N      N13         *      type 1 ext.   16     RT5       RT5
 N      N14         *      type 1 ext.   16     RT5       RT5
 N      N15         *      type 1 ext.   23     RT5       RT7
                Table 13: Router RT4's routing table
                     in the presence of areas.
      intra-area path through Area 1).  This would yield a cost of 1
      for the virtual link.  The routing table entries changes that
      would be caused by the addition of this virtual link are shown
      in Table 14.

12. Link State Advertisements

  Each router in the Autonomous System originates one or more link
  state advertisements.  There are five distinct types of link state
  advertisements, which are described in Section 4.3.  The collection
  of link state advertisements forms the link state or topological
  database.  Each separate type of advertisement has a separate

Moy [Page 100] RFC 1583 OSPF Version 2 March 1994

  Type   Dest        Area   Path  Type   Cost   Next     Adv.
                                                Hop(s)   Router(s)
  ________________________________________________________________
  N      Ib          0      intra-area   16     RT3      *
  N      Ia          0      intra-area   21     RT3      *
  BR     RT3         0      intra-area   1      *        *
  BR     RT10        0      intra-area   16     RT3      *
  BR     RT11        0      intra-area   19     RT3      *
  ________________________________________________________________
  N      N9-N11,H1   0      inter-area   20     RT3      RT11
                Table 14: Changes resulting from an
                      additional virtual link.
  function.  Router links and network links advertisements describe
  how an area's routers and networks are interconnected.  Summary link
  advertisements provide a way of condensing an area's routing
  information.  AS external advertisements provide a way of
  transparently advertising externally-derived routing information
  throughout the Autonomous System.
  Each link state advertisement begins with a standard 20-byte header.
  This link state advertisement header is discussed below.
  12.1.  The Link State Advertisement Header
      The link state advertisement header contains the LS type, Link
      State ID and Advertising Router fields.  The combination of
      these three fields uniquely identifies the link state
      advertisement.
      There may be several instances of an advertisement present in
      the Autonomous System, all at the same time.  It must then be
      determined which instance is more recent.  This determination is
      made by examining the LS sequence, LS checksum and LS age
      fields.  These fields are also contained in the 20-byte link
      state advertisement header.
      Several of the OSPF packet types list link state advertisements.
      When the instance is not important, an advertisement is referred
      to by its LS type, Link State ID and Advertising Router (see
      Link State Request Packets).  Otherwise, the LS sequence number,
      LS age and LS checksum fields must also be referenced.

Moy [Page 101] RFC 1583 OSPF Version 2 March 1994

      A detailed explanation of the fields contained in the link state
      advertisement header follows.
      12.1.1.  LS age
          This field is the age of the link state advertisement in
          seconds.  It should be processed as an unsigned 16-bit
          integer.  It is set to 0 when the link state advertisement
          is originated.  It must be incremented by InfTransDelay on
          every hop of the flooding procedure.  Link state
          advertisements are also aged as they are held in each
          router's database.
          The age of a link state advertisement is never incremented
          past MaxAge.  Advertisements having age MaxAge are not used
          in the routing table calculation.  When an advertisement's
          age first reaches MaxAge, it is reflooded.  A link state
          advertisement of age MaxAge is finally flushed from the
          database when it is no longer needed to ensure database
          synchronization.  For more information on the aging of link
          state advertisements, consult Section 14.
          The LS age field is examined when a router receives two
          instances of a link state advertisement, both having
          identical LS sequence numbers and LS checksums.  An instance
          of age MaxAge is then always accepted as most recent; this
          allows old advertisements to be flushed quickly from the
          routing domain.  Otherwise, if the ages differ by more than
          MaxAgeDiff, the instance having the smaller age is accepted
          as most recent.[11] See Section 13.1 for more details.
      12.1.2.  Options
          The Options field in the link state advertisement header
          indicates which optional capabilities are associated with
          the advertisement.  OSPF's optional capabilities are
          described in Section 4.5.  There are currently two optional
          capabilities defined; they are represented by the T-bit and
          E-bit found in the Options field.  The rest of the Options
          field should be set to zero.
          The E-bit represents OSPF's ExternalRoutingCapability.  This
          bit should be set in all advertisements associated with the
          backbone, and all advertisements associated with non-stub
          areas (see Section 3.6).  It should also be set in all AS
          external link advertisements.  It should be reset in all

Moy [Page 102] RFC 1583 OSPF Version 2 March 1994

          router links, network links and summary link advertisements
          associated with a stub area.  For all link state
          advertisements, the setting of the E-bit is for
          informational purposes only; it does not affect the routing
          table calculation.
          The T-bit represents OSPF's TOS routing capability.  This
          bit should be set in a router links advertisement if and
          only if the router is capable of calculating separate routes
          for each IP TOS (see Section 2.4).  The T-bit should always
          be set in network links advertisements.  It should be set in
          summary link and AS external link advertisements if and only
          if the advertisement describes paths for all TOS values,
          instead of just the TOS 0 path.  Note that, with the T-bit
          set, there may still be only a single metric in the
          advertisement (the TOS 0 metric).  This would mean that
          paths for non-zero TOS exist, but are equivalent to the TOS
          0 path.  A link state advertisement's T-bit is examined when
          calculating the routing table's non-zero TOS paths (see
          Section 16.9).
      12.1.3.  LS type
          The LS type field dictates the format and function of the
          link state advertisement.  Advertisements of different types
          have different names (e.g., router links or network links).
          All advertisement types, except the AS external link
          advertisements (LS type = 5), are flooded throughout a
          single area only.  AS external link advertisements are
          flooded throughout the entire Autonomous System, excepting
          stub areas (see Section 3.6).  Each separate advertisement
          type is briefly described below in Table 15.
      12.1.4.  Link State ID
          This field identifies the piece of the routing domain that
          is being described by the advertisement.  Depending on the
          advertisement's LS type, the Link State ID takes on the
          values listed in Table 16.
          Actually, for Type 3 summary link (LS type = 3)
          advertisements and AS external link (LS type = 5)
          advertisements, the Link State ID may additionally have one
          or more of the destination network's "host" bits set. For
          example, when originating an AS external link for the
          network 10.0.0.0 with mask of 255.0.0.0, the Link State ID

Moy [Page 103] RFC 1583 OSPF Version 2 March 1994

         LS Type   Advertisement description
         __________________________________________________
         1         These are the router links
                   advertisements. They describe the
                   collected states of the router's
                   interfaces. For more information,
                   consult Section 12.4.1.
         __________________________________________________
         2         These are the network links
                   advertisements. They describe the set
                   of routers attached to the network. For
                   more information, consult
                   Section 12.4.2.
         __________________________________________________
         3 or 4    These are the summary link
                   advertisements. They describe
                   inter-area routes, and enable the
                   condensation of routing information at
                   area borders. Originated by area border
                   routers, the Type 3 advertisements
                   describe routes to networks while the
                   Type 4 advertisements describe routes to
                   AS boundary routers.
         __________________________________________________
         5         These are the AS external link
                   advertisements. Originated by AS
                   boundary routers, they describe routes
                   to destinations external to the
                   Autonomous System. A default route for
                   the Autonomous System can also be
                   described by an AS external link
                   advertisement.
             Table 15: OSPF link state advertisements.

Moy [Page 104] RFC 1583 OSPF Version 2 March 1994

          LS Type   Link State ID
          _______________________________________________
          1         The originating router's Router ID.
          2         The IP interface address of the
                    network's Designated Router.
          3         The destination network's IP address.
          4         The Router ID of the described AS
                    boundary router.
          5         The destination network's IP address.
            Table 16: The advertisement's Link State ID.
          can be set to anything in the range 10.0.0.0 through
          10.255.255.255 inclusive (although 10.0.0.0 should be used
          whenever possible). The freedom to set certain host bits
          allows a router to originate separate advertisements for two
          networks having the same address but different masks. See
          Appendix F for details.
          When the link state advertisement is describing a network
          (LS type = 2, 3 or 5), the network's IP address is easily
          derived by masking the Link State ID with the network/subnet
          mask contained in the body of the link state advertisement.
          When the link state advertisement is describing a router (LS
          type = 1 or 4), the Link State ID is always the described
          router's OSPF Router ID.
          When an AS external advertisement (LS Type = 5) is
          describing a default route, its Link State ID is set to
          DefaultDestination (0.0.0.0).
      12.1.5.  Advertising Router
          This field specifies the OSPF Router ID of the
          advertisement's originator.  For router links
          advertisements, this field is identical to the Link State ID
          field.  Network link advertisements are originated by the
          network's Designated Router.  Summary link advertisements
          are originated by area border routers.  AS external link
          advertisements are originated by AS boundary routers.
      12.1.6.  LS sequence number
          The sequence number field is a signed 32-bit integer.  It is
          used to detect old and duplicate link state advertisements.

Moy [Page 105] RFC 1583 OSPF Version 2 March 1994

          The space of sequence numbers is linearly ordered.  The
          larger the sequence number (when compared as signed 32-bit
          integers) the more recent the advertisement.  To describe to
          sequence number space more precisely, let N refer in the
          discussion below to the constant 2**31.
          The sequence number -N (0x80000000) is reserved (and
          unused).  This leaves -N + 1 (0x80000001) as the smallest
          (and therefore oldest) sequence number.  A router uses this
          sequence number the first time it originates any link state
          advertisement.  Afterwards, the advertisement's sequence
          number is incremented each time the router originates a new
          instance of the advertisement.  When an attempt is made to
          increment the sequence number past the maximum value of N -
          1 (0x7fffffff), the current instance of the advertisement
          must first be flushed from the routing domain.  This is done
          by prematurely aging the advertisement (see Section 14.1)
          and reflooding it.  As soon as this flood has been
          acknowledged by all adjacent neighbors, a new instance can
          be originated with sequence number of -N + 1 (0x80000001).
          The router may be forced to promote the sequence number of
          one of its advertisements when a more recent instance of the
          advertisement is unexpectedly received during the flooding
          process.  This should be a rare event.  This may indicate
          that an out-of-date advertisement, originated by the router
          itself before its last restart/reload, still exists in the
          Autonomous System.  For more information see Section 13.4.
      12.1.7.  LS checksum
          This field is the checksum of the complete contents of the
          advertisement, excepting the LS age field.  The LS age field
          is excepted so that an advertisement's age can be
          incremented without updating the checksum.  The checksum
          used is the same that is used for ISO connectionless
          datagrams; it is commonly referred to as the Fletcher
          checksum.  It is documented in Annex B of [RFC 905].  The
          link state advertisement header also contains the length of
          the advertisement in bytes; subtracting the size of the LS
          age field (two bytes) yields the amount of data to checksum.
          The checksum is used to detect data corruption of an
          advertisement.  This corruption can occur while an
          advertisement is being flooded, or while it is being held in
          a router's memory.  The LS checksum field cannot take on the
          value of zero; the occurrence of such a value should be

Moy [Page 106] RFC 1583 OSPF Version 2 March 1994

          considered a checksum failure.  In other words, calculation
          of the checksum is not optional.
          The checksum of a link state advertisement is verified in
          two cases: a) when it is received in a Link State Update
          Packet and b) at times during the aging of the link state
          database.  The detection of a checksum failure leads to
          separate actions in each case.  See Sections 13 and 14 for
          more details.
          Whenever the LS sequence number field indicates that two
          instances of an advertisement are the same, the LS checksum
          field is examined.  If there is a difference, the instance
          with the larger LS checksum is considered to be most
          recent.[12] See Section 13.1 for more details.
  12.2.  The link state database
      A router has a separate link state database for every area to
      which it belongs.  The link state database has been referred to
      elsewhere in the text as the topological database.  All routers
      belonging to the same area have identical topological databases
      for the area.
      The databases for each individual area are always dealt with
      separately.  The shortest path calculation is performed
      separately for each area (see Section 16).  Components of the
      area topological database are flooded throughout the area only.
      Finally, when an adjacency (belonging to Area A) is being
      brought up, only the database for Area A is synchronized between
      the two routers.
      The area database is composed of router links advertisements,
      network links advertisements, and summary link advertisements
      (all listed in the area data structure).  In addition, external
      routes (AS external advertisements) are included in all non-stub
      area databases (see Section 3.6).
      An implementation of OSPF must be able to access individual
      pieces of an area database.  This lookup function is based on an
      advertisement's LS type, Link State ID and Advertising
      Router.[13] There will be a single instance (the most up-to-
      date) of each link state advertisement in the database.  The
      database lookup function is invoked during the link state
      flooding procedure (Section 13) and the routing table
      calculation (Section 16).  In addition, using this lookup
      function the router can determine whether it has itself ever

Moy [Page 107] RFC 1583 OSPF Version 2 March 1994

      originated a particular link state advertisement, and if so,
      with what LS sequence number.
      A link state advertisement is added to a router's database when
      either a) it is received during the flooding process (Section
      13) or b) it is originated by the router itself (Section 12.4).
      A link state advertisement is deleted from a router's database
      when either a) it has been overwritten by a newer instance
      during the flooding process (Section 13) or b) the router
      originates a newer instance of one of its self-originated
      advertisements (Section 12.4) or c) the advertisement ages out
      and is flushed from the routing domain (Section 14).  Whenever a
      link state advertisement is deleted from the database it must
      also be removed from all neighbors' Link state retransmission
      lists (see Section 10).
  12.3.  Representation of TOS
      All OSPF link state advertisements (with the exception of
      network links advertisements) specify metrics.  In router links
      advertisements, the metrics indicate the costs of the described
      interfaces.  In summary link and AS external link
      advertisements, the metric indicates the cost of the described
      path.  In all of these advertisements, a separate metric can be
      specified for each IP TOS.  The encoding of TOS in OSPF link
      state advertisements is specified in Table 17. That table
      relates the OSPF encoding to the IP packet header's TOS field
      (defined in [RFC 1349]).  The OSPF encoding is expressed as a
      decimal integer, and the IP packet header's TOS field is
      expressed in the binary TOS values used in [RFC 1349].

Moy [Page 108] RFC 1583 OSPF Version 2 March 1994

                  OSPF encoding   RFC 1349 TOS values
                  ___________________________________________
                  0               0000 normal service
                  2               0001 minimize monetary cost
                  4               0010 maximize reliability
                  6               0011
                  8               0100 maximize throughput
                  10              0101
                  12              0110
                  14              0111
                  16              1000 minimize delay
                  18              1001
                  20              1010
                  22              1011
                  24              1100
                  26              1101
                  28              1110
                  30              1111
                      Table 17: Representing TOS in OSPF.
      Each OSPF link state advertisement must specify the TOS 0
      metric.  Other TOS metrics, if they appear, must appear in order
      of increasing TOS encoding.  For example, the TOS 8 (maximize
      throughput) metric must always appear before the TOS 16
      (minimize delay) metric when both are specified.  If a metric
      for some non-zero TOS is not specified, its cost defaults to the
      cost for TOS 0, unless the T-bit is reset in the advertisement's
      Options field (see Section 12.1.2 for more details).
  12.4.  Originating link state advertisements
      Into any given OSPF area, a router will originate several link
      state advertisements.  Each router originates a router links
      advertisement.  If the router is also the Designated Router for
      any of the area's networks, it will originate network links
      advertisements for those networks.
      Area border routers originate a single summary link
      advertisement for each known inter-area destination.  AS
      boundary routers originate a single AS external link
      advertisement for each known AS external destination.
      Destinations are advertised one at a time so that the change in
      any single route can be flooded without reflooding the entire

Moy [Page 109] RFC 1583 OSPF Version 2 March 1994

      collection of routes.  During the flooding procedure, many link
      state advertisements can be carried by a single Link State
      Update packet.
      As an example, consider Router RT4 in Figure 6.  It is an area
      border router, having a connection to Area 1 and the backbone.
      Router RT4 originates 5 distinct link state advertisements into
      the backbone (one router links, and one summary link for each of
      the networks N1-N4).  Router RT4 will also originate 8 distinct
      link state advertisements into Area 1 (one router links and
      seven summary link advertisements as pictured in Figure 7).  If
      RT4 has been selected as Designated Router for Network N3, it
      will also originate a network links advertisement for N3 into
      Area 1.
      In this same figure, Router RT5 will be originating 3 distinct
      AS external link advertisements (one for each of the networks
      N12-N14).  These will be flooded throughout the entire AS,
      assuming that none of the areas have been configured as stubs.
      However, if area 3 has been configured as a stub area, the
      external advertisements for networks N12-N14 will not be flooded
      into area 3 (see Section 3.6).  Instead, Router RT11 would
      originate a default summary link advertisement that would be
      flooded throughout area 3 (see Section 12.4.3).  This instructs
      all of area 3's internal routers to send their AS external
      traffic to RT11.
      Whenever a new instance of a link state advertisement is
      originated, its LS sequence number is incremented, its LS age is
      set to 0, its LS checksum is calculated, and the advertisement
      is added to the link state database and flooded out the
      appropriate interfaces.  See Section 13.2 for details concerning
      the installation of the advertisement into the link state
      database.  See Section 13.3 for details concerning the flooding
      of newly originated advertisements.
      The ten events that can cause a new instance of a link state
      advertisement to be originated are:
      (1) The LS age field of one of the router's self-originated
          advertisements reaches the value LSRefreshTime. In this
          case, a new instance of the link state advertisement is
          originated, even though the contents of the advertisement
          (apart from the link state advertisement header) will be the
          same.  This guarantees periodic originations of all link
          state advertisements. This periodic updating of link state

Moy [Page 110] RFC 1583 OSPF Version 2 March 1994

          advertisements adds robustness to the link state algorithm.
          Link state advertisements that solely describe unreachable
          destinations should not be refreshed, but should instead be
          flushed from the routing domain (see Section 14.1).
      When whatever is being described by a link state advertisement
      changes, a new advertisement is originated.  However, two
      instances of the same link state advertisement may not be
      originated within the time period MinLSInterval.  This may
      require that the generation of the next instance be delayed by
      up to MinLSInterval.  The following events may cause the
      contents of a link state advertisement to change.  These events
      should cause new originations if and only if the contents of the
      new advertisement would be different:
      (2) An interface's state changes (see Section 9.1).  This may
          mean that it is necessary to produce a new instance of the
          router links advertisement.
      (3) An attached network's Designated Router changes.  A new
          router links advertisement should be originated.  Also, if
          the router itself is now the Designated Router, a new
          network links advertisement should be produced.  If the
          router itself is no longer the Designated Router, any
          network links advertisement that it might have originated
          for the network should be flushed from the routing domain
          (see Section 14.1).
      (4) One of the neighboring routers changes to/from the FULL
          state.  This may mean that it is necessary to produce a new
          instance of the router links advertisement.  Also, if the
          router is itself the Designated Router for the attached
          network, a new network links advertisement should be
          produced.
      The next four events concern area border routers only:
      (5) An intra-area route has been added/deleted/modified in the
          routing table.  This may cause a new instance of a summary
          links advertisement (for this route) to be originated in
          each attached area (possibly including the backbone).
      (6) An inter-area route has been added/deleted/modified in the
          routing table.  This may cause a new instance of a summary

Moy [Page 111] RFC 1583 OSPF Version 2 March 1994

          links advertisement (for this route) to be originated in
          each attached area (but NEVER for the backbone).
      (7) The router becomes newly attached to an area.  The router
          must then originate summary link advertisements into the
          newly attached area for all pertinent intra-area and inter-
          area routes in the router's routing table.  See Section
          12.4.3 for more details.
      (8) When the state of one of the router's configured virtual
          links changes, it may be necessary to originate a new router
          links advertisement into the virtual link's transit area
          (see the discussion of the router links advertisement's bit
          V in Section 12.4.1), as well as originating a new router
          links advertisement into the backbone.
      The last two events concern AS boundary routers (and former AS
      boundary routers) only:
      (9) An external route gained through direct experience with an
          external routing protocol (like EGP) changes.  This will
          cause an AS boundary router to originate a new instance of
          an AS external link advertisement.
      (10)
          A router ceases to be an AS boundary router, perhaps after
          restarting. In this situation the router should flush all AS
          external link advertisements that it had previously
          originated.  These advertisements can be flushed via the
          premature aging procedure specified in Section 14.1.
      The construction of each type of link state advertisement is
      explained in detail below.  In general, these sections describe
      the contents of the advertisement body (i.e., the part coming
      after the 20-byte advertisement header).  For information
      concerning the building of the link state advertisement header,
      see Section 12.1.
      12.4.1.  Router links
          A router originates a router links advertisement for each
          area that it belongs to.  Such an advertisement describes
          the collected states of the router's links to the area.  The
          advertisement is flooded throughout the particular area, and
          no further.

Moy [Page 112] RFC 1583 OSPF Version 2 March 1994

                ....................................
                . 192.1.2                   Area 1 .
                .     +                            .
                .     |                            .
                .     | 3+---+1                    .
                .  N1 |--|RT1|-----+               .
                .     |  +---+                    .
                .     |                _______N3  .
                .     +               /          .  1+---+
                .                     * 192.1.1 *------|RT4|
                .     +               /_______/   .   +---+
                .     |              /     |       .
                .     | 3+---+1     /      |       .
                .  N2 |--|RT2|-----+      1|       .
                .     |  +---+           +---+8    .         6+---+
                .     |                  |RT3|----------------|RT6|
                .     +                  +---+     .          +---+
                . 192.1.3                  |2      .   18.10.0.6|7
                .                          |       .            |
                .                   +------------+ .
                .                     192.1.4 (N4) .
                ....................................
                  Figure 15: Area 1 with IP addresses shown
          The format of a router links advertisement is shown in
          Appendix A (Section A.4.2).  The first 20 bytes of the
          advertisement consist of the generic link state
          advertisement header that was discussed in Section 12.1.
          Router links advertisements have LS type = 1.  The router
          indicates whether it is willing to calculate separate routes
          for each IP TOS by setting (or resetting) the T-bit of the
          link state advertisement's Options field.
          A router also indicates whether it is an area border router,
          or an AS boundary router, by setting the appropriate bits
          (bit B and bit E, respectively) in its router links
          advertisements. This enables paths to those types of routers
          to be saved in the routing table, for later processing of
          summary link advertisements and AS external link
          advertisements.  Bit B should be set whenever the router is
          actively attached to two or more areas, even if the router
          is not currently attached to the OSPF backbone area.  Bit E
          should never be set in a router links advertisement for a
          stub area (stub areas cannot contain AS boundary routers).
          In addition, the router sets bit V in its router links

Moy [Page 113] RFC 1583 OSPF Version 2 March 1994

          advertisement for Area A if and only if it is the endpoint
          of an active virtual link using Area A as its Transit area.
          This enables the other routers attached to Area A to
          discover whether the area supports any virtual links (i.e.,
          is a transit area).
          The router links advertisement then describes the router's
          working connections (i.e., interfaces or links) to the area.
          Each link is typed according to the kind of attached
          network.  Each link is also labelled with its Link ID.  This
          Link ID gives a name to the entity that is on the other end
          of the link.  Table 18 summarizes the values used for the
          Type and Link ID fields.
                 Link type   Description       Link ID
                 __________________________________________________
                 1           Point-to-point    Neighbor Router ID
                             link
                 2           Link to transit   Interface address of
                             network           Designated Router
                 3           Link to stub      IP network number
                             network
                 4           Virtual link      Neighbor Router ID
                         Table 18: Link descriptions in the
                            router links advertisement.
          In addition, the Link Data field is specified for each link.
          This field gives 32 bits of extra information for the link.
          For links to transit networks, numbered links to routers and
          virtual links, this field specifies the IP interface address
          of the associated router interface (this is needed by the
          routing table calculation, see Section 16.1.1).  For links
          to stub networks, this field specifies the network's IP
          address mask.  For unnumbered point-to-point networks, the
          Link Data field should be set to the unnumbered interface's
          MIB-II [RFC 1213] ifIndex value.
          Finally, the cost of using the link for output (possibly
          specifying a different cost for each Type of Service) is
          specified.  The output cost of a link is configurable.  It
          must always be non-zero.
          To further describe the process of building the list of link

Moy [Page 114] RFC 1583 OSPF Version 2 March 1994

          descriptions, suppose a router wishes to build a router
          links advertisement for Area A.  The router examines its
          collection of interface data structures.  For each
          interface, the following steps are taken:
          o   If the attached network does not belong to Area A, no
              links are added to the advertisement, and the next
              interface should be examined.
          o   Else, if the state of the interface is Down, no links
              are added.
          o   Else, if the state of the interface is Point-to-Point,
              then add links according to the following:
  1. If the neighboring router is fully adjacent, add a

Type 1 link (point-to-point) if this is an interface

                  to a point-to-point network, or add a Type 4 link
                  (virtual link) if this is a virtual link.  The Link
                  ID should be set to the Router ID of the neighboring
                  router. For virtual links and numbered point-to-
                  point networks, the Link Data should specify the IP
                  interface address. For unnumbered point-to-point
                  networks, the Link Data field should specify the
                  interface's MIB-II [RFC 1213] ifIndex value.
  1. If this is a numbered point-to-point network (i.e,

not a virtual link and not an unnumbered point-to-

                  point network) and the neighboring router's IP
                  address is known, add a Type 3 link (stub network)
                  whose Link ID is the neighbor's IP address, whose
                  Link Data is the mask 0xffffffff indicating a host
                  route, and whose cost is the interface's configured
                  output cost.
          o   Else if the state of the interface is Loopback, add a
              Type 3 link (stub network) as long as this is not an
              interface to an unnumbered serial line.  The Link ID
              should be set to the IP interface address, the Link Data
              set to the mask 0xffffffff (indicating a host route),
              and the cost set to 0.
          o   Else if the state of the interface is Waiting, add a
              Type 3 link (stub network) whose Link ID is the IP
              network number of the attached network and whose Link
              Data is the attached network's address mask.

Moy [Page 115] RFC 1583 OSPF Version 2 March 1994

          o   Else, there has been a Designated Router selected for
              the attached network.  If the router is fully adjacent
              to the Designated Router, or if the router itself is
              Designated Router and is fully adjacent to at least one
              other router, add a single Type 2 link (transit network)
              whose Link ID is the IP interface address of the
              attached network's Designated Router (which may be the
              router itself) and whose Link Data is the router's own
              IP interface address.  Otherwise, add a link as if the
              interface state were Waiting (see above).
          Unless otherwise specified, the cost of each link generated
          by the above procedure is equal to the output cost of the
          associated interface.  Note that in the case of serial
          lines, multiple links may be generated by a single
          interface.
          After consideration of all the router interfaces, host links
          are added to the advertisement by examining the list of
          attached hosts.  A host route is represented as a Type 3
          link (stub network) whose Link ID is the host's IP address
          and whose Link Data is the mask of all ones (0xffffffff).
          As an example, consider the router links advertisements
          generated by Router RT3, as pictured in Figure 6.  The area
          containing Router RT3 (Area 1) has been redrawn, with actual
          network addresses, in Figure 15.  Assume that the last byte
          of all of RT3's interface addresses is 3, giving it the
          interface addresses 192.1.1.3 and 192.1.4.3, and that the
          other routers have similar addressing schemes.  In addition,
          assume that all links are functional, and that Router IDs
          are assigned as the smallest IP interface address.
          RT3 originates two router links advertisements, one for Area
          1 and one for the backbone.  Assume that Router RT4 has been
          selected as the Designated router for network 192.1.1.0.
          RT3's router links advertisement for Area 1 is then shown
          below.  It indicates that RT3 has two connections to Area 1,
          the first a link to the transit network 192.1.1.0 and the
          second a link to the stub network 192.1.4.0.  Note that the
          transit network is identified by the IP interface of its
          Designated Router (i.e., the Link ID = 192.1.1.4 which is
          the Designated Router RT4's IP interface to 192.1.1.0).
          Note also that RT3 has indicated that it is capable of
          calculating separate routes based on IP TOS, through setting
          the T-bit in the Options field.  It has also indicated that
          it is an area border router.

Moy [Page 116] RFC 1583 OSPF Version 2 March 1994

            ; RT3's router links advertisement for Area 1
            LS age = 0                     ;always true on origination
            Options = (T-bit|E-bit)        ;TOS-capable
            LS type = 1                    ;indicates router links
            Link State ID = 192.1.1.3      ;RT3's Router ID
            Advertising Router = 192.1.1.3 ;RT3's Router ID
            bit E = 0                      ;not an AS boundary router
            bit B = 1                      ;area border router
            #links = 2
                   Link ID = 192.1.1.4     ;IP address of Desig. Rtr.
                   Link Data = 192.1.1.3   ;RT3's IP interface to net
                   Type = 2                ;connects to transit network
                   # other metrics = 0
                   TOS 0 metric = 1
                   Link ID = 192.1.4.0     ;IP Network number
                   Link Data = 0xffffff00  ;Network mask
                   Type = 3                ;connects to stub network
                   # other metrics = 0
                   TOS 0 metric = 2
          Next RT3's router links advertisement for the backbone is
          shown.  It indicates that RT3 has a single attachment to the
          backbone.  This attachment is via an unnumbered point-to-
          point link to Router RT6.  RT3 has again indicated that it
          is TOS-capable, and that it is an area border router.
            ; RT3's router links advertisement for the backbone
            LS age = 0                     ;always true on origination
            Options = (T-bit|E-bit)        ;TOS-capable
            LS type = 1                    ;indicates router links
            Link State ID = 192.1.1.3      ;RT3's router ID
            Advertising Router = 192.1.1.3 ;RT3's router ID
            bit E = 0                      ;not an AS boundary router
            bit B = 1                      ;area border router
            #links = 1
                   Link ID = 18.10.0.6     ;Neighbor's Router ID
                   Link Data = 0.0.0.3     ;MIB-II ifIndex of P-P link
                   Type = 1                ;connects to router
                   # other metrics = 0
                   TOS 0 metric = 8
          Even though Router RT3 has indicated that it is TOS-capable
          in the above examples, only a single metric (the TOS 0
          metric) has been specified for each interface.  Different
          metrics can be specified for each TOS.  The encoding of TOS

Moy [Page 117] RFC 1583 OSPF Version 2 March 1994

          in OSPF link state advertisements is described in Section
          12.3.
          As an example, suppose the point-to-point link between
          Routers RT3 and RT6 in Figure 15 is a satellite link.  The
          AS administrator may want to encourage the use of the line
          for high bandwidth traffic.  This would be done by setting
          the metric artificially low for the appropriate TOS value.
          Router RT3 would then originate the following router links
          advertisement for the backbone (TOS 8 = maximize
          throughput):
            ; RT3's router links advertisement for the backbone
            LS age = 0                  ;always true on origination
            Options = (T-bit|E-bit)     ;TOS-capable
            LS type = 1                 ;indicates router links
            Link State ID = 192.1.1.3   ;RT3's Router ID
            Advertising Router = 192.1.1.3
            bit E = 0                   ;not an AS boundary router
            bit B = 1                   ;area border router
            #links = 1
                   Link ID = 18.10.0.6  ;Neighbor's Router ID
                   Link Data = 0.0.0.3  ;MIB-II ifIndex of P-P link
                   Type = 1             ;connects to router
                   # other metrics = 1
                   TOS 0 metric = 8
                           TOS = 8      ;maximize throughput
                           metric = 1   ;traffic preferred
      12.4.2.  Network links
          A network links advertisement is generated for every transit
          multi-access network.  (A transit network is a network
          having two or more attached routers).  The network links
          advertisement describes all the routers that are attached to
          the network.
          The Designated Router for the network originates the
          advertisement.  The Designated Router originates the
          advertisement only if it is fully adjacent to at least one
          other router on the network.  The network links
          advertisement is flooded throughout the area that contains
          the transit network, and no further.  The networks links
          advertisement lists those routers that are fully adjacent to
          the Designated Router; each fully adjacent router is
          identified by its OSPF Router ID.  The Designated Router

Moy [Page 118] RFC 1583 OSPF Version 2 March 1994

          includes itself in this list.
          The Link State ID for a network links advertisement is the
          IP interface address of the Designated Router.  This value,
          masked by the network's address mask (which is also
          contained in the network links advertisement) yields the
          network's IP address.
          A router that has formerly been the Designated Router for a
          network, but is no longer, should flush the network links
          advertisement that it had previously originated.  This
          advertisement is no longer used in the routing table
          calculation.  It is flushed by prematurely incrementing the
          advertisement's age to MaxAge and reflooding (see Section
          14.1). In addition, in those rare cases where a router's
          Router ID has changed, any network links advertisements that
          were originated with the router's previous Router ID must be
          flushed. Since the router may have no idea what it's
          previous Router ID might have been, these network links
          advertisements are indicated by having their Link State ID
          equal to one of the router's IP interface addresses and
          their Advertising Router not equal to the router's current
          Router ID (see Section 13.4 for more details).
          As an example of a network links advertisement, again
          consider the area configuration in Figure 6.  Network links
          advertisements are originated for Network N3 in Area 1,
          Networks N6 and N8 in Area 2, and Network N9 in Area 3.
          Assuming that Router RT4 has been selected as the Designated
          Router for Network N3, the following network links
          advertisement is generated by RT4 on behalf of Network N3
          (see Figure 15 for the address assignments):
            ; network links advertisement for Network N3
            LS age = 0                     ;always true on origination
            Options = (T-bit|E-bit)        ;TOS-capable
            LS type = 2                    ;indicates network links
            Link State ID = 192.1.1.4      ;IP address of Desig. Rtr.
            Advertising Router = 192.1.1.4 ;RT4's Router ID
            Network Mask = 0xffffff00
                   Attached Router = 192.1.1.4    ;Router ID
                   Attached Router = 192.1.1.1    ;Router ID
                   Attached Router = 192.1.1.2    ;Router ID
                   Attached Router = 192.1.1.3    ;Router ID

Moy [Page 119] RFC 1583 OSPF Version 2 March 1994

      12.4.3.  Summary links
          Each summary link advertisement describes a route to a
          single destination.  Summary link advertisements are flooded
          throughout a single area only.  The destination described is
          one that is external to the area, yet still belonging to the
          Autonomous System.
          Summary link advertisements are originated by area border
          routers.  The precise summary routes to advertise into an
          area are determined by examining the routing table structure
          (see Section 11) in accordance with the algorithm described
          below. Note that only intra-area routes are advertised into
          the backbone, while both intra-area and inter-area routes
          are advertised into the other areas.
          To determine which routes to advertise into an attached Area
          A, each routing table entry is processed as follows.
          Remember that each routing table entry describes a set of
          equal-cost best paths to a particular destination:
          o   Only Destination Types of network and AS boundary router
              are advertised in summary link advertisements.  If the
              routing table entry's Destination Type is area border
              router, examine the next routing table entry.
          o   AS external routes are never advertised in summary link
              advertisements.  If the routing table entry has Path-
              type of type 1 external or type 2 external, examine the
              next routing table entry.
          o   Else, if the area associated with this set of paths is
              the Area A itself, do not generate a summary link
              advertisement for the route.[14]
          o   Else, if the next hops associated with this set of paths
              belong to Area A itself, do not generate a summary link
              advertisement for the route.[15] This is the logical
              equivalent of a Distance Vector protocol's split horizon
              logic.
          o   Else, if the routing table cost equals or exceeds the
              value LSInfinity, a summary link advertisement cannot be
              generated for this route.
          o   Else, if the destination of this route is an AS boundary
              router, generate a Type 4 link state advertisement for

Moy [Page 120] RFC 1583 OSPF Version 2 March 1994

              the destination, with Link State ID equal to the AS
              boundary router's Router ID and metric equal to the
              routing table entry's cost.  These advertisements should
              not be generated if Area A has been configured as a stub
              area.
          o   Else, the Destination type is network. If this is an
              inter-area route, generate a Type 3 advertisement for
              the destination, with Link State ID equal to the
              network's address (if necessary, the Link State ID can
              also have one or more of the network's host bits set;
              see Appendix F for details) and metric equal to the
              routing table cost.
          o   The one remaining case is an intra-area route to a
              network.  This means that the network is contained in
              one of the router's directly attached areas.  In
              general, this information must be condensed before
              appearing in summary link advertisements.  Remember that
              an area has been defined as a list of address ranges,
              each range consisting of an [address,mask] pair and a
              status indication of either Advertise or DoNotAdvertise.
              At most a single Type 3 advertisement is made for each
              range. When the range's status indicates Advertise, a
              Type 3 advertisement is generated with Link State ID
              equal to the range's address (if necessary, the Link
              State ID can also have one or more of the range's "host"
              bits set; see Appendix F for details) and cost equal to
              the smallest cost of any of the component networks. When
              the range's status indicates DoNotAdvertise, the Type 3
              advertisement is suppressed and the component networks
              remain hidden from other areas.
              By default, if a network is not contained in any
              explicitly configured address range, a Type 3
              advertisement is generated with Link State ID equal to
              the network's address (if necessary, the Link State ID
              can also have one or more of the network's "host" bits
              set; see Appendix F for details) and metric equal to the
              network's routing table cost.
              If virtual links are being used to provide/increase
              connectivity of the backbone, routing information
              concerning the backbone networks should not be condensed
              before being summarized into the virtual links' Transit
              areas. Nor should the advertisement of backbone networks
              into Transit areas be suppressed.  In other words, the
              backbone's configured ranges should be ignored when

Moy [Page 121] RFC 1583 OSPF Version 2 March 1994

              originating summary links into Transit areas.  The
              existence of virtual links is determined during the
              shortest path calculation for the Transit areas (see
              Section 16.1).
          If a router advertises a summary advertisement for a
          destination which then becomes unreachable, the router must
          then flush the advertisement from the routing domain by
          setting its age to MaxAge and reflooding (see Section 14.1).
          Also, if the destination is still reachable, yet can no
          longer be advertised according to the above procedure (e.g.,
          it is now an inter-area route, when it used to be an intra-
          area route associated with some non-backbone area; it would
          thus no longer be advertisable to the backbone), the
          advertisement should also be flushed from the routing
          domain.
          For an example of summary link advertisements, consider
          again the area configuration in Figure 6.  Routers RT3, RT4,
          RT7, RT10 and RT11 are all area border routers, and
          therefore are originating summary link advertisements.
          Consider in particular Router RT4.  Its routing table was
          calculated as the example in Section 11.3.  RT4 originates
          summary link advertisements into both the backbone and Area
          1.  Into the backbone, Router RT4 originates separate
          advertisements for each of the networks N1-N4.  Into Area 1,
          Router RT4 originates separate advertisements for networks
          N6-N8 and the AS boundary routers RT5,RT7.  It also
          condenses host routes Ia and Ib into a single summary link
          advertisement.  Finally, the routes to networks N9,N10,N11
          and Host H1 are advertised by a single summary link
          advertisement.  This condensation was originally performed
          by the router RT11.
          These advertisements are illustrated graphically in Figures
          7 and 8.  Two of the summary link advertisements originated
          by Router RT4 follow.  The actual IP addresses for the
          networks and routers in question have been assigned in
          Figure 15.
            ; summary link advertisement for Network N1,
            ; originated by Router RT4 into the backbone
            LS age = 0                  ;always true on origination
            Options = (T-bit|E-bit)     ;TOS-capable
            LS type = 3                 ;summary link to IP net
            Link State ID = 192.1.2.0   ;N1's IP network number
            Advertising Router = 192.1.1.4       ;RT4's ID

Moy [Page 122] RFC 1583 OSPF Version 2 March 1994

                   TOS = 0
                   metric = 4
            ; summary link advertisement for AS boundary router RT7
            ; originated by Router RT4 into Area 1
            LS age = 0                  ;always true on origination
            Options = (T-bit|E-bit)     ;TOS-capable
            LS type = 4                 ;summary link to ASBR
            Link State ID = Router RT7's ID
            Advertising Router = 192.1.1.4       ;RT4's ID
                   TOS = 0
                   metric = 14
          Summary link advertisements pertain to a single destination
          (IP network or AS boundary router).  However, for a single
          destination there may be separate sets of paths, and
          therefore separate routing table entries, for each Type of
          Service.  All these entries must be considered when building
          the summary link advertisement for the destination; a single
          advertisement must specify the separate costs (if they
          exist) for each TOS.  The encoding of TOS in OSPF link state
          advertisements is described in Section 12.3.
          Clearing the T-bit in the Options field of a summary link
          advertisement indicates that there is a TOS 0 path to the
          destination, but no paths for non-zero TOS.  This can happen
          when non-TOS-capable routers exist in the routing domain
          (see Section 2.4).
      12.4.4.  Originating summary links into stub areas
          The algorithm in Section 12.4.3 is optional when Area A is
          an OSPF stub area. Area border routers connecting to a stub
          area can originate summary link advertisements into the area
          according to the above Section's algorithm, or can choose to
          originate only a subset of the advertisements, possibly
          under configuration control.  The fewer advertisements
          originated, the smaller the stub area's link state database,
          further reducing the demands on its routers' resources.
          However, omitting advertisements may also lead to sub-
          optimal inter-area routing, although routing will continue
          to function.
          As specified in Section 12.4.3, Type 4 link state
          advertisements (ASBR summary links) are never originated
          into stub areas.

Moy [Page 123] RFC 1583 OSPF Version 2 March 1994

          In a stub area, instead of importing external routes each
          area border router originates a "default summary link" into
          the area. The Link State ID for the default summary link is
          set to DefaultDestination, and the metric set to the (per-
          area) configurable parameter StubDefaultCost.  Note that
          StubDefaultCost need not be configured identically in all of
          the stub area's area border routers.
      12.4.5.  AS external links
          AS external link advertisements describe routes to
          destinations external to the Autonomous System.  Most AS
          external link advertisements describe routes to specific
          external destinations; in these cases the advertisement's
          Link State ID is set to the destination network's IP address
          (if necessary, the Link State ID can also have one or more
          of the network's "host" bits set; see Appendix F for
          details).  However, a default route for the Autonomous
          System can be described in an AS external link advertisement
          by setting the advertisement's Link State ID to
          DefaultDestination (0.0.0.0).  AS external link
          advertisements are originated by AS boundary routers.  An AS
          boundary router originates a single AS external link
          advertisement for each external route that it has learned,
          either through another routing protocol (such as EGP), or
          through configuration information.
          In general, AS external link advertisements are the only
          type of link state advertisements that are flooded
          throughout the entire Autonomous System; all other types of
          link state advertisements are specific to a single area.
          However, AS external link advertisements are not flooded
          into/throughout stub areas (see Section 3.6).  This enables
          a reduction in link state database size for routers internal
          to stub areas.
          The metric that is advertised for an external route can be
          one of two types.  Type 1 metrics are comparable to the link
          state metric.  Type 2 metrics are assumed to be larger than
          the cost of any intra-AS path.  As with summary link
          advertisements, if separate paths exist based on TOS,
          separate TOS costs can be included in the AS external link
          advertisement.  The encoding of TOS in OSPF link state
          advertisements is described in Section 12.3.  If the T-bit
          of the advertisement's Options field is clear, no non-zero
          TOS paths to the destination exist.
          If a router advertises an AS external link advertisement for

Moy [Page 124] RFC 1583 OSPF Version 2 March 1994

          a destination which then becomes unreachable, the router
          must then flush the advertisement from the routing domain by
          setting its age to MaxAge and reflooding (see Section 14.1).
          For an example of AS external link advertisements, consider
          once again the AS pictured in Figure 6.  There are two AS
          boundary routers: RT5 and RT7.  Router RT5 originates three
          external link advertisements, for networks N12-N14.  Router
          RT7 originates two external link advertisements, for
          networks N12 and N15.  Assume that RT7 has learned its route
          to N12 via EGP, and that it wishes to advertise a Type 2
          metric to the AS.  RT7 would then originate the following
          advertisement for N12:
            ; AS external link advertisement for Network N12,
            ; originated by Router RT7
            LS age = 0                  ;always true on origination
            Options = (T-bit|E-bit)     ;TOS-capable
            LS type = 5                 ;indicates AS external link
            Link State ID = N12's IP network number
            Advertising Router = Router RT7's ID
                   bit E = 1            ;Type 2 metric
                   TOS = 0
                   metric = 2
                   Forwarding address = 0.0.0.0
          In the above example, the forwarding address field has been
          set to 0.0.0.0, indicating that packets for the external
          destination should be forwarded to the advertising OSPF
          router (RT7).  This is not always desirable.  Consider the
          example pictured in Figure 16.  There are three OSPF routers
          (RTA, RTB and RTC) connected to a common network.  Only one
          of these routers, RTA, is exchanging EGP information with
          the non-OSPF router RTX.  RTA must then originate AS
          external link advertisements for those destinations it has
          learned from RTX.  By using the AS external link
          advertisement's forwarding address field, RTA can specify
          that packets for these destinations be forwarded directly to
          RTX.  Without this feature, Routers RTB and RTC would take
          an extra hop to get to these destinations.
          Note that when the forwarding address field is non-zero, it
          should point to a router belonging to another Autonomous
          System.
          A forwarding address can also be specified for the default
          route.  For example, in figure 16 RTA may want to specify

Moy [Page 125] RFC 1583 OSPF Version 2 March 1994

          that all externally-destined packets should by default be
          forwarded to its EGP peer RTX.  The resulting AS external
          link advertisement is pictured below.  Note that the Link
          State ID is set to DefaultDestination.
            ; Default route, originated by Router RTA
            ; Packets forwarded through RTX
            LS age = 0                  ;always true on origination
            Options = (T-bit|E-bit)          ;TOS-capable
            LS type = 5                 ;indicates AS external link
            Link State ID = DefaultDestination  ; default route
            Advertising Router = Router RTA's ID
                   bit E = 1            ;Type 2 metric
                   TOS = 0
                   metric = 1
                   Forwarding address = RTX's IP address
          In figure 16, suppose instead that both RTA and RTB exchange
          EGP information with RTX.  In this case, RTA and RTB would
          originate the same set of AS external link advertisements.
          These advertisements, if they specify the same metric, would
          be functionally equivalent since they would specify the same
          destination and forwarding address (RTX).  This leads to a
          clear duplication of effort.  If only one of RTA or RTB
          originated the set of external advertisements, the routing
          would remain the same, and the size of the link state
          database would decrease.  However, it must be unambiguously
          defined as to which router originates the advertisements
          (otherwise neither may, or the identity of the originator
          may oscillate).  The following rule is thereby established:
          if two routers, both reachable from one another, originate
          functionally equivalent AS external advertisements (i.e.,
          same destination, cost and non-zero forwarding address),
          then the advertisement originated by the router having the
          highest OSPF Router ID is used.  The router having the lower
          OSPF Router ID can then flush its advertisement.  Flushing a
          link state advertisement is discussed in Section 14.1.

13. The Flooding Procedure

  Link State Update packets provide the mechanism for flooding link
  state advertisements.  A Link State Update packet may contain
  several distinct advertisements, and floods each advertisement one
  hop further from its point of origination.  To make the flooding
  procedure reliable, each advertisement must be acknowledged
  separately.  Acknowledgments are transmitted in Link State
  Acknowledgment packets.  Many separate acknowledgments can also be

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                              +
                              |
                    +---+.....|.EGP
                    |RTA|-----|.....+---+
                    +---+     |-----|RTX|
                              |     +---+
                    +---+     |
                    |RTB|-----|
                    +---+     |
                              |
                    +---+     |
                    |RTC|-----|
                    +---+     |
                              |
                              +
             Figure 16: Forwarding address example
  grouped together into a single packet.
  The flooding procedure starts when a Link State Update packet has
  been received.  Many consistency checks have been made on the
  received packet before being handed to the flooding procedure (see
  Section 8.2).  In particular, the Link State Update packet has been
  associated with a particular neighbor, and a particular area.  If
  the neighbor is in a lesser state than Exchange, the packet should
  be dropped without further processing.
  All types of link state advertisements, other than AS external link
  advertisements, are associated with a specific area.  However, link
  state advertisements do not contain an area field.  A link state
  advertisement's area must be deduced from the Link State Update
  packet header.
  For each link state advertisement contained in the packet, the
  following steps are taken:
  (1) Validate the advertisement's LS checksum.  If the checksum turns
      out to be invalid, discard the advertisement and get the next
      one from the Link State Update packet.
  (2) Examine the link state advertisement's LS type.  If the LS type
      is unknown, discard the advertisement and get the next one from
      the Link State Update Packet.  This specification defines LS
      types 1-5 (see Section 4.3).

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  (3) Else if this is a AS external link advertisement (LS type = 5),
      and the area has been configured as a stub area, discard the
      advertisement and get the next one from the Link State Update
      Packet.  AS external link advertisements are not flooded
      into/throughout stub areas (see Section 3.6).
  (4) Else if the advertisement's LS age is equal to MaxAge, and there
      is currently no instance of the advertisement in the router's
      link state database, then take the following actions:
      (a) Acknowledge the receipt of the advertisement by sending a
          Link State Acknowledgment packet back to the sending
          neighbor (see Section 13.5).
      (b) Purge all outstanding requests for equal or previous
          instances of the advertisement from the sending neighbor's
          Link State Request list (see Section 10).
      (c) If the sending neighbor is in state Exchange or in state
          Loading, then install the MaxAge advertisement in the link
          state database.  Otherwise, simply discard the
          advertisement.  In either case, examine the next
          advertisement (if any) listed in the Link State Update
          packet.
  (5) Otherwise, find the instance of this advertisement that is
      currently contained in the router's link state database.  If
      there is no database copy, or the received advertisement is more
      recent than the database copy (see Section 13.1 below for the
      determination of which advertisement is more recent) the
      following steps must be performed:
      (a) If there is already a database copy, and if the database
          copy was installed less than MinLSInterval seconds ago,
          discard the new advertisement (without acknowledging it) and
          examine the next advertisement (if any) listed in the Link
          State Update packet.
      (b) Otherwise immediately flood the new advertisement out some
          subset of the router's interfaces (see Section 13.3).  In
          some cases (e.g., the state of the receiving interface is DR
          and the advertisement was received from a router other than
          the Backup DR) the advertisement will be flooded back out
          the receiving interface.  This occurrence should be noted
          for later use by the acknowledgment process (Section 13.5).
      (c) Remove the current database copy from all neighbors' Link
          state retransmission lists.

Moy [Page 128] RFC 1583 OSPF Version 2 March 1994

      (d) Install the new advertisement in the link state database
          (replacing the current database copy).  This may cause the
          routing table calculation to be scheduled.  In addition,
          timestamp the new advertisement with the current time (i.e.,
          the time it was received).  The flooding procedure cannot
          overwrite the newly installed advertisement until
          MinLSInterval seconds have elapsed.  The advertisement
          installation process is discussed further in Section 13.2.
      (e) Possibly acknowledge the receipt of the advertisement by
          sending a Link State Acknowledgment packet back out the
          receiving interface.  This is explained below in Section
          13.5.
      (f) If this new link state advertisement indicates that it was
          originated by the receiving router itself (i.e., is
          considered a self-originated advertisement), the router must
          take special action, either updating the advertisement or in
          some cases flushing it from the routing domain. For a
          description of how self-originated advertisements are
          detected and subsequently handled, see Section 13.4.
  (6) Else, if there is an instance of the advertisement on the
      sending neighbor's Link state request list, an error has
      occurred in the Database Exchange process.  In this case,
      restart the Database Exchange process by generating the neighbor
      event BadLSReq for the sending neighbor and stop processing the
      Link State Update packet.
  (7) Else, if the received advertisement is the same instance as the
      database copy (i.e., neither one is more recent) the following
      two steps should be performed:
      (a) If the advertisement is listed in the Link state
          retransmission list for the receiving adjacency, the router
          itself is expecting an acknowledgment for this
          advertisement.  The router should treat the received
          advertisement as an acknowledgment, by removing the
          advertisement from the Link state retransmission list.  This
          is termed an "implied acknowledgment".  Its occurrence
          should be noted for later use by the acknowledgment process
          (Section 13.5).
      (b) Possibly acknowledge the receipt of the advertisement by
          sending a Link State Acknowledgment packet back out the
          receiving interface.  This is explained below in Section
          13.5.

Moy [Page 129] RFC 1583 OSPF Version 2 March 1994

  (8) Else, the database copy is more recent.  Note an unusual event
      to network management, discard the advertisement and process the
      next link state advertisement contained in the Link State Update
      packet.
  13.1.  Determining which link state is newer
      When a router encounters two instances of a link state
      advertisement, it must determine which is more recent.  This
      occurred above when comparing a received advertisement to its
      database copy.  This comparison must also be done during the
      Database Exchange procedure which occurs during adjacency
      bring-up.
      A link state advertisement is identified by its LS type, Link
      State ID and Advertising Router.  For two instances of the same
      advertisement, the LS sequence number, LS age, and LS checksum
      fields are used to determine which instance is more recent:
      o   The advertisement having the newer LS sequence number is
          more recent.  See Section 12.1.6 for an explanation of the
          LS sequence number space.  If both instances have the same
          LS sequence number, then:
      o   If the two instances have different LS checksums, then the
          instance having the larger LS checksum (when considered as a
          16-bit unsigned integer) is considered more recent.
      o   Else, if only one of the instances has its LS age field set
          to MaxAge, the instance of age MaxAge is considered to be
          more recent.
      o   Else, if the LS age fields of the two instances differ by
          more than MaxAgeDiff, the instance having the smaller
          (younger) LS age is considered to be more recent.
      o   Else, the two instances are considered to be identical.
  13.2.  Installing link state advertisements in the database
      Installing a new link state advertisement in the database,
      either as the result of flooding or a newly self-originated
      advertisement, may cause the OSPF routing table structure to be
      recalculated.  The contents of the new advertisement should be
      compared to the old instance, if present.  If there is no

Moy [Page 130] RFC 1583 OSPF Version 2 March 1994

      difference, there is no need to recalculate the routing table.
      (Note that even if the contents are the same, the LS checksum
      will probably be different, since the checksum covers the LS
      sequence number.)
      If the contents are different, the following pieces of the
      routing table must be recalculated, depending on the new
      advertisement's LS type field:
      Router links and network links advertisements
          The entire routing table must be recalculated, starting with
          the shortest path calculations for each area (not just the
          area whose topological database has changed).  The reason
          that the shortest path calculation cannot be restricted to
          the single changed area has to do with the fact that AS
          boundary routers may belong to multiple areas.  A change in
          the area currently providing the best route may force the
          router to use an intra-area route provided by a different
          area.[16]
      Summary link advertisements
          The best route to the destination described by the summary
          link advertisement must be recalculated (see Section 16.5).
          If this destination is an AS boundary router, it may also be
          necessary to re-examine all the AS external link
          advertisements.
      AS external link advertisements
          The best route to the destination described by the AS
          external link advertisement must be recalculated (see
          Section 16.6).
      Also, any old instance of the advertisement must be removed from
      the database when the new advertisement is installed.  This old
      instance must also be removed from all neighbors' Link state
      retransmission lists (see Section 10).
  13.3.  Next step in the flooding procedure
      When a new (and more recent) advertisement has been received, it
      must be flooded out some set of the router's interfaces.  This
      section describes the second part of flooding procedure (the
      first part being the processing that occurred in Section 13),
      namely, selecting the outgoing interfaces and adding the
      advertisement to the appropriate neighbors' Link state

Moy [Page 131] RFC 1583 OSPF Version 2 March 1994

      retransmission lists.  Also included in this part of the
      flooding procedure is the maintenance of the neighbors' Link
      state request lists.
      This section is equally applicable to the flooding of an
      advertisement that the router itself has just originated (see
      Section 12.4).  For these advertisements, this section provides
      the entirety of the flooding procedure (i.e., the processing of
      Section 13 is not performed, since, for example, the
      advertisement has not been received from a neighbor and
      therefore does not need to be acknowledged).
      Depending upon the advertisement's LS type, the advertisement
      can be flooded out only certain interfaces.  These interfaces,
      defined by the following, are called the eligible interfaces:
      AS external link advertisements (LS Type = 5)
          AS external link advertisements are flooded throughout the
          entire AS, with the exception of stub areas (see Section
          3.6).  The eligible interfaces are all the router's
          interfaces, excluding virtual links and those interfaces
          attaching to stub areas.
      All other LS types
          All other types are specific to a single area (Area A).  The
          eligible interfaces are all those interfaces attaching to
          the Area A.  If Area A is the backbone, this includes all
          the virtual links.
      Link state databases must remain synchronized over all
      adjacencies associated with the above eligible interfaces.  This
      is accomplished by executing the following steps on each
      eligible interface.  It should be noted that this procedure may
      decide not to flood a link state advertisement out a particular
      interface, if there is a high probability that the attached
      neighbors have already received the advertisement.  However, in
      these cases the flooding procedure must be absolutely sure that
      the neighbors eventually do receive the advertisement, so the
      advertisement is still added to each adjacency's Link state
      retransmission list.  For each eligible interface:
      (1) Each of the neighbors attached to this interface are
          examined, to determine whether they must receive the new
          advertisement.  The following steps are executed for each
          neighbor:

Moy [Page 132] RFC 1583 OSPF Version 2 March 1994

          (a) If the neighbor is in a lesser state than Exchange, it
              does not participate in flooding, and the next neighbor
              should be examined.
          (b) Else, if the adjacency is not yet full (neighbor state
              is Exchange or Loading), examine the Link state request
              list associated with this adjacency.  If there is an
              instance of the new advertisement on the list, it
              indicates that the neighboring router has an instance of
              the advertisement already.  Compare the new
              advertisement to the neighbor's copy:
              o   If the new advertisement is less recent, then
                  examine the next neighbor.
              o   If the two copies are the same instance, then delete
                  the advertisement from the Link state request list,
                  and examine the next neighbor.[17]
              o   Else, the new advertisement is more recent.  Delete
                  the advertisement from the Link state request list.
          (c) If the new advertisement was received from this
              neighbor, examine the next neighbor.
          (d) At this point we are not positive that the neighbor has
              an up-to-date instance of this new advertisement.  Add
              the new advertisement to the Link state retransmission
              list for the adjacency.  This ensures that the flooding
              procedure is reliable; the advertisement will be
              retransmitted at intervals until an acknowledgment is
              seen from the neighbor.
      (2) The router must now decide whether to flood the new link
          state advertisement out this interface.  If in the previous
          step, the link state advertisement was NOT added to any of
          the Link state retransmission lists, there is no need to
          flood the advertisement out the interface and the next
          interface should be examined.
      (3) If the new advertisement was received on this interface, and
          it was received from either the Designated Router or the
          Backup Designated Router, chances are that all the neighbors
          have received the advertisement already.  Therefore, examine
          the next interface.
      (4) If the new advertisement was received on this interface, and
          the interface state is Backup (i.e., the router itself is

Moy [Page 133] RFC 1583 OSPF Version 2 March 1994

          the Backup Designated Router), examine the next interface.
          The Designated Router will do the flooding on this
          interface.  If the Designated Router fails, this router will
          end up retransmitting the updates.
      (5) If this step is reached, the advertisement must be flooded
          out the interface.  Send a Link State Update packet (with
          the new advertisement as contents) out the interface.  The
          advertisement's LS age must be incremented by InfTransDelay
          (which must be > 0) when copied into the outgoing Link State
          Update packet (until the LS age field reaches its maximum
          value of MaxAge).
          On broadcast networks, the Link State Update packets are
          multicast.  The destination IP address specified for the
          Link State Update Packet depends on the state of the
          interface.  If the interface state is DR or Backup, the
          address AllSPFRouters should be used.  Otherwise, the
          address AllDRouters should be used.
          On non-broadcast, multi-access networks, separate Link State
          Update packets must be sent, as unicasts, to each adjacent
          neighbor (i.e., those in state Exchange or greater).  The
          destination IP addresses for these packets are the
          neighbors' IP addresses.
  13.4.  Receiving self-originated link state
      It is a common occurrence for a router to receive self-
      originated link state advertisements via the flooding procedure.
      A self-originated advertisement is detected when either 1) the
      advertisement's Advertising Router is equal to the router's own
      Router ID or 2) the advertisement is a network links
      advertisement and its Link State ID is equal to one of the
      router's own IP interface addresses.
      However, if the received self-originated advertisement is newer
      than the last instance that the router actually originated, the
      router must take special action.  The reception of such an
      advertisement indicates that there are link state advertisements
      in the routing domain that were originated before the last time
      the router was restarted. In most cases, the router must then
      advance the advertisement's LS sequence number one past the
      received LS sequence number, and originate a new instance of the
      advertisement.
      It may be the case the router no longer wishes to originate the

Moy [Page 134] RFC 1583 OSPF Version 2 March 1994

      received advertisement. Possible examples include: 1) the
      advertisement is a summary link or AS external link and the
      router no longer has an (advertisable) route to the destination,
      2) the advertisement is a network links advertisement but the
      router is no longer Designated Router for the network or 3) the
      advertisement is a network links advertisement whose Link State
      ID is one of the router's own IP interface addresses but whose
      Advertising Router is not equal to the router's own Router ID
      (this latter case should be rare, and it indicates that the
      router's Router ID has changed since originating the
      advertisement).  In all these cases, instead of updating the
      advertisement, the advertisement should be flushed from the
      routing domain by incrementing the received advertisement's LS
      age to MaxAge and reflooding (see Section 14.1).
  13.5.  Sending Link State Acknowledgment packets
      Each newly received link state advertisement must be
      acknowledged.  This is usually done by sending Link State
      Acknowledgment packets.  However, acknowledgments can also be
      accomplished implicitly by sending Link State Update packets
      (see step 7a of Section 13).
      Many acknowledgments may be grouped together into a single Link
      State Acknowledgment packet.  Such a packet is sent back out the
      interface that has received the advertisements.  The packet can
      be sent in one of two ways: delayed and sent on an interval
      timer, or sent directly (as a unicast) to a particular neighbor.
      The particular acknowledgment strategy used depends on the
      circumstances surrounding the receipt of the advertisement.
      Sending delayed acknowledgments accomplishes several things: it
      facilitates the packaging of multiple acknowledgments in a
      single Link State Acknowledgment packet; it enables a single
      Link State Acknowledgment packet to indicate acknowledgments to
      several neighbors at once (through multicasting); and it
      randomizes the Link State Acknowledgment packets sent by the
      various routers attached to a multi-access network.  The fixed
      interval between a router's delayed transmissions must be short
      (less than RxmtInterval) or needless retransmissions will ensue.
      Direct acknowledgments are sent to a particular neighbor in
      response to the receipt of duplicate link state advertisements.
      These acknowledgments are sent as unicasts, and are sent
      immediately when the duplicate is received.
      The precise procedure for sending Link State Acknowledgment

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      packets is described in Table 19.  The circumstances surrounding
      the receipt of the advertisement are listed in the left column.
      The acknowledgment action then taken is listed in one of the two
      right columns.  This action depends on the state of the
      concerned interface; interfaces in state Backup behave
      differently from interfaces in all other states.  Delayed
      acknowledgments must be delivered to all adjacent routers
      associated with the interface.  On broadcast networks, this is
      accomplished by sending the delayed Link State Acknowledgment
      packets as multicasts.  The Destination IP address used depends
      on the state of the interface.  If the state is DR or Backup,
      the destination AllSPFRouters is used.  In other states, the
      destination AllDRouters is used.  On non-broadcast networks,
      delayed Link State Acknowledgment packets must be unicast
      separately over each adjacency (i.e., neighbor whose state is >=
      Exchange).
      The reasoning behind sending the above packets as multicasts is
      best explained by an example.  Consider the network
      configuration depicted in Figure 15.  Suppose RT4 has been
      elected as Designated Router, and RT3 as Backup Designated
      Router for the network N3.  When Router RT4 floods a new
      advertisement to Network N3, it is received by routers RT1, RT2,
      and RT3.  These routers will not flood the advertisement back
      onto net N3, but they still must ensure that their topological
      databases remain synchronized with their adjacent neighbors.  So
      RT1, RT2, and RT4 are waiting to see an acknowledgment from RT3.
      Likewise, RT4 and RT3 are both waiting to see acknowledgments
      from RT1 and RT2.  This is best achieved by sending the
      acknowledgments as multicasts.
      The reason that the acknowledgment logic for Backup DRs is
      slightly different is because they perform differently during
      the flooding of link state advertisements (see Section 13.3,
      step 4).
  13.6.  Retransmitting link state advertisements
      Advertisements flooded out an adjacency are placed on the
      adjacency's Link state retransmission list.  In order to ensure
      that flooding is reliable, these advertisements are
      retransmitted until they are acknowledged.  The length of time
      between retransmissions is a configurable per-interface value,
      RxmtInterval.  If this is set too low for an interface, needless
      retransmissions will ensue.  If the value is set too high, the
      speed of the flooding, in the face of lost packets, may be

Moy [Page 136] RFC 1583 OSPF Version 2 March 1994

                                  Action taken in state
  Circumstances          Backup                All other states
  _______________________________________________________________
  Advertisement  has     No  acknowledgment    No  acknowledgment
  been  flooded back     sent.                 sent.
  out receiving  in-
  terface  (see Sec-
  tion 13, step 5b).
  _______________________________________________________________
  Advertisement   is     Delayed acknowledg-   Delayed       ack-
  more  recent  than     ment sent if adver-   nowledgment sent.
  database copy, but     tisement   received
  was   not  flooded     from    Designated
  back out receiving     Router,  otherwise
  interface              do nothing
  _______________________________________________________________
  Advertisement is a     Delayed acknowledg-   No  acknowledgment
  duplicate, and was     ment sent if adver-   sent.
  treated as an  im-     tisement   received
  plied  acknowledg-     from    Designated
  ment (see  Section     Router,  otherwise
  13, step 7a).          do nothing
  _______________________________________________________________
  Advertisement is a     Direct acknowledg-    Direct acknowledg-
  duplicate, and was     ment sent.            ment sent.
  not treated as  an
  implied       ack-
  nowledgment.
  _______________________________________________________________
  Advertisement's LS     Direct acknowledg-    Direct acknowledg-
  age is equal to        ment sent.            ment sent.
  MaxAge, and there is
  no current instance
  of the advertisement
  in the link state
  database (see
  Section 13, step 4).
           Table 19: Sending link state acknowledgements.

Moy [Page 137] RFC 1583 OSPF Version 2 March 1994

      affected.
      Several retransmitted advertisements may fit into a single Link
      State Update packet.  When advertisements are to be
      retransmitted, only the number fitting in a single Link State
      Update packet should be transmitted.  Another packet of
      retransmissions can be sent when some of the advertisements are
      acknowledged, or on the next firing of the retransmission timer.
      Link State Update Packets carrying retransmissions are always
      sent as unicasts (directly to the physical address of the
      neighbor).  They are never sent as multicasts.  Each
      advertisement's LS age must be incremented by InfTransDelay
      (which must be > 0) when copied into the outgoing Link State
      Update packet (until the LS age field reaches its maximum value
      of MaxAge).
      If the adjacent router goes down, retransmissions may occur
      until the adjacency is destroyed by OSPF's Hello Protocol.  When
      the adjacency is destroyed, the Link state retransmission list
      is cleared.
  13.7.  Receiving link state acknowledgments
      Many consistency checks have been made on a received Link State
      Acknowledgment packet before it is handed to the flooding
      procedure.  In particular, it has been associated with a
      particular neighbor.  If this neighbor is in a lesser state than
      Exchange, the Link State Acknowledgment packet is discarded.
      Otherwise, for each acknowledgment in the Link State
      Acknowledgment packet, the following steps are performed:
      o   Does the advertisement acknowledged have an instance on the
          Link state retransmission list for the neighbor?  If not,
          examine the next acknowledgment.  Otherwise:
      o   If the acknowledgment is for the same instance that is
          contained on the list, remove the item from the list and
          examine the next acknowledgment.  Otherwise:
      o   Log the questionable acknowledgment, and examine the next
          one.

Moy [Page 138] RFC 1583 OSPF Version 2 March 1994

14. Aging The Link State Database

  Each link state advertisement has an LS age field.  The LS age is
  expressed in seconds.  An advertisement's LS age field is
  incremented while it is contained in a router's database.  Also,
  when copied into a Link State Update Packet for flooding out a
  particular interface, the advertisement's LS age is incremented by
  InfTransDelay.
  An advertisement's LS age is never incremented past the value
  MaxAge.  Advertisements having age MaxAge are not used in the
  routing table calculation.  As a router ages its link state
  database, an advertisement's LS age may reach MaxAge.[18] At this
  time, the router must attempt to flush the advertisement from the
  routing domain.  This is done simply by reflooding the MaxAge
  advertisement just as if it was a newly originated advertisement
  (see Section 13.3).
  When creating a Database summary list for a newly forming adjacency,
  any MaxAge advertisements present in the link state database are
  added to the neighbor's Link state retransmission list instead of
  the neighbor's Database summary list.  See Section 10.3 for more
  details.
  A MaxAge advertisement must be removed immediately from the router's
  link state database as soon as both a) it is no longer contained on
  any neighbor Link state retransmission lists and b) none of the
  router's neighbors are in states Exchange or Loading.
  When, in the process of aging the link state database, an
  advertisement's LS age hits a multiple of CheckAge, its LS checksum
  should be verified.  If the LS checksum is incorrect, a program or
  memory error has been detected, and at the very least the router
  itself should be restarted.
  14.1.  Premature aging of advertisements
      A link state advertisement can be flushed from the routing
      domain by setting its LS age to MaxAge and reflooding the
      advertisement.  This procedure follows the same course as
      flushing an advertisement whose LS age has naturally reached the
      value MaxAge (see Section 14).  In particular, the MaxAge
      advertisement is removed from the router's link state database
      as soon as a) it is no longer contained on any neighbor Link
      state retransmission lists and b) none of the router's neighbors
      are in states Exchange or Loading.  We call the setting of an
      advertisement's LS age to MaxAge premature aging.

Moy [Page 139] RFC 1583 OSPF Version 2 March 1994

      Premature aging is used when it is time for a self-originated
      advertisement's sequence number field to wrap.  At this point,
      the current advertisement instance (having LS sequence number of
      0x7fffffff) must be prematurely aged and flushed from the
      routing domain before a new instance with sequence number
      0x80000001 can be originated.  See Section 12.1.6 for more
      information.
      Premature aging can also be used when, for example, one of the
      router's previously advertised external routes is no longer
      reachable.  In this circumstance, the router can flush its
      external advertisement from the routing domain via premature
      aging. This procedure is preferable to the alternative, which is
      to originate a new advertisement for the destination specifying
      a metric of LSInfinity.  Premature aging is also be used when
      unexpectedly receiving self-originated advertisements during the
      flooding procedure (see Section 13.4).
      A router may only prematurely age its own self-originated link
      state advertisements. The router may not prematurely age
      advertisements that have been originated by other routers. An
      advertisement is considered self-originated when either 1) the
      advertisement's Advertising Router is equal to the router's own
      Router ID or 2) the advertisement is a network links
      advertisement and its Link State ID is equal to one of the
      router's own IP interface addresses.

15. Virtual Links

  The single backbone area (Area ID = 0.0.0.0) cannot be disconnected,
  or some areas of the Autonomous System will become unreachable.  To
  establish/maintain connectivity of the backbone, virtual links can
  be configured through non-backbone areas.  Virtual links serve to
  connect physically separate components of the backbone.  The two
  endpoints of a virtual link are area border routers.  The virtual
  link must be configured in both routers.  The configuration
  information in each router consists of the other virtual endpoint
  (the other area border router), and the non-backbone area the two
  routers have in common (called the transit area).  Virtual links
  cannot be configured through stub areas (see Section 3.6).
  The virtual link is treated as if it were an unnumbered point-to-
  point network (belonging to the backbone) joining the two area
  border routers.  An attempt is made to establish an adjacency over
  the virtual link.  When this adjacency is established, the virtual
  link will be included in backbone router links advertisements, and
  OSPF packets pertaining to the backbone area will flow over the

Moy [Page 140] RFC 1583 OSPF Version 2 March 1994

  adjacency.  Such an adjacency has been referred to in this document
  as a "virtual adjacency".
  In each endpoint router, the cost and viability of the virtual link
  is discovered by examining the routing table entry for the other
  endpoint router.  (The entry's associated area must be the
  configured transit area).  Actually, there may be a separate routing
  table entry for each Type of Service.  These are called the virtual
  link's corresponding routing table entries.  The InterfaceUp event
  occurs for a virtual link when its corresponding TOS 0 routing table
  entry becomes reachable.  Conversely, the InterfaceDown event occurs
  when its TOS 0 routing table entry becomes unreachable.[19] In other
  words, the virtual link's viability is determined by the existence
  of an intra-area path, through the transit area, between the two
  endpoints.  Note that a virtual link whose underlying path has cost
  greater than hexadecimal 0xffff (the maximum size of an interface
  cost in a router links advertisement) should be considered
  inoperational (i.e., treated the same as if the path did not exist).
  The other details concerning virtual links are as follows:
  o   AS external links are NEVER flooded over virtual adjacencies.
      This would be duplication of effort, since the same AS external
      links are already flooded throughout the virtual link's transit
      area.  For this same reason, AS external link advertisements are
      not summarized over virtual adjacencies during the Database
      Exchange process.
  o   The cost of a virtual link is NOT configured.  It is defined to
      be the cost of the intra-area path between the two defining area
      border routers.  This cost appears in the virtual link's
      corresponding routing table entry.  When the cost of a virtual
      link changes, a new router links advertisement should be
      originated for the backbone area.
  o   Just as the virtual link's cost and viability are determined by
      the routing table build process (through construction of the
      routing table entry for the other endpoint), so are the IP
      interface address for the virtual interface and the virtual
      neighbor's IP address.  These are used when sending OSPF
      protocol packets over the virtual link. Note that when one (or
      both) of the virtual link endpoints connect to the transit area
      via an unnumbered point-to-point link, it may be impossible to
      calculate either the virtual interface's IP address and/or the
      virtual neighbor's IP address, thereby causing the virtual link
      to fail.

Moy [Page 141] RFC 1583 OSPF Version 2 March 1994

  o   In each endpoint's router links advertisement for the backbone,
      the virtual link is represented as a Type 4 link whose Link ID
      is set to the virtual neighbor's OSPF Router ID and whose Link
      Data is set to the virtual interface's IP address.  See Section
      12.4.1 for more information. Note that it may be the case that
      there is a TOS 0 path, but no non-zero TOS paths, between the
      two endpoint routers.  In this case, both routers must revert to
      being non-TOS-capable, clearing the T-bit in the Options field
      of their backbone router links advertisements.
  o   When virtual links are configured for the backbone, information
      concerning backbone networks should not be condensed before
      being summarized for the transit areas.  In other words, each
      backbone network should be advertised into the transit areas in
      a separate summary link advertisement, regardless of the
      backbone's configured area address ranges.  See Section 12.4.3
      for more information.
  o   The time between link state retransmissions, RxmtInterval, is
      configured for a virtual link.  This should be well over the
      expected round-trip delay between the two routers.  This may be
      hard to estimate for a virtual link; it is better to err on the
      side of making it too large.

16. Calculation Of The Routing Table

  This section details the OSPF routing table calculation.  Using its
  attached areas' link state databases as input, a router runs the
  following algorithm, building its routing table step by step.  At
  each step, the router must access individual pieces of the link
  state databases (e.g., a router links advertisement originated by a
  certain router).  This access is performed by the lookup function
  discussed in Section 12.2.  The lookup process may return a link
  state advertisement whose LS age is equal to MaxAge.  Such an
  advertisement should not be used in the routing table calculation,
  and is treated just as if the lookup process had failed.
  The OSPF routing table's organization is explained in Section 11.
  Two examples of the routing table build process are presented in
  Sections 11.2 and 11.3.  This process can be broken into the
  following steps:
  (1) The present routing table is invalidated.  The routing table is
      built again from scratch.  The old routing table is saved so
      that changes in routing table entries can be identified.

Moy [Page 142] RFC 1583 OSPF Version 2 March 1994

  (2) The intra-area routes are calculated by building the shortest-
      path tree for each attached area.  In particular, all routing
      table entries whose Destination Type is "area border router" are
      calculated in this step.  This step is described in two parts.
      At first the tree is constructed by only considering those links
      between routers and transit networks.  Then the stub networks
      are incorporated into the tree. During the area's shortest-path
      tree calculation, the area's TransitCapability is also
      calculated for later use in Step 4.
  (3) The inter-area routes are calculated, through examination of
      summary link advertisements.  If the router is attached to
      multiple areas (i.e., it is an area border router), only
      backbone summary link advertisements are examined.
  (4) In area border routers connecting to one or more transit areas
      (i.e, non-backbone areas whose TransitCapability is found to be
      TRUE), the transit areas' summary link advertisements are
      examined to see whether better paths exist using the transit
      areas than were found in Steps 2-3 above.
  (5) Routes to external destinations are calculated, through
      examination of AS external link advertisements.  The locations
      of the AS boundary routers (which originate the AS external link
      advertisements) have been determined in steps 2-4.
  Steps 2-5 are explained in further detail below.  The explanations
  describe the calculations for TOS 0 only.  It may also be necessary
  to perform each step (separately) for each of the non-zero TOS
  values.[20] For more information concerning the building of non-zero
  TOS routes see Section 16.9.
  Changes made to routing table entries as a result of these
  calculations can cause the OSPF protocol to take further actions.
  For example, a change to an intra-area route will cause an area
  border router to originate new summary link advertisements (see
  Section 12.4).  See Section 16.7 for a complete list of the OSPF
  protocol actions resulting from routing table changes.
  16.1.  Calculating the shortest-path tree for an area
      This calculation yields the set of intra-area routes associated
      with an area (called hereafter Area A).  A router calculates the
      shortest-path tree using itself as the root.[21] The formation
      of the shortest path tree is done here in two stages.  In the
      first stage, only links between routers and transit networks are

Moy [Page 143] RFC 1583 OSPF Version 2 March 1994

      considered.  Using the Dijkstra algorithm, a tree is formed from
      this subset of the link state database.  In the second stage,
      leaves are added to the tree by considering the links to stub
      networks.
      The procedure will be explained using the graph terminology that
      was introduced in Section 2.  The area's link state database is
      represented as a directed graph.  The graph's vertices are
      routers, transit networks and stub networks.  The first stage of
      the procedure concerns only the transit vertices (routers and
      transit networks) and their connecting links.  Throughout the
      shortest path calculation, the following data is also associated
      with each transit vertex:
      Vertex (node) ID
          A 32-bit number uniquely identifying the vertex.  For router
          vertices this is the router's OSPF Router ID.  For network
          vertices, this is the IP address of the network's Designated
          Router.
      A link state advertisement
          Each transit vertex has an associated link state
          advertisement.  For router vertices, this is a router links
          advertisement.  For transit networks, this is a network
          links advertisement (which is actually originated by the
          network's Designated Router).  In any case, the
          advertisement's Link State ID is always equal to the above
          Vertex ID.
      List of next hops
          The list of next hops for the current set of shortest paths
          from the root to this vertex.  There can be multiple
          shortest paths due to the equal-cost multipath capability.
          Each next hop indicates the outgoing router interface to use
          when forwarding traffic to the destination.  On multi-access
          networks, the next hop also includes the IP address of the
          next router (if any) in the path towards the destination.
      Distance from root
          The link state cost of the current set of shortest paths
          from the root to the vertex.  The link state cost of a path
          is calculated as the sum of the costs of the path's
          constituent links (as advertised in router links and network
          links advertisements).  One path is said to be "shorter"
          than another if it has a smaller link state cost.

Moy [Page 144] RFC 1583 OSPF Version 2 March 1994

      The first stage of the procedure (i.e., the Dijkstra algorithm)
      can now be summarized as follows. At each iteration of the
      algorithm, there is a list of candidate vertices.  Paths from
      the root to these vertices have been found, but not necessarily
      the shortest ones.  However, the paths to the candidate vertex
      that is closest to the root are guaranteed to be shortest; this
      vertex is added to the shortest-path tree, removed from the
      candidate list, and its adjacent vertices are examined for
      possible addition to/modification of the candidate list.  The
      algorithm then iterates again.  It terminates when the candidate
      list becomes empty.
      The following steps describe the algorithm in detail.  Remember
      that we are computing the shortest path tree for Area A.  All
      references to link state database lookup below are from Area A's
      database.
      (1) Initialize the algorithm's data structures.  Clear the list
          of candidate vertices.  Initialize the shortest-path tree to
          only the root (which is the router doing the calculation).
          Set Area A's TransitCapability to FALSE.
      (2) Call the vertex just added to the tree vertex V.  Examine
          the link state advertisement associated with vertex V.  This
          is a lookup in the Area A's link state database based on the
          Vertex ID.  If this is a router links advertisement, and bit
          V of the router links advertisement (see Section A.4.2) is
          set, set Area A's TransitCapability to TRUE.  In any case,
          each link described by the advertisement gives the cost to
          an adjacent vertex.  For each described link, (say it joins
          vertex V to vertex W):
          (a) If this is a link to a stub network, examine the next
              link in V's advertisement.  Links to stub networks will
              be considered in the second stage of the shortest path
              calculation.
          (b) Otherwise, W is a transit vertex (router or transit
              network).  Look up the vertex W's link state
              advertisement (router links or network links) in Area
              A's link state database.  If the advertisement does not
              exist, or its LS age is equal to MaxAge, or it does not
              have a link back to vertex V, examine the next link in
              V's advertisement.[22]
          (c) If vertex W is already on the shortest-path tree,
              examine the next link in the advertisement.

Moy [Page 145] RFC 1583 OSPF Version 2 March 1994

          (d) Calculate the link state cost D of the resulting path
              from the root to vertex W.  D is equal to the sum of the
              link state cost of the (already calculated) shortest
              path to vertex V and the advertised cost of the link
              between vertices V and W.  If D is:
              o   Greater than the value that already appears for
                  vertex W on the candidate list, then examine the
                  next link.
              o   Equal to the value that appears for vertex W on the
                  candidate list, calculate the set of next hops that
                  result from using the advertised link.  Input to
                  this calculation is the destination (W), and its
                  parent (V).  This calculation is shown in Section
                  16.1.1.  This set of hops should be added to the
                  next hop values that appear for W on the candidate
                  list.
              o   Less than the value that appears for vertex W on the
                  candidate list, or if W does not yet appear on the
                  candidate list, then set the entry for W on the
                  candidate list to indicate a distance of D from the
                  root.  Also calculate the list of next hops that
                  result from using the advertised link, setting the
                  next hop values for W accordingly.  The next hop
                  calculation is described in Section 16.1.1; it takes
                  as input the destination (W) and its parent (V).
      (3) If at this step the candidate list is empty, the shortest-
          path tree (of transit vertices) has been completely built
          and this stage of the procedure terminates.  Otherwise,
          choose the vertex belonging to the candidate list that is
          closest to the root, and add it to the shortest-path tree
          (removing it from the candidate list in the process). Note
          that when there is a choice of vertices closest to the root,
          network vertices must be chosen before router vertices in
          order to necessarily find all equal-cost paths. This is
          consistent with the tie-breakers that were introduced in the
          modified Dijkstra algorithm used by OSPF's Multicast routing
          extensions (MOSPF).
      (4) Possibly modify the routing table.  For those routing table
          entries modified, the associated area will be set to Area A,
          the path type will be set to intra-area, and the cost will
          be set to the newly discovered shortest path's calculated
          distance.

Moy [Page 146] RFC 1583 OSPF Version 2 March 1994

          If the newly added vertex is an area border router (call it
          ABR), a routing table entry is added whose destination type
          is "area border router". The Options field found in the
          associated router links advertisement is copied into the
          routing table entry's Optional capabilities field. If in
          addition ABR is the endpoint of one of the calculating
          router's configured virtual links that uses Area A as its
          Transit area: the virtual link is declared up, the IP
          address of the virtual interface is set to the IP address of
          the outgoing interface calculated above for ABR, and the
          virtual neighbor's IP address is set to the ABR interface
          address (contained in ABR's router links advertisement) that
          points back to the root of the shortest-path tree;
          equivalently, this is the interface that points back to
          ABR's parent vertex on the shortest-path tree (similar to
          the calculation in Section 16.1.1).
          If the newly added vertex is an AS boundary router, the
          routing table entry of type "AS boundary router" for the
          destination is located.  Since routers can belong to more
          than one area, it is possible that several sets of intra-
          area paths exist to the AS boundary router, each set using a
          different area.  However, the AS boundary router's routing
          table entry must indicate a set of paths which utilize a
          single area.  The area leading to the routing table entry is
          selected as follows: The area providing the shortest path is
          always chosen; if more than one area provides paths with the
          same minimum cost, the area with the largest OSPF Area ID
          (when considered as an unsigned 32-bit integer) is chosen.
          Note that whenever an AS boundary router's routing table
          entry is added/modified, the Options found in the associated
          router links advertisement is copied into the routing table
          entry's Optional capabilities field.
          If the newly added vertex is a transit network, the routing
          table entry for the network is located.  The entry's
          Destination ID is the IP network number, which can be
          obtained by masking the Vertex ID (Link State ID) with its
          associated subnet mask (found in the body of the associated
          network links advertisement).  If the routing table entry
          already exists (i.e., there is already an intra-area route
          to the destination installed in the routing table), multiple
          vertices have mapped to the same IP network.  For example,
          this can occur when a new Designated Router is being
          established.  In this case, the current routing table entry
          should be overwritten if and only if the newly found path is
          just as short and the current routing table entry's Link
          State Origin has a smaller Link State ID than the newly

Moy [Page 147] RFC 1583 OSPF Version 2 March 1994

          added vertex' link state advertisement.
          If there is no routing table entry for the network (the
          usual case), a routing table entry for the IP network should
          be added.  The routing table entry's Link State Origin
          should be set to the newly added vertex' link state
          advertisement.
      (5) Iterate the algorithm by returning to Step 2.
      The stub networks are added to the tree in the procedure's
      second stage.  In this stage, all router vertices are again
      examined.  Those that have been determined to be unreachable in
      the above first phase are discarded.  For each reachable router
      vertex (call it V), the associated router links advertisement is
      found in the link state database.  Each stub network link
      appearing in the advertisement is then examined, and the
      following steps are executed:
      (1) Calculate the distance D of stub network from the root.  D
          is equal to the distance from the root to the router vertex
          (calculated in stage 1), plus the stub network link's
          advertised cost.  Compare this distance to the current best
          cost to the stub network.  This is done by looking up the
          stub network's current routing table entry.  If the
          calculated distance D is larger, go on to examine the next
          stub network link in the advertisement.
      (2) If this step is reached, the stub network's routing table
          entry must be updated.  Calculate the set of next hops that
          would result from using the stub network link.  This
          calculation is shown in Section 16.1.1; input to this
          calculation is the destination (the stub network) and the
          parent vertex (the router vertex).  If the distance D is the
          same as the current routing table cost, simply add this set
          of next hops to the routing table entry's list of next hops.
          In this case, the routing table already has a Link State
          Origin.  If this Link State Origin is a router links
          advertisement whose Link State ID is smaller than V's Router
          ID, reset the Link State Origin to V's router links
          advertisement.
          Otherwise D is smaller than the routing table cost.
          Overwrite the current routing table entry by setting the
          routing table entry's cost to D, and by setting the entry's
          list of next hops to the newly calculated set.  Set the

Moy [Page 148] RFC 1583 OSPF Version 2 March 1994

          routing table entry's Link State Origin to V's router links
          advertisement.  Then go on to examine the next stub network
          link.
      For all routing table entries added/modified in the second
      stage, the associated area will be set to Area A and the path
      type will be set to intra-area.  When the list of reachable
      router links is exhausted, the second stage is completed.  At
      this time, all intra-area routes associated with Area A have
      been determined.
      The specification does not require that the above two stage
      method be used to calculate the shortest path tree.  However, if
      another algorithm is used, an identical tree must be produced.
      For this reason, it is important to note that links between
      transit vertices must be bidirectional in ordered to be included
      in the above tree.  It should also be mentioned that more
      efficient algorithms exist for calculating the tree; for
      example, the incremental SPF algorithm described in [BBN].
      16.1.1.  The next hop calculation
          This section explains how to calculate the current set of
          next hops to use for a destination.  Each next hop consists
          of the outgoing interface to use in forwarding packets to
          the destination together with the next hop router (if any).
          The next hop calculation is invoked each time a shorter path
          to the destination is discovered.  This can happen in either
          stage of the shortest-path tree calculation (see Section
          16.1).  In stage 1 of the shortest-path tree calculation a
          shorter path is found as the destination is added to the
          candidate list, or when the destination's entry on the
          candidate list is modified (Step 2d of Stage 1).  In stage 2
          a shorter path is discovered each time the destination's
          routing table entry is modified (Step 2 of Stage 2).
          The set of next hops to use for the destination may be
          recalculated several times during the shortest-path tree
          calculation, as shorter and shorter paths are discovered.
          In the end, the destination's routing table entry will
          always reflect the next hops resulting from the absolute
          shortest path(s).
          Input to the next hop calculation is a) the destination and
          b) its parent in the current shortest path between the root
          (the calculating router) and the destination.  The parent is

Moy [Page 149] RFC 1583 OSPF Version 2 March 1994

          always a transit vertex (i.e., always a router or a transit
          network).
          If there is at least one intervening router in the current
          shortest path between the destination and the root, the
          destination simply inherits the set of next hops from the
          parent.  Otherwise, there are two cases.  In the first case,
          the parent vertex is the root (the calculating router
          itself).  This means that the destination is either a
          directly connected network or directly connected router.
          The next hop in this case is simply the OSPF interface
          connecting to the network/router; no next hop router is
          required. If the connecting OSPF interface in this case is a
          virtual link, the setting of the next hop should be deferred
          until the calculation in Section 16.3.
          In the second case, the parent vertex is a network that
          directly connects the calculating router to the destination
          router.  The list of next hops is then determined by
          examining the destination's router links advertisement.  For
          each link in the advertisement that points back to the
          parent network, the link's Link Data field provides the IP
          address of a next hop router.  The outgoing interface to use
          can then be derived from the next hop IP address (or it can
          be inherited from the parent network).
  16.2.  Calculating the inter-area routes
      The inter-area routes are calculated by examining summary link
      advertisements.  If the router has active attachments to
      multiple areas, only backbone summary link advertisements are
      examined.  Routers attached to a single area examine that area's
      summary links.  In either case, the summary links examined below
      are all part of a single area's link state database (call it
      Area A).
      Summary link advertisements are originated by the area border
      routers.  Each summary link advertisement in Area A is
      considered in turn.  Remember that the destination described by
      a summary link advertisement is either a network (Type 3 summary
      link advertisements) or an AS boundary router (Type 4 summary
      link advertisements).  For each summary link advertisement:
      (1) If the cost specified by the advertisement is LSInfinity, or
          if the advertisement's LS age is equal to MaxAge, then
          examine the the next advertisement.

Moy [Page 150] RFC 1583 OSPF Version 2 March 1994

      (2) If the advertisement was originated by the calculating
          router itself, examine the next advertisement.
      (3) If the collection of destinations described by the summary
          link advertisement falls into one of the router's configured
          area address ranges (see Section 3.5) and the particular
          area address range is active, the summary link advertisement
          should be ignored.  Active means that there are one or more
          reachable (by intra-area paths) networks contained in the
          area range.  In this case, all addresses in the area range
          are assumed to be either reachable via intra-area paths, or
          else to be unreachable by any other means.
      (4) Else, call the destination described by the advertisement N
          (for Type 3 summary links, N's address is obtained by
          masking the advertisement's Link State ID with the
          network/subnet mask contained in the body of the
          advertisement), and the area border originating the
          advertisement BR.  Look up the routing table entry for BR
          having Area A as its associated area.  If no such entry
          exists for router BR (i.e., BR is unreachable in Area A), do
          nothing with this advertisement and consider the next in the
          list.  Else, this advertisement describes an inter-area path
          to destination N, whose cost is the distance to BR plus the
          cost specified in the advertisement. Call the cost of this
          inter-area path IAC.
      (5) Next, look up the routing table entry for the destination N.
          (The entry's Destination Type is either Network or AS
          boundary router.)  If no entry exists for N or if the
          entry's path type is "type 1 external" or "type 2 external",
          then install the inter-area path to N, with associated area
          Area A, cost IAC, next hop equal to the list of next hops to
          router BR, and Advertising router equal to BR.
      (6) Else, if the paths present in the table are intra-area
          paths, do nothing with the advertisement (intra-area paths
          are always preferred).
      (7) Else, the paths present in the routing table are also
          inter-area paths.  Install the new path through BR if it is
          cheaper, overriding the paths in the routing table.
          Otherwise, if the new path is the same cost, add it to the
          list of paths that appear in the routing table entry.

Moy [Page 151] RFC 1583 OSPF Version 2 March 1994

  16.3.  Examining transit areas' summary links
      This step is only performed by area border routers attached to
      one or more transit areas. Transit areas are those areas
      supporting one or more virtual links; their TransitCapability
      parameter has been set to TRUE in Step 2 of the Dijkstra
      algorithm (see Section 16.1). They are the only non-backbone
      areas that can carry data traffic that neither originates nor
      terminates in the area itself.
      The purpose of the calculation below is to examine the transit
      areas to see whether they provide any better (shorter) paths
      than the paths previously calculated in Sections 16.1 and 16.2.
      Any paths found that are better than or equal to previously
      discovered paths are installed in the routing table.
      The calculation proceeds as follows. All the transit areas'
      summary link advertisements are examined in turn.  Each such
      summary link advertisement describes a route through a transit
      area Area A to a Network N (N's address is obtained by masking
      the advertisement's Link State ID with the network/subnet mask
      contained in the body of the advertisement) or in the case of a
      Type 4 summary link advertisement, to an AS boundary router N.
      Suppose also that the summary link advertisement was originated
      by an area border router BR.
      (1) If the cost advertised by the summary link advertisement is
          LSInfinity, or if the advertisement's LS age is equal to
          MaxAge, then examine the next advertisement.
      (2) If the summary link advertisement was originated by the
          calculating router itself, examine the next advertisement.
      (3) Look up the routing table entry for N. If it does not exist,
          or if the route type is other than intra-area or inter-area,
          or if the area associated with the routing table entry is
          not the backbone area, then examine the next advertisement.
          In other words, this calculation only updates backbone
          intra-area routes found in Section 16.1 and inter-area
          routes found in Section 16.2.
      (4) Look up the routing table entry for the advertising router
          BR associated with the Area A. If it is unreachable, examine
          the next advertisement. Otherwise, the cost to destination N
          is the sum of the cost in BR's Area A routing table entry
          and the cost advertised in the advertisement. Call this cost
          IAC.

Moy [Page 152] RFC 1583 OSPF Version 2 March 1994

      (5) If this cost is less than the cost occurring in N's routing
          table entry, overwrite N's list of next hops with those used
          for BR, and set N's routing table cost to IAC. Else, if IAC
          is the same as N's current cost, add BR's list of next hops
          to N's list of next hops. In any case, the area associated
          with N's routing table entry must remain the backbone area,
          and the path type (either intra-area or inter-area) must
          also remain the same.
      It is important to note that the above calculation never makes
      unreachable destinations reachable, but instead just potentially
      finds better paths to already reachable destinations. Also,
      unlike Section 16.3 of [RFC 1247], the above calculation
      installs any better cost found into the routing table entry,
      from which it may be readvertised in summary link advertisements
      to other areas.
      As an example of the calculation, consider the Autonomous System
      pictured in Figure 17.  There is a single non-backbone area
      (Area 1) that physically divides the backbone into two separate
      pieces. To maintain connectivity of the backbone, a virtual link
      has been configured between routers RT1 and RT4. On the right
      side of the figure, Network N1 belongs to the backbone. The
      dotted lines indicate that there is a much shorter intra-area
                    ........................
                    . Area 1 (transit)     .            +
                    .                      .            |
                    .      +---+1        1+---+100      |
                    .      |RT2|----------|RT4|=========|
                    .    1/+---+********* +---+         |
                    .    /*******          .            |
                    .  1/*Virtual          .            |
                 1+---+/*  Link            .         Net|work
           =======|RT1|*                   .            | N1
                  +---+\                   .            |
                    .   \                  .            |
                    .    \                 .            |
                    .    1\+---+1        1+---+20       |
                    .      |RT3|----------|RT5|=========|
                    .      +---+          +---+         |
                    .                      .            |
                    ........................            +
                  Figure 17: Routing through transit areas

Moy [Page 153] RFC 1583 OSPF Version 2 March 1994

      backbone path between router RT5 and Network N1 (cost 20) than
      there is between Router RT4 and Network N1 (cost 100). Both
      Router RT4 and Router RT5 will inject summary link
      advertisements for Network N1 into Area 1.
      After the shortest-path tree has been calculated for the
      backbone in Section 16.1, Router RT1 (left end of the virtual
      link) will have calculated a path through Router RT4 for all
      data traffic destined for Network N1. However, since Router RT5
      is so much closer to Network N1, all routers internal to Area 1
      (e.g., Routers RT2 and RT3) will forward their Network N1
      traffic towards Router RT5, instead of RT4. And indeed, after
      examining Area 1's summary link advertisements by the above
      calculation, Router RT1 will also forward Network N1 traffic
      towards RT5. Note that in this example the virtual link enables
      Network N1 traffic to be forwarded through the transit area Area
      1, but the actual path the data traffic takes does not follow
      the virtual link.  In other words, virtual links allow transit
      traffic to be forwarded through an area, but do not dictate the
      precise path that the traffic will take.
  16.4.  Calculating AS external routes
      AS external routes are calculated by examining AS external link
      advertisements.  Each of the AS external link advertisements is
      considered in turn.  Most AS external link advertisements
      describe routes to specific IP destinations.  An AS external
      link advertisement can also describe a default route for the
      Autonomous System (Destination ID = DefaultDestination,
      network/subnet mask = 0x00000000).  For each AS external link
      advertisement:
      (1) If the cost specified by the advertisement is LSInfinity, or
          if the advertisement's LS age is equal to MaxAge, then
          examine the next advertisement.
      (2) If the advertisement was originated by the calculating
          router itself, examine the next advertisement.
      (3) Call the destination described by the advertisement N.  N's
          address is obtained by masking the advertisement's Link
          State ID with the network/subnet mask contained in the body
          of the advertisement.  Look up the routing table entry for
          the AS boundary router (ASBR) that originated the
          advertisement. If no entry exists for router ASBR (i.e.,
          ASBR is unreachable), do nothing with this advertisement and
          consider the next in the list.

Moy [Page 154] RFC 1583 OSPF Version 2 March 1994

          Else, this advertisement describes an AS external path to
          destination N.  Examine the forwarding address specified in
          the AS external link advertisement.  This indicates the IP
          address to which packets for the destination should be
          forwarded.  If the forwarding address is set to 0.0.0.0,
          packets should be sent to the ASBR itself.  Otherwise, look
          up the forwarding address in the routing table.[23] An
          intra-area or inter-area path must exist to the forwarding
          address.  If no such path exists, do nothing with the
          advertisement and consider the next in the list.
          Call the routing table distance to the forwarding address X
          (when the forwarding address is set to 0.0.0.0, this is the
          distance to the ASBR itself), and the cost specified in the
          advertisement Y.  X is in terms of the link state metric,
          and Y is a type 1 or 2 external metric.
      (4) Next, look up the routing table entry for the destination N.
          If no entry exists for N, install the AS external path to N,
          with next hop equal to the list of next hops to the
          forwarding address, and advertising router equal to ASBR.
          If the external metric type is 1, then the path-type is set
          to type 1 external and the cost is equal to X+Y.  If the
          external metric type is 2, the path-type is set to type 2
          external, the link state component of the route's cost is X,
          and the type 2 cost is Y.
      (5) Else, if the paths present in the table are not type 1 or
          type 2 external paths, do nothing (AS external paths have
          the lowest priority).
      (6) Otherwise, compare the cost of this new AS external path to
          the ones present in the table.  Type 1 external paths are
          always shorter than type 2 external paths.  Type 1 external
          paths are compared by looking at the sum of the distance to
          the forwarding address and the advertised type 1 metric
          (X+Y).  Type 2 external paths are compared by looking at the
          advertised type 2 metrics, and then if necessary, the
          distance to the forwarding addresses.
          If the new path is shorter, it replaces the present paths in
          the routing table entry.  If the new path is the same cost,
          it is added to the routing table entry's list of paths.

Moy [Page 155] RFC 1583 OSPF Version 2 March 1994

  16.5.  Incremental updates -- summary link advertisements
      When a new summary link advertisement is received, it is not
      necessary to recalculate the entire routing table.  Call the
      destination described by the summary link advertisement N (N's
      address is obtained by masking the advertisement's Link State ID
      with the network/subnet mask contained in the body of the
      advertisement), and let Area A be the area to which the
      advertisement belongs. There are then two separate cases:
      Case 1: Area A is the backbone and/or the router is not an area
          border router.
          In this case, the following calculations must be performed.
          First, if there is presently an inter-area route to the
          destination N, N's routing table entry is invalidated,
          saving the entry's values for later comparisons. Then the
          calculation in Section 16.2 is run again for the single
          destination N. In this calculation, all of Area A's summary
          link advertisements that describe a route to N are examined.
          In addition, if the router is an area border router attached
          to one or more transit areas, the calculation in Section
          16.3 must be run again for the single destination.  If the
          results of these calculations have changed the cost/path to
          an AS boundary router (as would be the case for a Type 4
          summary link advertisement) or to any forwarding addresses,
          all AS external link advertisements will have to be
          reexamined by rerunning the calculation in Section 16.4.
          Otherwise, if N is now newly unreachable, the calculation in
          Section 16.4 must be rerun for the single destination N, in
          case an alternate external route to N exists.
      Case 2: Area A is a transit area and the router is an area
          border router.
          In this case, the following calculations must be performed.
          First, if N's routing table entry presently contains one or
          more inter-area paths that utilize the transit area Area A,
          these paths should be removed. If this removes all paths
          from the routing table entry, the entry should be
          invalidated.  The entry's old values should be saved for
          later comparisons. Next the calculation in Section 16.3 must
          be run again for the single destination N. If the results of
          this calculation have caused the cost to N to increase, the
          complete routing table calculation must be rerun starting
          with the Dijkstra algorithm specified in Section 16.1.
          Otherwise, if the cost/path to an AS boundary router (as
          would be the case for a Type 4 summary link advertisement)
          or to any forwarding addresses has changed, all AS external
          link advertisements will have to be reexamined by rerunning

Moy [Page 156] RFC 1583 OSPF Version 2 March 1994

          the calculation in Section 16.4.  Otherwise, if N is now
          newly unreachable, the calculation in Section 16.4 must be
          rerun for the single destination N, in case an alternate
          external route to N exists.
  16.6.  Incremental updates -- AS external link advertisements
      When a new AS external link advertisement is received, it is not
      necessary to recalculate the entire routing table.  Call the
      destination described by the AS external link advertisement N.
      N's address is obtained by masking the advertisement's Link
      State ID with the network/subnet mask contained in the body of
      the advertisement. If there is already an intra-area or inter-
      area route to the destination, no recalculation is necessary
      (internal routes take precedence).
      Otherwise, the procedure in Section 16.4 will have to be
      performed, but only for those AS external link advertisements
      whose destination is N.  Before this procedure is performed, the
      present routing table entry for N should be invalidated.
  16.7.  Events generated as a result of routing table changes
      Changes to routing table entries sometimes cause the OSPF area
      border routers to take additional actions.  These routers need
      to act on the following routing table changes:
      o   The cost or path type of a routing table entry has changed.
          If the destination described by this entry is a Network or
          AS boundary router, and this is not simply a change of AS
          external routes, new summary link advertisements may have to
          be generated (potentially one for each attached area,
          including the backbone).  See Section 12.4.3 for more
          information.  If a previously advertised entry has been
          deleted, or is no longer advertisable to a particular area,
          the advertisement must be flushed from the routing domain by
          setting its LS age to MaxAge and reflooding (see Section
          14.1).
      o   A routing table entry associated with a configured virtual
          link has changed.  The destination of such a routing table
          entry is an area border router.  The change indicates a
          modification to the virtual link's cost or viability.
          If the entry indicates that the area border router is newly
          reachable (via TOS 0), the corresponding virtual link is now

Moy [Page 157] RFC 1583 OSPF Version 2 March 1994

          operational.  An InterfaceUp event should be generated for
          the virtual link, which will cause a virtual adjacency to
          begin to form (see Section 10.3).  At this time the virtual
          link's IP interface address and the virtual neighbor's
          Neighbor IP address are also calculated.
          If the entry indicates that the area border router is no
          longer reachable (via TOS 0), the virtual link and its
          associated adjacency should be destroyed.  This means an
          InterfaceDown event should be generated for the associated
          virtual link.
          If the cost of the entry has changed, and there is a fully
          established virtual adjacency, a new router links
          advertisement for the backbone must be originated.  This in
          turn may cause further routing table changes.
  16.8.  Equal-cost multipath
      The OSPF protocol maintains multiple equal-cost routes to all
      destinations.  This can be seen in the steps used above to
      calculate the routing table, and in the definition of the
      routing table structure.
      Each one of the multiple routes will be of the same type
      (intra-area, inter-area, type 1 external or type 2 external),
      cost, and will have the same associated area.  However, each
      route specifies a separate next hop and Advertising router.
      There is no requirement that a router running OSPF keep track of
      all possible equal-cost routes to a destination.  An
      implementation may choose to keep only a fixed number of routes
      to any given destination.  This does not affect any of the
      algorithms presented in this specification.
  16.9.  Building the non-zero-TOS portion of the routing table
      The OSPF protocol can calculate a different set of routes for
      each IP TOS (see Section 2.4).  Support for TOS-based routing is
      optional.  TOS-capable and non-TOS-capable routers can be mixed
      in an OSPF routing domain.  Routers not supporting TOS calculate
      only the TOS 0 route to each destination.  These routes are then
      used to forward all data traffic, regardless of the TOS
      indications in the data packet's IP header.  A router that does
      not support TOS indicates this fact to the other OSPF routers by
      clearing the T-bit in the Options field of its router links

Moy [Page 158] RFC 1583 OSPF Version 2 March 1994

      advertisement.
      The above sections detailing the routing table calculations
      handle the TOS 0 case only.  In general, for routers supporting
      TOS-based routing, each piece of the routing table calculation
      must be rerun separately for the non-zero TOS values.  When
      calculating routes for TOS X, only TOS X metrics can be used.
      Any link state advertisement may specify a separate cost for
      each TOS (a cost for TOS 0 must always be specified).  The
      encoding of TOS in OSPF link state advertisements is described
      in Section 12.3.
      An advertisement can specify that it is restricted to TOS 0
      (i.e., non-zero TOS is not handled) by clearing the T-bit in the
      link state advertisement's Option field.  Such advertisements
      are not used when calculating routes for non-zero TOS.  For this
      reason, it is possible that a destination is unreachable for
      some non-zero TOS.  In this case, the TOS 0 path is used when
      forwarding packets (see Section 11.1).
      The following lists the modifications needed when running the
      routing table calculation for a non-zero TOS value (called TOS
      X).  In general, routers and advertisements that do not support
      TOS are omitted from the calculation.
      Calculating the shortest-path tree (Section  16.1).
          Routers that do not support TOS-based routing should be
          omitted from the shortest-path tree calculation.  These
          routers are identified as those having the T-bit reset in
          the Options field of their router links advertisements.
          Such routers should never be added to the Dijktra
          algorithm's candidate list, nor should their router links
          advertisements be examined when adding the stub networks to
          the tree.  In particular, if the T-bit is reset in the
          calculating router's own router links advertisement, it does
          not run the shortest-path tree calculation for non-zero TOS
          values.
      Calculating the inter-area routes (Section  16.2).
          Inter-area paths are the concatenation of a path to an area
          border router with a summary link.  When calculating TOS X
          routes, both path components must also specify TOS X.  In
          other words, only TOS X paths to the area border router are
          examined, and the area border router must be advertising a
          TOS X route to the destination.  Note that this means that
          summary link advertisements having the T-bit reset in their
          Options field are not considered.

Moy [Page 159] RFC 1583 OSPF Version 2 March 1994

      Examining transit areas' summary links (Section 16.3).
          This calculation again considers the concatenation of a path
          to an area border router with a summary link.  As with
          inter-area routes, only TOS X paths to the area border
          router are examined, and the area border router must be
          advertising a TOS X route to the destination.
      Calculating AS external routes (Section 16.4).
          This calculation considers the concatenation of a path to a
          forwarding address with an AS external link.  Only TOS X
          paths to the forwarding address are examined, and the AS
          boundary router must be advertising a TOS X route to the
          destination.  Note that this means that AS external link
          advertisements having the T-bit reset in their Options field
          are not considered.
          In addition, the advertising AS boundary router must also be
          reachable for its advertisements to be considered (see
          Section 16.4).  However, if the advertising router and the
          forwarding address are not one in the same, the advertising
          router need only be reachable via TOS 0.

Moy [Page 160] RFC 1583 OSPF Version 2 March 1994

Footnotes

  [1]The graph's vertices represent either routers, transit networks,
  or stub networks.  Since routers may belong to multiple areas, it is
  not possible to color the graph's vertices.
  [2]It is possible for all of a router's interfaces to be unnumbered
  point-to-point links.  In this case, an IP address must be assigned
  to the router.  This address will then be advertised in the router's
  router links advertisement as a host route.
  [3]Note that in these cases both interfaces, the non-virtual and the
  virtual, would have the same IP address.
  [4]Note that no host route is generated for, and no IP packets can
  be addressed to, interfaces to unnumbered point-to-point networks.
  This is regardless of such an interface's state.
  [5]It is instructive to see what happens when the Designated Router
  for the network crashes.  Call the Designated Router for the network
  RT1, and the Backup Designated Router RT2.  If Router RT1 crashes
  (or maybe its interface to the network dies), the other routers on
  the network will detect RT1's absence within RouterDeadInterval
  seconds.  All routers may not detect this at precisely the same
  time; the routers that detect RT1's absence before RT2 does will,
  for a time, select RT2 to be both Designated Router and Backup
  Designated Router.  When RT2 detects that RT1 is gone it will move
  itself to Designated Router.  At this time, the remaining router
  having highest Router Priority will be selected as Backup Designated
  Router.
  [6]On point-to-point networks, the lower level protocols indicate
  whether the neighbor is up and running.  Likewise, existence of the
  neighbor on virtual links is indicated by the routing table
  calculation.  However, in both these cases, the Hello Protocol is
  still used.  This ensures that communication between the neighbors
  is bidirectional, and that each of the neighbors has a functioning
  routing protocol layer.
  [7]When the identity of the Designated Router is changing, it may be
  quite common for a neighbor in this state to send the router a
  Database Description packet; this means that there is some momentary
  disagreement on the Designated Router's identity.
  [8]Note that it is possible for a router to resynchronize any of its
  fully established adjacencies by setting the adjacency's state back
  to ExStart.  This will cause the other end of the adjacency to

Moy [Page 161] RFC 1583 OSPF Version 2 March 1994

  process a SeqNumberMismatch event, and therefore to also go back to
  ExStart state.
  [9]The address space of IP networks and the address space of OSPF
  Router IDs may overlap.  That is, a network may have an IP address
  which is identical (when considered as a 32-bit number) to some
  router's Router ID.
  [10]It is assumed that, for two different address ranges matching
  the destination, one range is more specific than the other. Non-
  contiguous subnet masks can be configured to violate this
  assumption. Such subnet mask configurations cannot be handled by the
  OSPF protocol.
  [11]MaxAgeDiff is an architectural constant.  It indicates the
  maximum dispersion of ages, in seconds, that can occur for a single
  link state instance as it is flooded throughout the routing domain.
  If two advertisements differ by more than this, they are assumed to
  be different instances of the same advertisement.  This can occur
  when a router restarts and loses track of the advertisement's
  previous LS sequence number.  See Section 13.4 for more details.
  [12]When two advertisements have different LS checksums, they are
  assumed to be separate instances.  This can occur when a router
  restarts, and loses track of the advertisement's previous LS
  sequence number.  In the case where the two advertisements have the
  same LS sequence number, it is not possible to determine which link
  state is actually newer.  If the wrong advertisement is accepted as
  newer, the originating router will originate another instance.  See
  Section 13.4 for further details.
  [13]There is one instance where a lookup must be done based on
  partial information.  This is during the routing table calculation,
  when a network links advertisement must be found based solely on its
  Link State ID.  The lookup in this case is still well defined, since
  no two network links advertisements can have the same Link State ID.
  [14]This clause covers the case: Inter-area routes are not
  summarized to the backbone.  This is because inter-area routes are
  always associated with the backbone area.
  [15]This clause is only invoked when Area A is a Transit area
  supporting one or more virtual links. For example, in the area
  configuration of Figure 6, Router RT11 need only originate a single
  summary link having the (collapsed) destination N9-N11,H1 into its
  connected Transit area Area 2, since all of its other eligible
  routes have next hops belonging to Area 2 (and as such only need be
  advertised by other area border routers; in this case, Routers RT10

Moy [Page 162] RFC 1583 OSPF Version 2 March 1994

  and RT7).
  [16]By keeping more information in the routing table, it is possible
  for an implementation to recalculate the shortest path tree only for
  a single area.  In fact, there are incremental algorithms that allow
  an implementation to recalculate only a portion of a single area's
  shortest path tree [BBN].  However, these algorithms are beyond the
  scope of this specification.
  [17]This is how the Link state request list is emptied, which
  eventually causes the neighbor state to transition to Full.  See
  Section 10.9 for more details.
  [18]It should be a relatively rare occurrence for an advertisement's
  LS age to reach MaxAge in this fashion.  Usually, the advertisement
  will be replaced by a more recent instance before it ages out.
  [19]Only the TOS 0 routes are important here because all OSPF
  protocol packets are sent with TOS = 0.  See Appendix A.
  [20]It may be the case that paths to certain destinations do not
  vary based on TOS.  For these destinations, the routing calculation
  need not be repeated for each TOS value.  In addition, there need
  only be a single routing table entry for these destinations (instead
  of a separate entry for each TOS value).
  [21]Strictly speaking, because of equal-cost multipath, the
  algorithm does not create a tree.  We continue to use the "tree"
  terminology because that is what occurs most often in the existing
  literature.
  [22]Note that the presence of any link back to V is sufficient; it
  need not be the matching half of the link under consideration from V
  to W. This is enough to ensure that, before data traffic flows
  between a pair of neighboring routers, their link state databases
  will be synchronized.
  [23]When the forwarding address is non-zero, it should point to a
  router belonging to another Autonomous System.  See Section 12.4.5
  for more details.

Moy [Page 163] RFC 1583 OSPF Version 2 March 1994

References

  [BBN]           McQuillan, J., I. Richer and E. Rosen, "ARPANET
                  Routing Algorithm Improvements", BBN Technical
                  Report 3803, April 1978.
  [DEC]           Digital Equipment Corporation, "Information
                  processing systems -- Data communications --
                  Intermediate System to Intermediate System Intra-
                  Domain Routing Protocol", October 1987.
  [McQuillan]     McQuillan, J. et.al., "The New Routing Algorithm for
                  the Arpanet", IEEE Transactions on Communications,
                  May 1980.
  [Perlman]       Perlman, R., "Fault-Tolerant Broadcast of Routing
                  Information", Computer Networks, December 1983.
  [RFC 791]       Postel, J., "Internet Protocol", STD 5, RFC 791,
                  USC/Information Sciences Institute, September 1981.
  [RFC 905]       McKenzie, A., "ISO Transport Protocol specification
                  ISO DP 8073", RFC 905, ISO, April 1984.
  [RFC 1112]      Deering, S., "Host extensions for IP multicasting",
                  STD 5, RFC 1112, Stanford University, May 1988.
  [RFC 1213]      McCloghrie, K., and M. Rose, "Management Information
                  Base for network management of TCP/IP-based
                  internets: MIB-II", STD 17, RFC 1213, Hughes LAN
                  Systems, Performance Systems International, March
                  1991.
  [RFC 1247]      Moy, J., "OSPF Version 2", RFC 1247, Proteon, Inc.,
                  July 1991.
  [RFC 1519]      Fuller, V., T. Li, J. Yu, and K. Varadhan,
                  "Classless Inter-Domain Routing (CIDR): an Address
                  Assignment and Aggregation Strategy", RFC1519,
                  BARRNet, cisco, MERIT, OARnet, September 1993.
  [RFC 1340]      Reynolds, J., and J. Postel, "Assigned Numbers", STD
                  2, RFC 1340, USC/Information Sciences Institute,
                  July 1992.
  [RFC 1349]      Almquist, P., "Type of Service in the Internet
                  Protocol Suite", RFC 1349, July 1992.

Moy [Page 164] RFC 1583 OSPF Version 2 March 1994

  [RS-85-153]     Leiner, B., et.al., "The DARPA Internet Protocol
                  Suite", DDN Protocol Handbook, April 1985.

Moy [Page 165] RFC 1583 OSPF Version 2 March 1994

A. OSPF data formats

  This appendix describes the format of OSPF protocol packets and OSPF
  link state advertisements.  The OSPF protocol runs directly over the
  IP network layer.  Before any data formats are described, the
  details of the OSPF encapsulation are explained.
  Next the OSPF Options field is described.  This field describes
  various capabilities that may or may not be supported by pieces of
  the OSPF routing domain. The OSPF Options field is contained in OSPF
  Hello packets, Database Description packets and in OSPF link state
  advertisements.
  OSPF packet formats are detailed in Section A.3.  A description of
  OSPF link state advertisements appears in Section A.4.

A.1 Encapsulation of OSPF packets

  OSPF runs directly over the Internet Protocol's network layer.  OSPF
  packets are therefore encapsulated solely by IP and local data-link
  headers.
  OSPF does not define a way to fragment its protocol packets, and
  depends on IP fragmentation when transmitting packets larger than
  the network MTU.  The OSPF packet types that are likely to be large
  (Database Description Packets, Link State Request, Link State
  Update, and Link State Acknowledgment packets) can usually be split
  into several separate protocol packets, without loss of
  functionality.  This is recommended; IP fragmentation should be
  avoided whenever possible.  Using this reasoning, an attempt should
  be made to limit the sizes of packets sent over virtual links to 576
  bytes.  However, if necessary, the length of OSPF packets can be up
  to 65,535 bytes (including the IP header).
  The other important features of OSPF's IP encapsulation are:
  o   Use of IP multicast.  Some OSPF messages are multicast, when
      sent over multi-access networks.  Two distinct IP multicast
      addresses are used.  Packets sent to these multicast addresses
      should never be forwarded; they are meant to travel a single hop
      only.  To ensure that these packets will not travel multiple
      hops, their IP TTL must be set to 1.
      AllSPFRouters
          This multicast address has been assigned the value
          224.0.0.5.  All routers running OSPF should be prepared to
          receive packets sent to this address.  Hello packets are
          always sent to this destination.  Also, certain OSPF

Moy [Page 166] RFC 1583 OSPF Version 2 March 1994

          protocol packets are sent to this address during the
          flooding procedure.
      AllDRouters
          This multicast address has been assigned the value
          224.0.0.6.  Both the Designated Router and Backup Designated
          Router must be prepared to receive packets destined to this
          address.  Certain OSPF protocol packets are sent to this
          address during the flooding procedure.
  o   OSPF is IP protocol number 89.  This number has been registered
      with the Network Information Center.  IP protocol number
      assignments are documented in [RFC 1340].
  o   Routing protocol packets are sent with IP TOS of 0.  The OSPF
      protocol supports TOS-based routing.  Routes to any particular
      destination may vary based on TOS.  However, all OSPF routing
      protocol packets are sent using the normal service TOS value of
      binary 0000 defined in [RFC 1349].
  o   Routing protocol packets are sent with IP precedence set to
      Internetwork Control.  OSPF protocol packets should be given
      precedence over regular IP data traffic, in both sending and
      receiving.  Setting the IP precedence field in the IP header to
      Internetwork Control [RFC 791] may help implement this
      objective.

Moy [Page 167] RFC 1583 OSPF Version 2 March 1994

A.2 The Options field

  The OSPF Options field is present in OSPF Hello packets, Database
  Description packets and all link state advertisements.  The Options
  field enables OSPF routers to support (or not support) optional
  capabilities, and to communicate their capability level to other
  OSPF routers.  Through this mechanism routers of differing
  capabilities can be mixed within an OSPF routing domain.
  When used in Hello packets, the Options field allows a router to
  reject a neighbor because of a capability mismatch.  Alternatively,
  when capabilities are exchanged in Database Description packets a
  router can choose not to forward certain link state advertisements
  to a neighbor because of its reduced functionality.  Lastly, listing
  capabilities in link state advertisements allows routers to route
  traffic around reduced functionality routers, by excluding them from
  parts of the routing table calculation.
  Two capabilities are currently defined.  For each capability, the
  effect of the capability's appearance (or lack of appearance) in
  Hello packets, Database Description packets and link state
  advertisements is specified below.  For example, the
  ExternalRoutingCapability (below called the E-bit) has meaning only
  in OSPF Hello Packets.  Routers should reset (i.e.  clear) the
  unassigned part of the capability field when sending Hello packets
  or Database Description packets and when originating link state
  advertisements.
  Additional capabilities may be assigned in the future.  Routers
  encountering unrecognized capabilities in received Hello Packets,
  Database Description packets or link state advertisements should
  ignore the capability and process the packet/advertisement normally.
                             +-+-+-+-+-+-+-+-+
                             | | | | | | |E|T|
                             +-+-+-+-+-+-+-+-+
                           The Options field
  T-bit
      This describes the router's TOS capability.  If the T-bit is
      reset, then the router supports only a single TOS (TOS 0).  Such
      a router is also said to be incapable of TOS-routing, and
      elsewhere in this document referred to as a TOS-0-only router.
      The absence of the T-bit in a router links advertisement causes
      the router to be skipped when building a non-zero TOS shortest-
      path tree (see Section 16.9).  In other words, routers incapable

Moy [Page 168] RFC 1583 OSPF Version 2 March 1994

      of TOS routing will be avoided as much as possible when
      forwarding data traffic requesting a non-zero TOS.  The absence
      of the T-bit in a summary link advertisement or an AS external
      link advertisement indicates that the advertisement is
      describing a TOS 0 route only (and not routes for non-zero TOS).
  E-bit
      This bit reflects the associated area's
      ExternalRoutingCapability.  AS external link advertisements are
      not flooded into/through OSPF stub areas (see Section 3.6).  The
      E-bit ensures that all members of a stub area agree on that
      area's configuration.  The E-bit is meaningful only in OSPF
      Hello packets.  When the E-bit is reset in the Hello packet sent
      out a particular interface, it means that the router will
      neither send nor receive AS external link state advertisements
      on that interface (in other words, the interface connects to a
      stub area).  Two routers will not become neighbors unless they
      agree on the state of the E-bit.

Moy [Page 169] RFC 1583 OSPF Version 2 March 1994

A.3 OSPF Packet Formats

  There are five distinct OSPF packet types.  All OSPF packet types
  begin with a standard 24 byte header.  This header is described
  first.  Each packet type is then described in a succeeding section.
  In these sections each packet's division into fields is displayed,
  and then the field definitions are enumerated.
  All OSPF packet types (other than the OSPF Hello packets) deal with
  lists of link state advertisements.  For example, Link State Update
  packets implement the flooding of advertisements throughout the OSPF
  routing domain.  Because of this, OSPF protocol packets cannot be
  parsed unless the format of link state advertisements is also
  understood.  The format of Link state advertisements is described in
  Section A.4.
  The receive processing of OSPF packets is detailed in Section 8.2.
  The sending of OSPF packets is explained in Section 8.1.

Moy [Page 170] RFC 1583 OSPF Version 2 March 1994

A.3.1 The OSPF packet header

  Every OSPF packet starts with a common 24 byte header.  This header
  contains all the necessary information to determine whether the
  packet should be accepted for further processing.  This
  determination is described in Section 8.2 of the specification.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Version #   |     Type      |         Packet length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Router ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Area ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Checksum            |             AuType            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Version #
      The OSPF version number.  This specification documents version 2
      of the protocol.
  Type
      The OSPF packet types are as follows.  The format of each of
      these packet types is described in a succeeding section.
                        Type   Description
                        ________________________________
                        1      Hello
                        2      Database Description
                        3      Link State Request
                        4      Link State Update
                        5      Link State Acknowledgment

Moy [Page 171] RFC 1583 OSPF Version 2 March 1994

  Packet length
      The length of the protocol packet in bytes.  This length
      includes the standard OSPF header.
  Router ID
      The Router ID of the packet's source.  In OSPF, the source and
      destination of a routing protocol packet are the two ends of an
      (potential) adjacency.
  Area ID
      A 32 bit number identifying the area that this packet belongs
      to.  All OSPF packets are associated with a single area.  Most
      travel a single hop only.  Packets travelling over a virtual
      link are labelled with the backbone Area ID of 0.0.0.0.
  Checksum
      The standard IP checksum of the entire contents of the packet,
      starting with the OSPF packet header but excluding the 64-bit
      authentication field.  This checksum is calculated as the 16-bit
      one's complement of the one's complement sum of all the 16-bit
      words in the packet, excepting the authentication field.  If the
      packet's length is not an integral number of 16-bit words, the
      packet is padded with a byte of zero before checksumming.
  AuType
      Identifies the authentication scheme to be used for the packet.
      Authentication is discussed in Appendix D of the specification.
      Consult Appendix D for a list of the currently defined
      authentication types.
  Authentication
      A 64-bit field for use by the authentication scheme.

Moy [Page 172] RFC 1583 OSPF Version 2 March 1994

A.3.2 The Hello packet

  Hello packets are OSPF packet type 1.  These packets are sent
  periodically on all interfaces (including virtual links) in order to
  establish and maintain neighbor relationships.  In addition, Hello
  Packets are multicast on those physical networks having a multicast
  or broadcast capability, enabling dynamic discovery of neighboring
  routers.
  All routers connected to a common network must agree on certain
  parameters (Network mask, HelloInterval and RouterDeadInterval).
  These parameters are included in Hello packets, so that differences
  can inhibit the forming of neighbor relationships.  A detailed
  explanation of the receive processing for Hello packets is presented
  in Section 10.5.  The sending of Hello packets is covered in Section
  9.5.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Version #   |       1       |         Packet length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Router ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Area ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Checksum            |             AuType            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Network Mask                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         HelloInterval         |    Options    |    Rtr Pri    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     RouterDeadInterval                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Designated Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Backup Designated Router                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Neighbor                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

Moy [Page 173] RFC 1583 OSPF Version 2 March 1994

  Network mask
      The network mask associated with this interface.  For example,
      if the interface is to a class B network whose third byte is
      used for subnetting, the network mask is 0xffffff00.
  Options
      The optional capabilities supported by the router, as documented
      in Section A.2.
  HelloInterval
      The number of seconds between this router's Hello packets.
  Rtr Pri
      This router's Router Priority.  Used in (Backup) Designated
      Router election.  If set to 0, the router will be ineligible to
      become (Backup) Designated Router.
  RouterDeadInterval
      The number of seconds before declaring a silent router down.
  Designated Router
      The identity of the Designated Router for this network, in the
      view of the advertising router.  The Designated Router is
      identified here by its IP interface address on the network.  Set
      to 0.0.0.0 if there is no Designated Router.
  Backup Designated Router
      The identity of the Backup Designated Router for this network,
      in the view of the advertising router.  The Backup Designated
      Router is identified here by its IP interface address on the
      network.  Set to 0.0.0.0 if there is no Backup Designated
      Router.
  Neighbor
      The Router IDs of each router from whom valid Hello packets have
      been seen recently on the network.  Recently means in the last
      RouterDeadInterval seconds.

Moy [Page 174] RFC 1583 OSPF Version 2 March 1994

A.3.3 The Database Description packet

  Database Description packets are OSPF packet type 2.  These packets
  are exchanged when an adjacency is being initialized.  They describe
  the contents of the topological database.  Multiple packets may be
  used to describe the database.  For this purpose a poll-response
  procedure is used.  One of the routers is designated to be master,
  the other a slave.  The master sends Database Description packets
  (polls) which are acknowledged by Database Description packets sent
  by the slave (responses).  The responses are linked to the polls via
  the packets' DD sequence numbers.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Version #   |       2       |         Packet length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Router ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Area ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Checksum            |             AuType            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       0       |       0       |    Options    |0|0|0|0|0|I|M|MS
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     DD sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                             A                                 |
     +-                 Link State Advertisement                    -+
     |                           Header                              |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
  The format of the Database Description packet is very similar to
  both the Link State Request and Link State Acknowledgment packets.
  The main part of all three is a list of items, each item describing

Moy [Page 175] RFC 1583 OSPF Version 2 March 1994

  a piece of the topological database.  The sending of Database
  Description Packets is documented in Section 10.8.  The reception of
  Database Description packets is documented in Section 10.6.
  0   These fields are reserved.  They must be 0.
  Options
      The optional capabilities supported by the router, as documented
      in Section A.2.
  I-bit
      The Init bit.  When set to 1, this packet is the first in the
      sequence of Database Description Packets.
  M-bit
      The More bit.  When set to 1, it indicates that more Database
      Description Packets are to follow.
  MS-bit
      The Master/Slave bit.  When set to 1, it indicates that the
      router is the master during the Database Exchange process.
      Otherwise, the router is the slave.
  DD sequence number
      Used to sequence the collection of Database Description Packets.
      The initial value (indicated by the Init bit being set) should
      be unique.  The DD sequence number then increments until the
      complete database description has been sent.
  The rest of the packet consists of a (possibly partial) list of the
  topological database's pieces.  Each link state advertisement in the
  database is described by its link state advertisement header.  The
  link state advertisement header is documented in Section A.4.1.  It
  contains all the information required to uniquely identify both the
  advertisement and the advertisement's current instance.

Moy [Page 176] RFC 1583 OSPF Version 2 March 1994

A.3.4 The Link State Request packet

  Link State Request packets are OSPF packet type 3.  After exchanging
  Database Description packets with a neighboring router, a router may
  find that parts of its topological database are out of date.  The
  Link State Request packet is used to request the pieces of the
  neighbor's database that are more up to date.  Multiple Link State
  Request packets may need to be used.  The sending of Link State
  Request packets is the last step in bringing up an adjacency.
  A router that sends a Link State Request packet has in mind the
  precise instance of the database pieces it is requesting, defined by
  LS sequence number, LS checksum, and LS age, although these fields
  are not specified in the Link State Request Packet itself.  The
  router may receive even more recent instances in response.
  The sending of Link State Request packets is documented in Section
  10.9.  The reception of Link State Request packets is documented in
  Section 10.7.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Version #   |       3       |         Packet length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Router ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Area ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Checksum            |             AuType            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          LS type                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Link State ID                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Advertising Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
  Each advertisement requested is specified by its LS type, Link State
  ID, and Advertising Router.  This uniquely identifies the
  advertisement, but not its instance.  Link State Request packets are

Moy [Page 177] RFC 1583 OSPF Version 2 March 1994

  understood to be requests for the most recent instance (whatever
  that might be).

Moy [Page 178] RFC 1583 OSPF Version 2 March 1994

A.3.5 The Link State Update packet

  Link State Update packets are OSPF packet type 4.  These packets
  implement the flooding of link state advertisements.  Each Link
  State Update packet carries a collection of link state
  advertisements one hop further from its origin.  Several link state
  advertisements may be included in a single packet.
  Link State Update packets are multicast on those physical networks
  that support multicast/broadcast.  In order to make the flooding
  procedure reliable, flooded advertisements are acknowledged in Link
  State Acknowledgment packets.  If retransmission of certain
  advertisements is necessary, the retransmitted advertisements are
  always carried by unicast Link State Update packets.  For more
  information on the reliable flooding of link state advertisements,
  consult Section 13.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Version #   |       4       |         Packet length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Router ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Area ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Checksum            |             AuType            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      # advertisements                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                            +-+
     |                  Link state advertisements                    |
     +-                                                            +-+
     |                              ...                              |
  # advertisements
      The number of link state advertisements included in this update.

Moy [Page 179] RFC 1583 OSPF Version 2 March 1994

  The body of the Link State Update packet consists of a list of link
  state advertisements.  Each advertisement begins with a common 20
  byte header, the link state advertisement header.  This header is
  described in Section A.4.1.  Otherwise, the format of each of the
  five types of link state advertisements is different.  Their formats
  are described in Section A.4.

Moy [Page 180] RFC 1583 OSPF Version 2 March 1994

A.3.6 The Link State Acknowledgment packet

  Link State Acknowledgment Packets are OSPF packet type 5.  To make
  the flooding of link state advertisements reliable, flooded
  advertisements are explicitly acknowledged.  This acknowledgment is
  accomplished through the sending and receiving of Link State
  Acknowledgment packets.  Multiple link state advertisements can be
  acknowledged in a single Link State Acknowledgment packet.
  Depending on the state of the sending interface and the source of
  the advertisements being acknowledged, a Link State Acknowledgment
  packet is sent either to the multicast address AllSPFRouters, to the
  multicast address AllDRouters, or as a unicast.  The sending of Link
  State Acknowledgement packets is documented in Section 13.5.  The
  reception of Link State Acknowledgement packets is documented in
  Section 13.7.
  The format of this packet is similar to that of the Data Description
  packet.  The body of both packets is simply a list of link state
  advertisement headers.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Version #   |       5       |         Packet length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Router ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Area ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Checksum            |             AuType            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Authentication                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                             A                                 |
     +-                 Link State Advertisement                    -+
     |                           Header                              |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

Moy [Page 181] RFC 1583 OSPF Version 2 March 1994

  Each acknowledged link state advertisement is described by its link
  state advertisement header.  The link state advertisement header is
  documented in Section A.4.1.  It contains all the information
  required to uniquely identify both the advertisement and the
  advertisement's current instance.

Moy [Page 182] RFC 1583 OSPF Version 2 March 1994

A.4 Link state advertisement formats

  There are five distinct types of link state advertisements.  Each
  link state advertisement begins with a standard 20-byte link state
  advertisement header.  This header is explained in Section A.4.1.
  Succeeding sections then diagram the separate link state
  advertisement types.
  Each link state advertisement describes a piece of the OSPF routing
  domain.  Every router originates a router links advertisement.  In
  addition, whenever the router is elected Designated Router, it
  originates a network links advertisement.  Other types of link state
  advertisements may also be originated (see Section 12.4).  All link
  state advertisements are then flooded throughout the OSPF routing
  domain.  The flooding algorithm is reliable, ensuring that all
  routers have the same collection of link state advertisements.  (See
  Section 13 for more information concerning the flooding algorithm).
  This collection of advertisements is called the link state (or
  topological) database.
  From the link state database, each router constructs a shortest path
  tree with itself as root.  This yields a routing table (see Section
  11).  For the details of the routing table build process, see
  Section 16.

Moy [Page 183] RFC 1583 OSPF Version 2 March 1994

A.4.1 The Link State Advertisement header

  All link state advertisements begin with a common 20 byte header.
  This header contains enough information to uniquely identify the
  advertisement (LS type, Link State ID, and Advertising Router).
  Multiple instances of the link state advertisement may exist in the
  routing domain at the same time.  It is then necessary to determine
  which instance is more recent.  This is accomplished by examining
  the LS age, LS sequence number and LS checksum fields that are also
  contained in the link state advertisement header.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            LS age             |    Options    |    LS type    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Link State ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Advertising Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     LS sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         LS checksum           |             length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  LS age
      The time in seconds since the link state advertisement was
      originated.
  Options
      The optional capabilities supported by the described portion of
      the routing domain.  OSPF's optional capabilities are documented
      in Section A.2.
  LS type
      The type of the link state advertisement.  Each link state type
      has a separate advertisement format.  The link state types are
      as follows (see Section 12.1.3 for further explanation):

Moy [Page 184] RFC 1583 OSPF Version 2 March 1994

                      LS Type   Description
                      ___________________________________
                      1         Router links
                      2         Network links
                      3         Summary link (IP network)
                      4         Summary link (ASBR)
                      5         AS external link
  Link State ID
      This field identifies the portion of the internet environment
      that is being described by the advertisement.  The contents of
      this field depend on the advertisement's LS type.  For example,
      in network links advertisements the Link State ID is set to the
      IP interface address of the network's Designated Router (from
      which the network's IP address can be derived).  The Link State
      ID is further discussed in Section 12.1.4.
  Advertising Router
      The Router ID of the router that originated the link state
      advertisement.  For example, in network links advertisements
      this field is set to the Router ID of the network's Designated
      Router.
  LS sequence number
      Detects old or duplicate link state advertisements.  Successive
      instances of a link state advertisement are given successive LS
      sequence numbers.  See Section 12.1.6 for more details.
  LS checksum
      The Fletcher checksum of the complete contents of the link state
      advertisement, including the link state advertisement header but
      excepting the LS age field. See Section 12.1.7 for more details.
  length
      The length in bytes of the link state advertisement.  This
      includes the 20 byte link state advertisement header.

Moy [Page 185] RFC 1583 OSPF Version 2 March 1994

A.4.2 Router links advertisements

  Router links advertisements are the Type 1 link state
  advertisements.  Each router in an area originates a router links
  advertisement.  The advertisement describes the state and cost of
  the router's links (i.e., interfaces) to the area.  All of the
  router's links to the area must be described in a single router
  links advertisement.  For details concerning the construction of
  router links advertisements, see Section 12.4.1.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            LS age             |     Options   |       1       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Link State ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Advertising Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     LS sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         LS checksum           |             length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    0    |V|E|B|        0      |            # links            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Link ID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Link Data                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     # TOS     |        TOS 0 metric           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      TOS      |        0      |            metric             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      TOS      |        0      |            metric             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Link ID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Link Data                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
  In router links advertisements, the Link State ID field is set to
  the router's OSPF Router ID.  The T-bit is set in the
  advertisement's Option field if and only if the router is able to

Moy [Page 186] RFC 1583 OSPF Version 2 March 1994

  calculate a separate set of routes for each IP TOS.  Router links
  advertisements are flooded throughout a single area only.
  bit V
      When set, the router is an endpoint of an active virtual link
      that is using the described area as a Transit area (V is for
      virtual link endpoint).
  bit E
      When set, the router is an AS boundary router (E is for
      external)
  bit B
      When set, the router is an area border router (B is for border)
  # links
      The number of router links described by this advertisement.
      This must be the total collection of router links (i.e.,
      interfaces) to the area.
  The following fields are used to describe each router link (i.e.,
  interface). Each router link is typed (see the below Type field).
  The Type field indicates the kind of link being described.  It may
  be a link to a transit network, to another router or to a stub
  network.  The values of all the other fields describing a router
  link depend on the link's Type.  For example, each link has an
  associated 32-bit data field.  For links to stub networks this field
  specifies the network's IP address mask.  For other link types the
  Link Data specifies the router's associated IP interface address.
  Type
      A quick description of the router link.  One of the following.
      Note that host routes are classified as links to stub networks
      whose network mask is 0xffffffff.
               Type   Description
               __________________________________________________
               1      Point-to-point connection to another router
               2      Connection to a transit network
               3      Connection to a stub network
               4      Virtual link

Moy [Page 187] RFC 1583 OSPF Version 2 March 1994

  Link ID
      Identifies the object that this router link connects to.  Value
      depends on the link's Type.  When connecting to an object that
      also originates a link state advertisement (i.e., another router
      or a transit network) the Link ID is equal to the neighboring
      advertisement's Link State ID.  This provides the key for
      looking up said advertisement in the link state database.  See
      Section 12.2 for more details.
                     Type   Link ID
                     ______________________________________
                     1      Neighboring router's Router ID
                     2      IP address of Designated Router
                     3      IP network/subnet number
                     4      Neighboring router's Router ID
  Link Data
      Contents again depend on the link's Type field. For connections
      to stub networks, it specifies the network's IP address mask.
      For unnumbered point-to-point connections, it specifies the
      interface's MIB-II [RFC 1213] ifIndex value. For the other link
      types it specifies the router's associated IP interface address.
      This latter piece of information is needed during the routing
      table build process, when calculating the IP address of the next
      hop. See Section 16.1.1 for more details.
  # TOS
      The number of different TOS metrics given for this link, not
      counting the required metric for TOS 0.  For example, if no
      additional TOS metrics are given, this field should be set to 0.
  TOS 0 metric
      The cost of using this router link for TOS 0.
  For each link, separate metrics may be specified for each Type of
  Service (TOS).  The metric for TOS 0 must always be included, and
  was discussed above.  Metrics for non-zero TOS are described below.
  The encoding of TOS in OSPF link state advertisements is described
  in Section 12.3.  Note that the cost for non-zero TOS values that
  are not specified defaults to the TOS 0 cost.  Metrics must be
  listed in order of increasing TOS encoding.  For example, the metric
  for TOS 16 must always follow the metric for TOS 8 when both are

Moy [Page 188] RFC 1583 OSPF Version 2 March 1994

  specified.
  TOS IP Type of Service that this metric refers to.  The encoding of
      TOS in OSPF link state advertisements is described in Section
      12.3.
  metric
      The cost of using this outbound router link, for traffic of the
      specified TOS.

Moy [Page 189] RFC 1583 OSPF Version 2 March 1994

A.4.3 Network links advertisements

  Network links advertisements are the Type 2 link state
  advertisements.  A network links advertisement is originated for
  each transit network in the area.  A transit network is a multi-
  access network that has more than one attached router.  The network
  links advertisement is originated by the network's Designated
  Router.  The advertisement describes all routers attached to the
  network, including the Designated Router itself.  The
  advertisement's Link State ID field lists the IP interface address
  of the Designated Router.
  The distance from the network to all attached routers is zero, for
  all Types of Service.  This is why the TOS and metric fields need
  not be specified in the network links advertisement.  For details
  concerning the construction of network links advertisements, see
  Section 12.4.2.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            LS age             |      Options  |      2        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Link State ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Advertising Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     LS sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         LS checksum           |             length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Network Mask                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Attached Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
  Network Mask
      The IP address mask for the network.  For example, a class A
      network would have the mask 0xff000000.
  Attached Router
      The Router IDs of each of the routers attached to the network.
      Actually, only those routers that are fully adjacent to the
      Designated Router are listed.  The Designated Router includes

Moy [Page 190] RFC 1583 OSPF Version 2 March 1994

      itself in this list.  The number of routers included can be
      deduced from the link state advertisement header's length field.

Moy [Page 191] RFC 1583 OSPF Version 2 March 1994

A.4.4 Summary link advertisements

  Summary link advertisements are the Type 3 and 4 link state
  advertisements.  These advertisements are originated by area border
  routers.  A separate summary link advertisement is made for each
  destination (known to the router) which belongs to the AS, yet is
  outside the area.  For details concerning the construction of
  summary link advertisements, see Section 12.4.3.
  Type 3 link state advertisements are used when the destination is an
  IP network.  In this case the advertisement's Link State ID field is
  an IP network number (if necessary, the Link State ID can also have
  one or more of the network's "host" bits set; see Appendix F for
  details). When the destination is an AS boundary router, a Type 4
  advertisement is used, and the Link State ID field is the AS
  boundary router's OSPF Router ID.  (To see why it is necessary to
  advertise the location of each ASBR, consult Section 16.4.)  Other
  than the difference in the Link State ID field, the format of Type 3
  and 4 link state advertisements is identical.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            LS age             |     Options   |    3 or 4     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Link State ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Advertising Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     LS sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         LS checksum           |             length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Network Mask                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TOS       |                  metric                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
  For stub areas, Type 3 summary link advertisements can also be used
  to describe a (per-area) default route.  Default summary routes are
  used in stub areas instead of flooding a complete set of external
  routes.  When describing a default summary route, the
  advertisement's Link State ID is always set to DefaultDestination
  (0.0.0.0) and the Network Mask is set to 0.0.0.0.

Moy [Page 192] RFC 1583 OSPF Version 2 March 1994

  Separate costs may be advertised for each IP Type of Service.  The
  encoding of TOS in OSPF link state advertisements is described in
  Section 12.3.  Note that the cost for TOS 0 must be included, and is
  always listed first.  If the T-bit is reset in the advertisement's
  Option field, only a route for TOS 0 is described by the
  advertisement.  Otherwise, routes for the other TOS values are also
  described; if a cost for a certain TOS is not included, its cost
  defaults to that specified for TOS 0.
  Network Mask
      For Type 3 link state advertisements, this indicates the
      destination network's IP address mask.  For example, when
      advertising the location of a class A network the value
      0xff000000 would be used.  This field is not meaningful and must
      be zero for Type 4 link state advertisements.
  For each specified Type of Service, the following fields are
  defined.  The number of TOS routes included can be calculated from
  the link state advertisement header's length field.  Values for TOS
  0 must be specified; they are listed first.  Other values must be
  listed in order of increasing TOS encoding.  For example, the cost
  for TOS 16 must always follow the cost for TOS 8 when both are
  specified.
  TOS The Type of Service that the following cost concerns.  The
      encoding of TOS in OSPF link state advertisements is described
      in Section 12.3.
  metric
      The cost of this route.  Expressed in the same units as the
      interface costs in the router links advertisements.

Moy [Page 193] RFC 1583 OSPF Version 2 March 1994

A.4.5 AS external link advertisements

  AS external link advertisements are the Type 5 link state
  advertisements.  These advertisements are originated by AS boundary
  routers.  A separate advertisement is made for each destination
  (known to the router) which is external to the AS.  For details
  concerning the construction of AS external link advertisements, see
  Section 12.4.3.
  AS external link advertisements usually describe a particular
  external destination.  For these advertisements the Link State ID
  field specifies an IP network number (if necessary, the Link State
  ID can also have one or more of the network's "host" bits set; see
  Appendix F for details).  AS external link advertisements are also
  used to describe a default route.  Default routes are used when no
  specific route exists to the destination.  When describing a default
  route, the Link State ID is always set to DefaultDestination
  (0.0.0.0) and the Network Mask is set to 0.0.0.0.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            LS age             |     Options   |      5        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Link State ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Advertising Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     LS sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         LS checksum           |             length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Network Mask                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |E|    TOS      |                  metric                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Forwarding address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      External Route Tag                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
  Separate costs may be advertised for each IP Type of Service.  The
  encoding of TOS in OSPF link state advertisements is described in
  Section 12.3.  Note that the cost for TOS 0 must be included, and is

Moy [Page 194] RFC 1583 OSPF Version 2 March 1994

  always listed first.  If the T-bit is reset in the advertisement's
  Option field, only a route for TOS 0 is described by the
  advertisement.  Otherwise, routes for the other TOS values are also
  described; if a cost for a certain TOS is not included, its cost
  defaults to that specified for TOS 0.
  Network Mask
      The IP address mask for the advertised destination.  For
      example, when advertising a class A network the mask 0xff000000
      would be used.
  For each specified Type of Service, the following fields are
  defined.  The number of TOS routes included can be calculated from
  the link state advertisement header's length field.  Values for TOS
  0 must be specified; they are listed first.  Other values must be
  listed in order of increasing TOS encoding.  For example, the cost
  for TOS 16 must always follow the cost for TOS 8 when both are
  specified.
  bit E
      The type of external metric.  If bit E is set, the metric
      specified is a Type 2 external metric.  This means the metric is
      considered larger than any link state path.  If bit E is zero,
      the specified metric is a Type 1 external metric.  This means
      that is is comparable directly (without translation) to the link
      state metric.
  Forwarding address
      Data traffic for the advertised destination will be forwarded to
      this address.  If the Forwarding address is set to 0.0.0.0, data
      traffic will be forwarded instead to the advertisement's
      originator (i.e., the responsible AS boundary router).
  TOS The Type of Service that the following cost concerns.  The
      encoding of TOS in OSPF link state advertisements is described
      in Section 12.3.
  metric
      The cost of this route.  Interpretation depends on the external
      type indication (bit E above).
  External Route Tag
      A 32-bit field attached to each external route.  This is not
      used by the OSPF protocol itself.  It may be used to communicate
      information between AS boundary routers; the precise nature of
      such information is outside the scope of this specification.

Moy [Page 195] RFC 1583 OSPF Version 2 March 1994

B. Architectural Constants

  Several OSPF protocol parameters have fixed architectural values.
  These parameters have been referred to in the text by names such as
  LSRefreshTime.  The same naming convention is used for the
  configurable protocol parameters.  They are defined in Appendix C.
  The name of each architectural constant follows, together with its
  value and a short description of its function.
  LSRefreshTime
      The maximum time between distinct originations of any particular
      link state advertisement.  When the LS age field of one of the
      router's self-originated advertisements reaches the value
      LSRefreshTime, a new instance of the link state advertisement is
      originated, even though the contents of the advertisement (apart
      from the link state header) will be the same.  The value of
      LSRefreshTime is set to 30 minutes.
  MinLSInterval
      The minimum time between distinct originations of any particular
      link state advertisement.  The value of MinLSInterval is set to
      5 seconds.
  MaxAge
      The maximum age that a link state advertisement can attain. When
      an advertisement's LS age field reaches MaxAge, it is reflooded
      in an attempt to flush the advertisement from the routing domain
      (See Section 14). Advertisements of age MaxAge are not used in
      the routing table calculation.  The value of MaxAge must be
      greater than LSRefreshTime.  The value of MaxAge is set to 1
      hour.
  CheckAge
      When the age of a link state advertisement (that is contained in
      the link state database) hits a multiple of CheckAge, the
      advertisement's checksum is verified.  An incorrect checksum at
      this time indicates a serious error.  The value of CheckAge is
      set to 5 minutes.
  MaxAgeDiff
      The maximum time dispersion that can occur, as a link state
      advertisement is flooded throughout the AS.  Most of this time
      is accounted for by the link state advertisements sitting on
      router output queues (and therefore not aging) during the
      flooding process.  The value of MaxAgeDiff is set to 15 minutes.

Moy [Page 196] RFC 1583 OSPF Version 2 March 1994

  LSInfinity
      The metric value indicating that the destination described by a
      link state advertisement is unreachable. Used in summary link
      advertisements and AS external link advertisements as an
      alternative to premature aging (see Section 14.1). It is defined
      to be the 24-bit binary value of all ones: 0xffffff.
  DefaultDestination
      The Destination ID that indicates the default route.  This route
      is used when no other matching routing table entry can be found.
      The default destination can only be advertised in AS external
      link advertisements and in stub areas' type 3 summary link
      advertisements.  Its value is the IP address 0.0.0.0.

Moy [Page 197] RFC 1583 OSPF Version 2 March 1994

C. Configurable Constants

  The OSPF protocol has quite a few configurable parameters.  These
  parameters are listed below.  They are grouped into general
  functional categories (area parameters, interface parameters, etc.).
  Sample values are given for some of the parameters.
  Some parameter settings need to be consistent among groups of
  routers.  For example, all routers in an area must agree on that
  area's parameters, and all routers attached to a network must agree
  on that network's IP network number and mask.
  Some parameters may be determined by router algorithms outside of
  this specification (e.g., the address of a host connected to the
  router via a SLIP line).  From OSPF's point of view, these items are
  still configurable.
  C.1 Global parameters
      In general, a separate copy of the OSPF protocol is run for each
      area.  Because of this, most configuration parameters are
      defined on a per-area basis.  The few global configuration
      parameters are listed below.
      Router ID
          This is a 32-bit number that uniquely identifies the router
          in the Autonomous System.  One algorithm for Router ID
          assignment is to choose the largest or smallest IP address
          assigned to the router.  If a router's OSPF Router ID is
          changed, the router's OSPF software should be restarted
          before the new Router ID takes effect. Before restarting in
          order to change its Router ID, the router should flush its
          self-originated link state advertisements from the routing
          domain (see Section 14.1), or they will persist for up to
          MaxAge minutes.
      TOS capability
          This item indicates whether the router will calculate
          separate routes based on TOS.  For more information, see
          Sections 4.5 and 16.9.
  C.2 Area parameters
      All routers belonging to an area must agree on that area's
      configuration.  Disagreements between two routers will lead to
      an inability for adjacencies to form between them, with a
      resulting hindrance to the flow of routing protocol and data

Moy [Page 198] RFC 1583 OSPF Version 2 March 1994

      traffic.  The following items must be configured for an area:
      Area ID
          This is a 32-bit number that identifies the area.  The Area
          ID of 0.0.0.0 is reserved for the backbone.  If the area
          represents a subnetted network, the IP network number of the
          subnetted network may be used for the Area ID.
      List of address ranges
          An OSPF area is defined as a list of address ranges. Each
          address range consists of the following items:
          [IP address, mask]
                  Describes the collection of IP addresses contained
                  in the address range. Networks and hosts are
                  assigned to an area depending on whether their
                  addresses fall into one of the area's defining
                  address ranges.  Routers are viewed as belonging to
                  multiple areas, depending on their attached
                  networks' area membership.
          Status  Set to either Advertise or DoNotAdvertise.  Routing
                  information is condensed at area boundaries.
                  External to the area, at most a single route is
                  advertised (via a summary link advertisement) for
                  each address range. The route is advertised if and
                  only if the address range's Status is set to
                  Advertise.  Unadvertised ranges allow the existence
                  of certain networks to be intentionally hidden from
                  other areas. Status is set to Advertise by default.
          As an example, suppose an IP subnetted network is to be its
          own OSPF area.  The area would be configured as a single
          address range, whose IP address is the address of the
          subnetted network, and whose mask is the natural class A, B,
          or C address mask.  A single route would be advertised
          external to the area, describing the entire subnetted
          network.
      AuType
          Each area can be configured for a separate type of
          authentication.  See Appendix D for a discussion of the
          defined authentication types.
      ExternalRoutingCapability
          Whether AS external advertisements will be flooded
          into/throughout the area.  If AS external advertisements are

Moy [Page 199] RFC 1583 OSPF Version 2 March 1994

          excluded from the area, the area is called a "stub".
          Internal to stub areas, routing to external destinations
          will be based solely on a default summary route.  The
          backbone cannot be configured as a stub area.  Also, virtual
          links cannot be configured through stub areas.  For more
          information, see Section 3.6.
      StubDefaultCost
          If the area has been configured as a stub area, and the
          router itself is an area border router, then the
          StubDefaultCost indicates the cost of the default summary
          link that the router should advertise into the area.  There
          can be a separate cost configured for each IP TOS.  See
          Section 12.4.3 for more information.
  C.3 Router interface parameters
      Some of the configurable router interface parameters (such as IP
      interface address and subnet mask) actually imply properties of
      the attached networks, and therefore must be consistent across
      all the routers attached to that network.  The parameters that
      must be configured for a router interface are:
      IP interface address
          The IP protocol address for this interface.  This uniquely
          identifies the router over the entire internet.  An IP
          address is not required on serial lines.  Such a serial line
          is called "unnumbered".
      IP interface mask
          Also referred to as the subnet mask, this indicates the
          portion of the IP interface address that identifies the
          attached network.  Masking the IP interface address with the
          IP interface mask yields the IP network number of the
          attached network.  On point-to-point networks and virtual
          links, the IP interface mask is not defined. On these
          networks, the link itself is not assigned an IP network
          number, and so the addresses of each side of the link are
          assigned independently, if they are assigned at all.
      Interface output cost(s)
          The cost of sending a packet on the interface, expressed in
          the link state metric.  This is advertised as the link cost
          for this interface in the router's router links
          advertisement.  There may be a separate cost for each IP
          Type of Service.  The interface output cost(s) must always
          be greater than 0.

Moy [Page 200] RFC 1583 OSPF Version 2 March 1994

      RxmtInterval
          The number of seconds between link state advertisement
          retransmissions, for adjacencies belonging to this
          interface.  Also used when retransmitting Database
          Description and Link State Request Packets.  This should be
          well over the expected round-trip delay between any two
          routers on the attached network.  The setting of this value
          should be conservative or needless retransmissions will
          result.  It will need to be larger on low speed serial lines
          and virtual links.  Sample value for a local area network: 5
          seconds.
      InfTransDelay
          The estimated number of seconds it takes to transmit a Link
          State Update Packet over this interface.  Link state
          advertisements contained in the update packet must have
          their age incremented by this amount before transmission.
          This value should take into account the transmission and
          propagation delays of the interface.  It must be greater
          than 0.  Sample value for a local area network: 1 second.
      Router Priority
          An 8-bit unsigned integer.  When two routers attached to a
          network both attempt to become Designated Router, the one
          with the highest Router Priority takes precedence.  If there
          is still a tie, the router with the highest Router ID takes
          precedence.  A router whose Router Priority is set to 0 is
          ineligible to become Designated Router on the attached
          network.  Router Priority is only configured for interfaces
          to multi-access networks.
      HelloInterval
          The length of time, in seconds, between the Hello Packets
          that the router sends on the interface.  This value is
          advertised in the router's Hello Packets.  It must be the
          same for all routers attached to a common network.  The
          smaller the HelloInterval, the faster topological changes
          will be detected, but more OSPF routing protocol traffic
          will ensue.  Sample value for a X.25 PDN network: 30
          seconds.  Sample value for a local area network: 10 seconds.
      RouterDeadInterval
          After ceasing to hear a router's Hello Packets, the number
          of seconds before its neighbors declare the router down.
          This is also advertised in the router's Hello Packets in
          their RouterDeadInterval field.  This should be some
          multiple of the HelloInterval (say 4).  This value again
          must be the same for all routers attached to a common

Moy [Page 201] RFC 1583 OSPF Version 2 March 1994

          network.
      Authentication key
          This configured data allows the authentication procedure to
          generate and/or verify the authentication field in the OSPF
          header.  This value again must be the same for all routers
          attached to a common network.  For example, if the AuType
          indicates simple password, the Authentication key would be a
          64-bit password. This key would be inserted directly into
          the OSPF header when originating routing protocol packets.
          There could be a separate password for each network.
  C.4 Virtual link parameters
      Virtual links are used to restore/increase connectivity of the
      backbone.  Virtual links may be configured between any pair of
      area border routers having interfaces to a common (non-backbone)
      area.  The virtual link appears as an unnumbered point-to-point
      link in the graph for the backbone.  The virtual link must be
      configured in both of the area border routers.
      A virtual link appears in router links advertisements (for the
      backbone) as if it were a separate router interface to the
      backbone.  As such, it has all of the parameters associated with
      a router interface (see Section C.3).  Although a virtual link
      acts like an unnumbered point-to-point link, it does have an
      associated IP interface address.  This address is used as the IP
      source in OSPF protocol packets it sends along the virtual link,
      and is set dynamically during the routing table build process.
      Interface output cost is also set dynamically on virtual links
      to be the cost of the intra-area path between the two routers.
      The parameter RxmtInterval must be configured, and should be
      well over the expected round-trip delay between the two routers.
      This may be hard to estimate for a virtual link; it is better to
      err on the side of making it too large.  Router Priority is not
      used on virtual links.
      A virtual link is defined by the following two configurable
      parameters: the Router ID of the virtual link's other endpoint,
      and the (non-backbone) area through which the virtual link runs
      (referred to as the virtual link's Transit area).  Virtual links
      cannot be configured through stub areas.
  C.5 Non-broadcast, multi-access network parameters
      OSPF treats a non-broadcast, multi-access network much like it
      treats a broadcast network.  Since there may be many routers
      attached to the network, a Designated Router is selected for the

Moy [Page 202] RFC 1583 OSPF Version 2 March 1994

      network.  This Designated Router then originates a networks
      links advertisement, which lists all routers attached to the
      non-broadcast network.
      However, due to the lack of broadcast capabilities, it is
      necessary to use configuration parameters in the Designated
      Router selection.  These parameters need only be configured in
      those routers that are themselves eligible to become Designated
      Router (i.e., those router's whose Router Priority for the
      network is non-zero):
      List of all other attached routers
          The list of all other routers attached to the non-broadcast
          network.  Each router is listed by its IP interface address
          on the network.  Also, for each router listed, that router's
          eligibility to become Designated Router must be defined.
          When an interface to a non-broadcast network comes up, the
          router sends Hello Packets only to those neighbors eligible
          to become Designated Router, until the identity of the
          Designated Router is discovered.
      PollInterval
          If a neighboring router has become inactive (Hello Packets
          have not been seen for RouterDeadInterval seconds), it may
          still be necessary to send Hello Packets to the dead
          neighbor.  These Hello Packets will be sent at the reduced
          rate PollInterval, which should be much larger than
          HelloInterval.  Sample value for a PDN X.25 network: 2
          minutes.
  C.6 Host route parameters
      Host routes are advertised in router links advertisements as
      stub networks with mask 0xffffffff.  They indicate either router
      interfaces to point-to-point networks, looped router interfaces,
      or IP hosts that are directly connected to the router (e.g., via
      a SLIP line).  For each host directly connected to the router,
      the following items must be configured:
      Host IP address
          The IP address of the host.
      Cost of link to host
          The cost of sending a packet to the host, in terms of the
          link state metric.  There may be multiple costs configured,
          one for each IP TOS.  However, since the host probably has

Moy [Page 203] RFC 1583 OSPF Version 2 March 1994

          only a single connection to the internet, the actual
          configured cost(s) in many cases is unimportant (i.e., will
          have no effect on routing).

Moy [Page 204] RFC 1583 OSPF Version 2 March 1994

D. Authentication

  All OSPF protocol exchanges are authenticated.  The OSPF packet
  header (see Section A.3.1) includes an authentication type field,
  and 64-bits of data for use by the appropriate authentication scheme
  (determined by the type field).
  The authentication type is configurable on a per-area basis.
  Additional authentication data is configurable on a per-interface
  basis.  For example, if an area uses a simple password scheme for
  authentication, a separate password may be configured for each
  network contained in the area.
  Authentication types 0 and 1 are defined by this specification.  All
  other authentication types are reserved for definition by the IANA
  (iana@ISI.EDU).  The current list of authentication types is
  described below in Table 20.
                AuType       Description
                ___________________________________________
                0            No authentication
                1            Simple password
                All others   Reserved for assignment by the
                             IANA (iana@ISI.EDU)
                    Table 20: OSPF authentication types.
  D.1 AuType 0 -- No authentication
      Use of this authentication type means that routing exchanges in
      the area are not authenticated.  The 64-bit field in the OSPF
      header can contain anything; it is not examined on packet
      reception.
  D.2 AuType 1 -- Simple password
      Using this authentication type, a 64-bit field is configured on
      a per-network basis.  All packets sent on a particular network
      must have this configured value in their OSPF header 64-bit
      authentication field.  This essentially serves as a "clear" 64-
      bit password.

Moy [Page 205] RFC 1583 OSPF Version 2 March 1994

      This guards against routers inadvertently joining the area.
      They must first be configured with their attached networks'
      passwords before they can participate in the routing domain.

Moy [Page 206] RFC 1583 OSPF Version 2 March 1994

E. Differences from RFC 1247

  This section documents the differences between this memo and RFC
  1247.  These differences include a fix for a problem involving OSPF
  virtual links, together with minor enhancements and clarifications
  to the protocol. All differences are backward-compatible.
  Implementations of this memo and of RFC 1247 will interoperate.
  E.1 A fix for a problem with OSPF Virtual links
      In RFC 1247, certain configurations of OSPF virtual links can
      cause routing loops. The root of the problem is that while there
      is an information mismatch at the boundary of any virtual link's
      Transit area, a backbone path can still cross the boundary. RFC
      1247 attempted to compensate for this information mismatch by
      adjusting any backbone path as it enters the transit area (see
      Section 16.3 in RFC 1247). However, this proved not to be
      enough. This memo fixes the problem by having all area border
      routers determine, by looking at summary links, whether better
      backbone paths can be found through the transit areas.
      This fix simplifies the OSPF virtual link logic, and consists of
      the following components:
      o   A new bit has been defined in the router links
          advertisement, called bit V. Bit V is set in a router's
          router links advertisement for Area A if and only if the
          router is an endpoint of an active virtual link that uses
          Area A as its Transit area (see Sections 12.4.1 and A.4.2).
          This enables the other routers attached to Area A to
          discover whether the area supports any virtual links (i.e.,
          is a transit area). This discovery is done during the
          calculation of Area A's shortest-path tree (see Section
          16.1).
      o   To aid in the description of the algorithm, a new parameter
          has been added to the OSPF area structure:
          TransitCapability. This parameter indicates whether the area
          supports any active virtual links. Equivalently, it
          indicates whether the area can carry traffic that neither
          originates nor terminates in the area itself.
      o   The calculation in Section 16.3 of RFC 1247 has been
          replaced. The new calculation, performed by area border
          routers only, examines the summary links belonging to all
          attached transit areas to see whether the transit areas can
          provide better paths than those already found in Sections
          16.1 and 16.2.

Moy [Page 207] RFC 1583 OSPF Version 2 March 1994

      o   The incremental calculations in Section 16.5 have been
          updated as a result of the new calculations in Section 16.3.
  E.2 Supporting supernetting and subnet 0
      In RFC 1247, an OSPF router cannot originate separate AS
      external link advertisements (or separate summary link
      advertisements) for two networks that have the same address but
      different masks. This situation can arise when subnet 0 of a
      network has been assigned (a practice that is generally
      discouraged), or when using supernetting as described in [RFC
      1519] (a practice that is generally encouraged to reduce the
      size of routing tables), or even when in transition from one
      mask to another on a subnet.  Using supernetting as an example,
      you might want to aggregate the four class C networks
      192.9.4.0-192.9.7.0, advertising one route for the aggregation
      and another for the single class C network 192.9.4.0.
      The reason behind this limitation is that in RFC 1247, the Link
      State ID of AS external link advertisements and summary link
      advertisements is set equal to the described network's IP
      address. In the above example, RFC 1247 would assign both
      advertisements the Link State ID of 192.9.4.0, making them in
      essence the same advertisement. This memo fixes the problem by
      relaxing the setting of the Link State ID so that any of the
      "host" bits of the network address can also be set. This allows
      you to disambiguate advertisements for networks having the same
      address but different masks. Given an AS external link
      advertisement (or a summary link advertisement), the described
      network's address can now be obtained by masking the Link State
      ID with the network mask carried in the body of the
      advertisement.  Again using the above example, the aggregate can
      now be advertised using a Link State ID of 192.9.4.0 and the
      single class C network advertised simultaneously using the Link
      State ID of 192.9.4.255.
      Appendix F gives one possible algorithm for setting one or more
      "host" bits in the Link State ID in order to disambiguate
      advertisements. It should be noted that this is a local
      decision. Each router in an OSPF system is free to use its own
      algorithm, since only those advertisements originated by the
      router itself are affected.
      It is believed that this change will be more or less compatible
      with implementations of RFC 1247. Implementations of RFC 1247
      will probably either a) install routing table entries that won't
      be used or b) do the correct processing as outlined in this memo
      or c) mark the advertisement as unusable when presented with a

Moy [Page 208] RFC 1583 OSPF Version 2 March 1994

      Link State ID that has one or more of the host bits set.
      However, in the interest of interoperability, implementations of
      this memo should only set the host bits in Link State IDs when
      absolutely necessary.
      The change affects Sections 12.1.4, 12.4.3, 12.4.5, 16.2, 16.3,
      16.4, 16.5, 16.6, A.4.4 and A.4.5.
  E.3 Obsoleting LSInfinity in router links advertisements
      The metric of LSInfinity can no longer be used in router links
      advertisements to indicate unusable links. This is being done
      for several reasons:
      o   It removes any possible confusion in an OSPF area as to just
          which routers/networks are reachable in the area. For
          example, the above virtual link fix relies on detecting the
          existence of virtual links when running the Dijkstra.
          However, when one-directional links (i.e., cost of
          LSInfinity in one direction, but not the other) are
          possible, some routers may detect the existence of virtual
          links while others may not. This may defeat the fix for the
          virtual link problem.
      o   It also helps OSPF's Multicast routing extensions (MOSPF),
          because one-way reachability can lead to places that are
          reachable via unicast but not multicast, or vice versa.
      The two prior justifications for using LSInfinity in router
      links advertisements were 1) it was a way to not support TOS
      before TOS was optional and 2) it went along with strong TOS
      interpretations. These justifications are no longer valid.
      However, LSInfinity will continue to mean "unreachable" in
      summary link advertisements and AS external link advertisements,
      as some implementations use this as an alternative to the
      premature aging procedure specified in Section 14.1.
      This change has one other side effect. When two routers are
      connected via a virtual link whose underlying path is non-TOS-
      capable, they must now revert to being non-TOS-capable routers
      themselves, instead of the previous behavior of advertising the
      non-zero TOS costs of the virtual link as LSInfinity. See
      Section 15 for details.
  E.4 TOS encoding updated
      The encoding of TOS in OSPF link state advertisements has been
      updated to reflect the new TOS value (minimize monetary cost)

Moy [Page 209] RFC 1583 OSPF Version 2 March 1994

      defined by [RFC 1349]. The OSPF encoding is defined in Section
      12.3, which is identical in content to Section A.5 of [RFC
      1349].
  E.5 Summarizing routes into transit areas
      RFC 1247 mandated that routes associated with Area A are never
      summarized back into Area A. However, this memo further reduces
      the number of summary links originated by refusing to summarize
      into Area A those routes having next hops belonging to Area A.
      This is an optimization over RFC 1247 behavior when virtual
      links are present.  For example, in the area configuration of
      Figure 6, Router RT11 need only originate a single summary link
      having the (collapsed) destination N9-N11,H1 into its connected
      transit area Area 2, since all of its other eligible routes have
      next hops belonging to Area 2 (and as such only need be
      advertised by other area border routers; in this case, Routers
      RT10 and RT7). This is the logical equivalent of a Distance
      Vector protocol's split horizon logic.
      This change appears in Section 12.4.3.
  E.6 Summarizing routes into stub areas
      RFC 1247 mandated that area border routers attached to stub
      areas must summarize all inter-area routes into the stub areas.
      However, while area border routers connected to OSPF stub areas
      must originate default summary links into the stub area, they
      need not summarize other routes into the stub area. The amount
      of summarization done into stub areas can instead be put under
      configuration control. The network administrator can then make
      the trade-off between optimal routing and database size.
      This change appears in Sections 12.4.3 and 12.4.4.
  E.7 Flushing anomalous network links advertisements
      Text was added indicating that a network links advertisement
      whose Link State ID is equal to one of the router's own IP
      interface addresses should be considered to be self-originated,
      regardless of the setting of the advertisement's Advertising
      Router. If the Advertising Router of such an advertisement is
      not equal to the router's own Router ID, the advertisement
      should be flushed from the routing domain using the premature
      aging procedure specified in Section 14.1. This case should be
      rare, and it indicates that the router's Router ID has changed
      since originating the advertisement.

Moy [Page 210] RFC 1583 OSPF Version 2 March 1994

      Failure to flush these anomalous advertisements could lead to
      multiple network links advertisements having the same Link State
      ID. This in turn could cause the Dijkstra calculation in Section
      16.1 to fail, since it would be impossible to tell which network
      links advertisement is valid (i.e., more recent).
      This change appears in Sections 13.4 and 14.1.
  E.8 Required Statistics appendix deleted
      Appendix D of RFC 1247, which specified a list of required
      statistics for an OSPF implementation, has been deleted. That
      appendix has been superseded by the two documents: the OSPF
      Version 2 Management Information Base and the OSPF Version 2
      Traps.
  E.9 Other changes
      The following small changes were also made to RFC 1247:
      o   When representing unnumbered point-to-point networks in
          router links advertisements, the corresponding Link Data
          field should be set to the unnumbered interface's MIB-II
          [RFC 1213] ifIndex value.
      o   A comment was added to Step 3 of the Dijkstra algorithm in
          Section 16.1. When removing vertices from the candidate
          list, and when there is a choice of vertices closest to the
          root, network vertices must be chosen before router vertices
          in order to necessarily find all equal-cost paths.
      o   A comment was added to Section 12.4.3 noting that a summary
          link advertisement cannot express a reachable destination
          whose path cost equals or exceeds LSInfinity.
      o   A comment was added to Section 15 noting that a virtual link
          whose underlying path has cost greater than hexadecimal
          0xffff (the maximum size of an interface cost in a router
          links advertisement) should be considered inoperational.
      o   An option was added to the definition of area address
          ranges, allowing the network administrator to specify that a
          particular range should not be advertised to other OSPF
          areas. This enables the existence of certain networks to be
          hidden from other areas. This change appears in Sections
          12.4.3 and C.2.

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      o   A note was added reminding implementors that bit E (the AS
          boundary router indication) should never be set in a router
          links advertisement for a stub area, since stub areas cannot
          contain AS boundary routers.  This change appears in Section
          12.4.1.

Moy [Page 212] RFC 1583 OSPF Version 2 March 1994

F. An algorithm for assigning Link State IDs

  In RFC 1247, the Link State ID in AS external link advertisements
  and summary link advertisements is set to the described network's IP
  address. This memo relaxes that requirement, allowing one or more of
  the network's host bits to be set in the Link State ID. This allows
  the router to originate separate advertisements for networks having
  the same addresses, yet different masks. Such networks can occur in
  the presence of supernetting and subnet 0s (see Section E.2 for more
  information).
  This appendix gives one possible algorithm for setting the host bits
  in Link State IDs.  The choice of such an algorithm is a local
  decision. Separate routers are free to use different algorithms,
  since the only advertisements affected are the ones that the router
  itself originates. The only requirement on the algorithms used is
  that the network's IP address should be used as the Link State ID
  (the RFC 1247 behavior) whenever possible.
  The algorithm below is stated for AS external link advertisements.
  This is only for clarity; the exact same algorithm can be used for
  summary link advertisements. Suppose that the router wishes to
  originate an AS external link advertisement for a network having
  address NA and mask NM1. The following steps are then used to
  determine the advertisement's Link State ID:
  (1) Determine whether the router is already originating an AS
      external link advertisement with Link State ID equal to NA (in
      such an advertisement the router itself will be listed as the
      advertisement's Advertising Router).  If not, set the Link State
      ID equal to NA (the RFC 1247 behavior) and the algorithm
      terminates. Otherwise,
  (2) Obtain the network mask from the body of the already existing AS
      external link advertisement. Call this mask NM2. There are then
      two cases:
      o   NM1 is longer (i.e., more specific) than NM2. In this case,
          set the Link State ID in the new advertisement to be the
          network [NA,NM1] with all the host bits set (i.e., equal to
          NA or'ed together with all the bits that are not set in NM1,
          which is network [NA,NM1]'s broadcast address).
      o   NM2 is longer than NM1. In this case, change the existing
          advertisement (having Link State ID of NA) to reference the
          new network [NA,NM1] by incrementing the sequence number,
          changing the mask in the body to NM1 and using the cost for
          the new network. Then originate a new advertisement for the

Moy [Page 213] RFC 1583 OSPF Version 2 March 1994

          old network [NA,NM2], with Link State ID equal to NA or'ed
          together with the bits that are not set in NM2 (i.e.,
          network [NA,NM2]'s broadcast address).
  The above algorithm assumes that all masks are contiguous; this
  ensures that when two networks have the same address, one mask is
  more specific than the other. The algorithm also assumes that no
  network exists having an address equal to another network's
  broadcast address. Given these two assumptions, the above algorithm
  always produces unique Link State IDs. The above algorithm can also
  be reworded as follows: When originating an AS external link state
  advertisement, try to use the network number as the Link State ID.
  If that produces a conflict, examine the two networks in conflict.
  One will be a subset of the other. For the less specific network,
  use the network number as the Link State ID and for the more
  specific use the network's broadcast address instead (i.e., flip all
  the "host" bits to 1).  If the most specific network was originated
  first, this will cause you to originate two link state
  advertisements at once.
  As an example of the algorithm, consider its operation when the
  following sequence of events occurs in a single router (Router A).
  (1) Router A wants to originate an AS external link advertisement
      for [10.0.0.0,255.255.255.0]:
      (a) A Link State ID of 10.0.0.0 is used.
  (2) Router A then wants to originate an AS external link
      advertisement for [10.0.0.0,255.255.0.0]:
      (a) The advertisement for [10.0.0,0,255.255.255.0] is
          reoriginated using a new Link State ID of 10.0.0.255.
      (b) A Link State ID of 10.0.0.0 is used for
          [10.0.0.0,255.255.0.0].
  (3) Router A then wants to originate an AS external link
      advertisement for [10.0.0.0,255.0.0.0]:
      (a) The advertisement for [10.0.0.0,255.255.0.0] is reoriginated
          using a new Link State ID of 10.0.255.255.
      (b) A Link State ID of 10.0.0.0 is used for
          [10.0.0.0,255.0.0.0].

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      (c) The network [10.0.0.0,255.255.255.0] keeps its Link State ID
          of 10.0.0.255.

Moy [Page 215] RFC 1583 OSPF Version 2 March 1994

Security Considerations

  All OSPF protocol exchanges are authenticated. This is accomplished
  through authentication fields contained in the OSPF packet header.
  For more information, see Sections 8.1, 8.2, and Appendix D.

Author's Address

  John Moy
  Proteon, Inc.
  9 Technology Drive
  Westborough, MA 01581
  Phone: 508-898-2800
  Fax:   508-898-3176
  Email: jmoy@proteon.com

Moy [Page 216]

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