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Network Working Group J. Moy Request for Comments: 2328 Ascend Communications, Inc. STD: 54 April 1998 Obsoletes: 2178 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.

Copyright Notice

  Copyright (C) The Internet Society (1998).  All Rights Reserved.

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.  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.
  The differences between this memo and RFC 2178 are explained in
  Appendix G. All differences are backward-compatible in nature.

Moy Standards Track [Page 1] RFC 2328 OSPF Version 2 April 1998

  Implementations of this memo and of RFCs 2178, 1583, and 1247 will
  interoperate.
  Please send comments to ospf@gated.cornell.edu.

Table of Contents

  1        Introduction ........................................... 6
  1.1      Protocol Overview ...................................... 6
  1.2      Definitions of commonly used terms ..................... 8
  1.3      Brief history of link-state routing technology ........ 11
  1.4      Organization of this document ......................... 12
  1.5      Acknowledgments ....................................... 12
  2        The link-state database: organization and calculations  13
  2.1      Representation of routers and networks ................ 13
  2.1.1    Representation of non-broadcast networks .............. 15
  2.1.2    An example link-state database ........................ 18
  2.2      The shortest-path tree ................................ 21
  2.3      Use of external routing information ................... 23
  2.4      Equal-cost multipath .................................. 26
  3        Splitting the AS into Areas ........................... 26
  3.1      The backbone of the Autonomous System ................. 27
  3.2      Inter-area routing .................................... 27
  3.3      Classification of routers ............................. 28
  3.4      A sample area configuration ........................... 29
  3.5      IP subnetting support ................................. 35
  3.6      Supporting stub areas ................................. 37
  3.7      Partitions of areas ................................... 38
  4        Functional Summary .................................... 40
  4.1      Inter-area routing .................................... 41
  4.2      AS external routes .................................... 41
  4.3      Routing protocol packets .............................. 42
  4.4      Basic implementation requirements ..................... 43
  4.5      Optional OSPF capabilities ............................ 46
  5        Protocol data structures .............................. 47
  6        The Area Data Structure ............................... 49
  7        Bringing Up Adjacencies ............................... 52
  7.1      The Hello Protocol .................................... 52
  7.2      The Synchronization of Databases ...................... 53
  7.3      The Designated Router ................................. 54
  7.4      The Backup Designated Router .......................... 56
  7.5      The graph of adjacencies .............................. 56

Moy Standards Track [Page 2] RFC 2328 OSPF Version 2 April 1998

  8        Protocol Packet Processing ............................ 58
  8.1      Sending protocol packets .............................. 58
  8.2      Receiving protocol packets ............................ 61
  9        The Interface Data Structure .......................... 63
  9.1      Interface states ...................................... 67
  9.2      Events causing interface state changes ................ 70
  9.3      The Interface state machine ........................... 72
  9.4      Electing the Designated Router ........................ 75
  9.5      Sending Hello packets ................................. 77
  9.5.1    Sending Hello packets on NBMA networks ................ 79
  10       The Neighbor Data Structure ........................... 80
  10.1     Neighbor states ....................................... 83
  10.2     Events causing neighbor state changes ................. 87
  10.3     The Neighbor state machine ............................ 89
  10.4     Whether to become adjacent ............................ 95
  10.5     Receiving Hello Packets ............................... 96
  10.6     Receiving Database Description Packets ................ 99
  10.7     Receiving Link State Request Packets ................. 102
  10.8     Sending Database Description Packets ................. 103
  10.9     Sending Link State Request Packets ................... 104
  10.10    An Example ........................................... 105
  11       The Routing Table Structure .......................... 107
  11.1     Routing table lookup ................................. 111
  11.2     Sample routing table, without areas .................. 111
  11.3     Sample routing table, with areas ..................... 112
  12       Link State Advertisements (LSAs) ..................... 115
  12.1     The LSA Header ....................................... 116
  12.1.1   LS age ............................................... 116
  12.1.2   Options .............................................. 117
  12.1.3   LS type .............................................. 117
  12.1.4   Link State ID ........................................ 117
  12.1.5   Advertising Router ................................... 119
  12.1.6   LS sequence number ................................... 120
  12.1.7   LS checksum .......................................... 121
  12.2     The link state database .............................. 121
  12.3     Representation of TOS ................................ 122
  12.4     Originating LSAs ..................................... 123
  12.4.1   Router-LSAs .......................................... 126
  12.4.1.1 Describing point-to-point interfaces ................. 130
  12.4.1.2 Describing broadcast and NBMA interfaces ............. 130
  12.4.1.3 Describing virtual links ............................. 131
  12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 131

Moy Standards Track [Page 3] RFC 2328 OSPF Version 2 April 1998

  12.4.1.5 Examples of router-LSAs .............................. 132
  12.4.2   Network-LSAs ......................................... 133
  12.4.2.1 Examples of network-LSAs ............................. 134
  12.4.3   Summary-LSAs ......................................... 135
  12.4.3.1 Originating summary-LSAs into stub areas ............. 137
  12.4.3.2 Examples of summary-LSAs ............................. 138
  12.4.4   AS-external-LSAs ..................................... 139
  12.4.4.1 Examples of AS-external-LSAs ......................... 140
  13       The Flooding Procedure ............................... 143
  13.1     Determining which LSA is newer ....................... 146
  13.2     Installing LSAs in the database ...................... 147
  13.3     Next step in the flooding procedure .................. 148
  13.4     Receiving self-originated LSAs ....................... 151
  13.5     Sending Link State Acknowledgment packets ............ 152
  13.6     Retransmitting LSAs .................................. 154
  13.7     Receiving link state acknowledgments ................. 155
  14       Aging The Link State Database ........................ 156
  14.1     Premature aging of LSAs .............................. 157
  15       Virtual Links ........................................ 158
  16       Calculation of the routing table ..................... 160
  16.1     Calculating the shortest-path tree for an area ....... 161
  16.1.1   The next hop calculation ............................. 167
  16.2     Calculating the inter-area routes .................... 178
  16.3     Examining transit areas' summary-LSAs ................ 170
  16.4     Calculating AS external routes ....................... 173
  16.4.1   External path preferences ............................ 175
  16.5     Incremental updates -- summary-LSAs .................. 175
  16.6     Incremental updates -- AS-external-LSAs .............. 177
  16.7     Events generated as a result of routing table changes  177
  16.8     Equal-cost multipath ................................. 178
           Footnotes ............................................ 179
           References ........................................... 183
  A        OSPF data formats .................................... 185
  A.1      Encapsulation of OSPF packets ........................ 185
  A.2      The Options field .................................... 187
  A.3      OSPF Packet Formats .................................. 189
  A.3.1    The OSPF packet header ............................... 190
  A.3.2    The Hello packet ..................................... 193
  A.3.3    The Database Description packet ...................... 195
  A.3.4    The Link State Request packet ........................ 197
  A.3.5    The Link State Update packet ......................... 199
  A.3.6    The Link State Acknowledgment packet ................. 201

Moy Standards Track [Page 4] RFC 2328 OSPF Version 2 April 1998

  A.4      LSA formats .......................................... 203
  A.4.1    The LSA header ....................................... 204
  A.4.2    Router-LSAs .......................................... 206
  A.4.3    Network-LSAs ......................................... 210
  A.4.4    Summary-LSAs ......................................... 212
  A.4.5    AS-external-LSAs ..................................... 214
  B        Architectural Constants .............................. 217
  C        Configurable Constants ............................... 219
  C.1      Global parameters .................................... 219
  C.2      Area parameters ...................................... 220
  C.3      Router interface parameters .......................... 221
  C.4      Virtual link parameters .............................. 224
  C.5      NBMA network parameters .............................. 224
  C.6      Point-to-MultiPoint network parameters ............... 225
  C.7      Host route parameters ................................ 226
  D        Authentication ....................................... 227
  D.1      Null authentication .................................. 227
  D.2      Simple password authentication ....................... 228
  D.3      Cryptographic authentication ......................... 228
  D.4      Message generation ................................... 231
  D.4.1    Generating Null authentication ....................... 231
  D.4.2    Generating Simple password authentication ............ 232
  D.4.3    Generating Cryptographic authentication .............. 232
  D.5      Message verification ................................. 234
  D.5.1    Verifying Null authentication ........................ 234
  D.5.2    Verifying Simple password authentication ............. 234
  D.5.3    Verifying Cryptographic authentication ............... 235
  E        An algorithm for assigning Link State IDs ............ 236
  F        Multiple interfaces to the same network/subnet ....... 239
  G        Differences from RFC 2178 ............................ 240
  G.1      Flooding modifications ............................... 240
  G.2      Changes to external path preferences ................. 241
  G.3      Incomplete resolution of virtual next hops ........... 241
  G.4      Routing table lookup ................................. 241
           Security Considerations .............................. 243
           Author's Address ..................................... 243
           Full Copyright Statement ............................. 244

Moy Standards Track [Page 5] RFC 2328 OSPF Version 2 April 1998

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 CIDR
  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.
  1.1.  Protocol overview
      OSPF routes IP packets based solely on the destination IP
      address 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.  This
      database is referred to as the link-state database. 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.

Moy Standards Track [Page 6] RFC 2328 OSPF Version 2 April 1998

      All routers run the exact same algorithm, in parallel.  From the
      link-state database, each router constructs a tree of shortest
      paths with itself as root.  This shortest-path tree gives the
      route to each destination in the Autonomous System.  Externally
      derived routing information appears on the tree as leaves.
      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; in
      fact, separate authentication schemes can be configured for each
      IP subnet.
      Externally derived routing data (e.g., routes learned from an
      Exterior Gateway Protocol such as BGP; see [Ref23]) is
      advertised 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 boundary of the Autonomous
      System.

Moy Standards Track [Page 7] RFC 2328 OSPF Version 2 April 1998

  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 [Ref13] for an introduction to IP.
      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.

Moy Standards Track [Page 8] RFC 2328 OSPF Version 2 April 1998

          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.
      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.
      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 OSPF
          protocol makes further use of multicast capabilities, if
          they exist.  Each pair of routers on a broadcast network is
          assumed to be able to communicate directly. 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 maintained
          on these nets using OSPF's Hello Protocol.  However, due to
          the lack of broadcast capability, some configuration
          information may be necessary to aid in the discovery of
          neighbors.  On non-broadcast 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.
          OSPF runs in one of two modes over non-broadcast networks.
          The first mode, called non-broadcast multi-access or NBMA,
          simulates the operation of OSPF on a broadcast network. The
          second mode, called Point-to-MultiPoint, treats the non-
          broadcast network as a collection of point-to-point links.
          Non-broadcast networks are referred to as NBMA networks or
          Point-to-MultiPoint networks, depending on OSPF's mode of
          operation over the network.

Moy Standards Track [Page 9] RFC 2328 OSPF Version 2 April 1998

      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
          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.
          Neighbor relationships are maintained by, and usually
          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
          Unit of data describing the local state of a router or
          network. For a router, 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 link state database.
          Throughout this memo, link state advertisement is
          abbreviated as LSA.
      Hello Protocol
          The part of the OSPF protocol used to establish and maintain
          neighbor relationships.  On broadcast networks the Hello
          Protocol can also dynamically discover neighboring routers.
      Flooding
          The part of the OSPF protocol that distributes and
          synchronizes the link-state database between OSPF routers.
      Designated Router
          Each broadcast and NBMA network that has at least two
          attached routers has a Designated Router.  The Designated

Moy Standards Track [Page 10] RFC 2328 OSPF Version 2 April 1998

          Router generates an LSA for the 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 broadcast or NBMA
          network.  This in turn reduces the amount of routing
          protocol traffic and the size of the link-state 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.
  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 [Ref3].  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 [Ref4].  These
      modifications dealt with increasing the fault tolerance of the
      routing protocol through, among other things, adding a checksum
      to the LSAs (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 LSA originations
      to be increased by an order of magnitude.

Moy Standards Track [Page 11] RFC 2328 OSPF Version 2 April 1998

      A link-state algorithm has also been proposed for use as an ISO
      IS-IS routing protocol.  This protocol is described in [Ref2].
      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 an LSA for the network.
      The OSPF Working Group 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 algorithms have been
      tailored 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
      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.
      Architectural constants are summarized in Appendix B.
      Configurable constants are summarized 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.
  1.5.  Acknowledgments
      The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
      Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
      Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui

Moy Standards Track [Page 12] RFC 2328 OSPF Version 2 April 1998

      Zhang and the rest of the OSPF Working Group for the ideas and
      support they have given to this project.
      The OSPF Point-to-MultiPoint interface is based on work done by
      Fred Baker.
      The OSPF Cryptographic Authentication option was developed by
      Fred Baker and Ran Atkinson.

2. The Link-state Database: organization and calculations

  The following subsections describe the organization of OSPF's link-
  state database, and the routing calculations that are performed on
  the database in order to produce a router's routing table.
  2.1.  Representation of routers and networks
      The Autonomous System's link-state 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. Networks can be either transit or
      stub networks. Transit networks are those capable of carrying
      data traffic that is neither locally originated nor locally
      destined. A transit network is represented by a graph vertex
      having both incoming and outgoing edges. A stub network's vertex
      has only incoming edges.
      The neighborhood of each network node in the graph depends on
      the network's type (point-to-point, broadcast, NBMA or Point-
      to-MultiPoint) and the number of routers having an interface to
      the network.  Three cases are depicted in Figure 1a.  Rectangles
      indicate routers.  Circles and oblongs indicate 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 networks with their
      connected routers, with the resulting graphs shown on the right.

Moy Standards Track [Page 13] RFC 2328 OSPF Version 2 April 1998

  • *FROM * |RT1|RT2| +—+Ia +—+ * ———— |RT1|——|RT2| T RT1| | X | +—+ Ib+—+ O RT2| X | | * Ia| | X | * Ib| X | | Physical point-to-point networks FROM +—+ * |RT7| * |RT7| N3| +—+ T ———— | O RT7| | | +———————-+ * N3| X | | N3 * Stub 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 | | +—+ +—+ Broadcast or NBMA networks Figure 1a: Network map components Moy Standards Track [Page 14] RFC 2328 OSPF Version 2 April 1998 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. The top of Figure 1a shows two routers connected by a point-to- point link. In the resulting link-state database graph, the two router vertices are directly connected by a pair of edges, one in each direction. Interfaces to point-to-point networks need not be assigned IP addresses. When interface addresses are assigned, they are modelled as stub links, with each router advertising a stub connection to the other router's interface address. Optionally, an IP subnet can be assigned to the point- to-point network. In this case, both routers advertise a stub link to the IP subnet, instead of advertising each others' IP interface addresses. The middle of Figure 1a shows a network with only one attached router (i.e., a stub network). In this case, the network appears on the end of a stub connection in the link-state database's graph. When multiple routers are attached to a broadcast network, the link-state database graph shows all routers bidirectionally connected to the network vertex. This is pictured at the bottom of Figure 1a. 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. 2.1.1. Representation of non-broadcast networks As mentioned previously, OSPF can run over non-broadcast networks in one of two modes: NBMA or Point-to-MultiPoint. The choice of mode determines the way that the Hello Moy Standards Track [Page 15] RFC 2328 OSPF Version 2 April 1998 protocol and flooding work over the non-broadcast network, and the way that the network is represented in the link- state database. In NBMA mode, OSPF emulates operation over a broadcast network: a Designated Router is elected for the NBMA network, and the Designated Router originates an LSA for the network. The graph representation for broadcast networks and NBMA networks is identical. This representation is pictured in the middle of Figure 1a. NBMA mode is the most efficient way to run OSPF over non- broadcast networks, both in terms of link-state database size and in terms of the amount of routing protocol traffic. However, it has one significant restriction: it requires all routers attached to the NBMA network to be able to communicate directly. This restriction may be met on some non-broadcast networks, such as an ATM subnet utilizing SVCs. But it is often not met on other non-broadcast networks, such as PVC-only Frame Relay networks. On non- broadcast networks where not all routers can communicate directly you can break the non-broadcast network into logical subnets, with the routers on each subnet being able to communicate directly, and then run each separate subnet as an NBMA network (see [Ref15]). This however requires quite a bit of administrative overhead, and is prone to misconfiguration. It is probably better to run such a non- broadcast network in Point-to-Multipoint mode. In Point-to-MultiPoint mode, OSPF treats all router-to- router connections over the non-broadcast network as if they were point-to-point links. No Designated Router is elected for the network, nor is there an LSA generated for the network. In fact, a vertex for the Point-to-MultiPoint network does not appear in the graph of the link-state database. Figure 1b illustrates the link-state database representation of a Point-to-MultiPoint network. On the left side of the figure, a Point-to-MultiPoint network is pictured. It is assumed that all routers can communicate directly, except for routers RT4 and RT5. I3 though I6 indicate the routers' Moy Standards Track [Page 16] RFC 2328 OSPF Version 2 April 1998 IP interface addresses on the Point-to-MultiPoint network. In the graphical representation of the link-state database, routers that can communicate directly over the Point-to- MultiPoint network are joined by bidirectional edges, and each router also has a stub connection to its own IP interface address (which is in contrast to the representation of real point-to-point links; see Figure 1a). On some non-broadcast networks, use of Point-to-MultiPoint mode and data-link protocols such as Inverse ARP (see [Ref14]) will allow autodiscovery of OSPF neighbors even though broadcast support is not available. FROM +—+ +—+ |RT3| |RT4| |RT3|RT4|RT5|RT6| +—+ +—+ * ——————– I3| N2 |I4 * RT3| | X | X | X | +———————-+ T RT4| X | | | X | I5| |I6 O RT5| X | | | X | +—+ +—+ * RT6| X | X | X | | |RT5| |RT6| * I3| X | | | | +—+ +—+ I4| | X | | | I5| | | X | | I6| | | | X | Figure 1b: Network map components Point-to-MultiPoint networks All routers can communicate directly over N2, except routers RT4 and RT5. I3 through I6 indicate IP interface addresses Moy Standards Track [Page 17] RFC 2328 OSPF Version 2 April 1998 2.1.2. An example link-state database 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 BGP connections to other Autonomous Systems. A set of BGP- 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 BGP-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 link-state database is pieced together from LSAs generated by the routers. In the associated graphical representation, the neighborhood of each router or transit network is represented in a single, separate LSA. Figure 4 shows these LSAs graphically. 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 LSA for Network N6 is actually generated by one of the network's attached routers: the router that has been elected Designated Router for the network. Moy Standards Track [Page 18] RFC 2328 OSPF Version 2 April 1998 + | 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 Moy Standards Track [Page 19] RFC 2328 OSPF Version 2 April 1998 Figure 2: A sample Autonomous System 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 Standards Track [Page 20] RFC 2328 OSPF Version 2 April 1998 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-LSA N9's network-LSA 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. 2.2. The shortest-path tree When no OSPF areas are configured, each router in the Autonomous System has an identical link-state database, leading to an 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 path 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 point-to-point network (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 Moy Standards Track [Page 21] RFC 2328 OSPF Version 2 April 1998 RT6(origin) RT5 o————o———–o Ib /|\ 6 |\ 7 8/8|8\ | \ / | \ 6| \ 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 2.3 Moy Standards Track [Page 22] RFC 2328 OSPF Version 2 April 1998 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. this externally derived routing information is considered in the next section. 2.3. 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 BGP, or be statically 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 expressed in the same units as OSPF interface cost Moy Standards Track [Page 23] RFC 2328 OSPF Version 2 April 1998 (i.e., in terms of the link state metric). Type 2 external metrics are an order of magnitude larger; any Type 2 metric is considered 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 advertised external route, the total cost from Router RT6 is calculated as the sum of the external route's advertised cost and the distance from Router RT6 to the advertising router. When two routers are advertising the same external destination, RT6 picks the advertising router providing the minimum total cost. RT6 then sets the next hop to the external destination equal to the next hop that would be used when routing packets to the chosen advertising router. In Figure 2, 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 Moy Standards Track [Page 24] RFC 2328 OSPF Version 2 April 1998 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 BGP 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 boundary router to specify a "forwarding address" in its AS- external-LSAs. 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 AS-external-LSAs. In each AS-external-LSA, Router RT6 would specify the correct Autonomous System exit point to use for the destination through appropriate setting of the LSA's "forwarding address" field. Moy Standards Track [Page 25] RFC 2328 OSPF Version 2 April 1998 2.4. 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. 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 link-state 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 link-state database. A router actually has a separate link-state 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 link-state databases. 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 Moy Standards Track [Page 26] RFC 2328 OSPF Version 2 April 1998 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 OSPF backbone is the special OSPF Area 0 (often written as Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP addresses). The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous. However, it need not be physically contiguous; backbone connectivity can be established/maintained through the configuration of 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 backbone 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. 3.2. Inter-area routing When routing a packet between two non-backbone 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 as forcing a star configuration on the Autonomous System, with the backbone as hub and each of the non-backbone areas as spokes. Moy Standards Track [Page 27] RFC 2328 OSPF Version 2 April 1998 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 inter-area destinations 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. 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. 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 area. 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 connecting to the backbone area are supported. Moy Standards Track [Page 28] RFC 2328 OSPF Version 2 April 1998 AS boundary routers A router that exchanges routing information with routers belonging to other Autonomous Systems. Such a router advertises AS external routing information throughout the Autonomous System. The paths to each AS boundary router are 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 link-state 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, AS-external-LSAs from RT5 and RT7 are flooded throughout the entire AS, and in particular throughout Area 1. These LSAs 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 Moy Standards Track [Page 29] RFC 2328 OSPF Version 2 April 1998 ……………………… . + . . | 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 . . | . . | . . +———+ . . +——–+ . Moy Standards Track [Page 30] RFC 2328 OSPF Version 2 April 1998 . N10 . . N7 . . . .Area 2 . .Area 3 . ………………………….. …………………….. Figure 6: A sample OSPF area configuration distribution to the backbone. Their backbone LSAs 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. The link-state 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. The area border routers RT3, RT4, RT7, RT10 and RT11 condense the routing information of their attached non-backbone 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. 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. Moy Standards Track [Page 31] RFC 2328 OSPF Version 2 April 1998 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| | |20|27| | | | N6| | |16|15| | | | N7| | |20|19| | | | N8| | |18|18| | | | N9-N11,H1| | |29|36| | | | 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 Standards Track [Page 32] RFC 2328 OSPF Version 2 April 1998 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| | | | | | |11| 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. 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 LSAs, and adding in the backbone distance to each advertising router. Moy Standards Track [Page 33] RFC 2328 OSPF Version 2 April 1998 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. Note that Table 6 assumes that an area range has been configured for the backbone which groups Ia and Ib into a single LSA. 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 dist from dist from 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. Moy Standards Track [Page 34] RFC 2328 OSPF Version 2 April 1998 Destination RT3 adv. RT4 adv. _ Ia,Ib 20 27 N6 16 15 N7 20 19 N8 18 18 N9-N11,H1 29 36 _ RT5 14 8 RT7 20 14 Table 6: Destinations advertised into Area 1 by Routers RT3 and RT4. load share between the two for traffic to Network N8. 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 AS-external-LSAs, 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. 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-LSA 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. Moy Standards Track [Page 35] RFC 2328 OSPF Version 2 April 1998 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. 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. Attaching an address mask to each route also enables the support of IP supernetting. For example, a single physical network segment could be assigned the [address,mask] pair [192.9.4.0,0xfffffc00]. The segment would then be single IP network, containing addresses from the four consecutive class C network numbers 192.9.4.0 through 192.9.7.0. Such addressing is now becoming commonplace with the advent of CIDR (see [Ref10]). Moy Standards Track [Page 36] RFC 2328 OSPF Version 2 April 1998 In order to get better aggregation at area boundaries, area address ranges can be employed (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 maximum cost to any of the networks falling in the specified range. For example, an IP subnetted network might be configured as a single OSPF area. In that case, a single address range could be configured: 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. However, 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 maximum of the set of costs to the component subnets. 3.6. Supporting stub areas In some Autonomous Systems, the majority of the link-state database may consist of AS-external-LSAs. An OSPF AS-external- LSA is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as "stub areas". AS- external-LSAs 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 link-state database size, and therefore the memory requirements, for a stub area's internal routers. 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-LSAs. 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 be used for any destination that is not Moy Standards Track [Page 37] RFC 2328 OSPF Version 2 April 1998 explicitly reachable by an intra-area or inter-area path (i.e., AS external destinations). An area can be configured as a 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-LSA), instead of flooding the AS-external-LSAs 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-LSAs. 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. However, in order to maintain full routing after the partition, an address range must not be split across multiple components of the area partition. Also, the backbone itself must not partition. If it does, parts of the Autonomous System will become unreachable. Backbone partitions can be repaired by 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 Moy Standards Track [Page 38] RFC 2328 OSPF Version 2 April 1998 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 Standards Track [Page 39] RFC 2328 OSPF Version 2 April 1998 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 may be necessary in order to discover neighbors. On broadcast and NBMA networks 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. Link-state databases are synchronized between pairs of adjacent routers. On broadcast and NBMA networks, the Designated Router determines which routers should become adjacent. Adjacencies control the distribution of routing information. Routing updates are sent and received only on 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 LSAs. This relationship between adjacencies and link state allows the protocol to detect dead routers in a timely fashion. LSAs are flooded throughout the area. The flooding algorithm is reliable, ensuring that all routers in an area have exactly the same link-state database. This database consists of the collection of LSAs originated by 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 Standards Track [Page 40] RFC 2328 OSPF Version 2 April 1998 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 non- backbone 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 inter-area destinations. 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 inter-area destinations. 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. Moy Standards Track [Page 41] RFC 2328 OSPF Version 2 April 1998 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 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 [Ref5]). 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. Moy Standards Track [Page 42] RFC 2328 OSPF Version 2 April 1998 Each Link State Update packet carries a set of new link state advertisements (LSAs) one hop further away from their point of origination. A single Link State Update packet may contain the LSAs of several routers. Each LSA is tagged with the ID of the originating router and a checksum of its link state contents. Each LSA also has a type field; the different types of OSPF LSAs are listed below in Table 9. OSPF routing packets (with the exception of Hellos) are sent only over adjacencies. 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 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. 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 interval timer at each firing. Moy Standards Track [Page 43] RFC 2328 OSPF Version 2 April 1998 LS LSA LSA description type name 1 Router-LSAs Originated by all routers. This LSA describes the collected states of the router's interfaces to an area. Flooded throughout a single area only. 2 Network-LSAs Originated for broadcast and NBMA networks by the Designated Router. This LSA contains the list of routers connected to the network. Flooded throughout a single area only. 3,4 Summary-LSAs Originated by area border routers, and flooded through- out the LSA's associated area. Each summary-LSA describes a route to a destination outside the area, yet still inside the AS (i.e., an inter-area route). Type 3 summary-LSAs describe routes to networks. Type 4 summary-LSAs describe routes to AS boundary routers. 5 AS-external-LSAs Originated by AS boundary routers, and flooded through- out the AS. Each AS-external-LSA describes a route to a destination in another Autonomous System. Default routes for the AS can also be described by AS-external-LSAs. Moy Standards Track [Page 44] RFC 2328 OSPF Version 2 April 1998 Table 9: OSPF link state advertisements (LSAs). 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 [Ref7]. 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 scaling of IP routing in the worldwide Internet. For more information on IP supernetting, see [Ref10]. 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 On non-broadcast networks, the OSPF Hello Protocol can be aided by providing an indication 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 Moy Standards Track [Page 45] RFC 2328 OSPF Version 2 April 1998 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 LSAs. For example, the collection of LSAs that will be retransmitted to an adjacent router until acknowledged are described as a list. Any particular LSA may be on many such lists. An OSPF implementation needs to be able to manipulate these lists, adding and deleting constituent LSAs 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 LSAs. This enables routers supporting a 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 Moy Standards Track [Page 46] RFC 2328 OSPF Version 2 April 1998 capability mismatch with a neighbor in this case will result in only a subset of the link state database 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 LSAs, routers incapable of certain functions can be avoided when building the shortest path tree. The OSPF optional capabilities defined in this memo 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-LSAs 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). 5. Protocol Data Structures The OSPF protocol is described herein 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. In this case the router should flush its self-originated LSAs from the routing domain (see Section 14.1) before restarting, or they will persist for up to MaxAge minutes. Moy Standards Track [Page 47] RFC 2328 OSPF Version 2 April 1998 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 OSPF algorithm. Remember that each area runs a separate copy of the basic OSPF algorithm. Backbone (area) structure The OSPF backbone area is responsible for the dissemination of inter-area routing information. 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 with another routing protocol (such as BGP), 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-LSAs. List of AS-external-LSAs Part of the link-state 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-LSAs have been self-originated. Moy Standards Track [Page 48] RFC 2328 OSPF Version 2 April 1998 The routing table Derived from the link-state database. Each entry in the routing table is indexed by a destination, and contains the destination's cost and a set of paths to use in forwarding packets to the destination. A path is described by its type and next hop. For more information, see Section 11. 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 OSPF routing algorithm. Each area maintains its own link-state 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 is the special OSPF area responsible for disseminating inter-area routing information. The area link-state database consists of the collection of router- LSAs, network-LSAs and summary-LSAs that have originated from the area's routers. This information is flooded throughout a single area only. The list of AS-external-LSAs (see Section 5) is also considered to be part of each area's link-state database. Area ID A 32-bit number identifying the area. The Area ID of 0.0.0.0 is reserved for the backbone. List of area address ranges In order to aggregate routing information at area boundaries, area address ranges can be employed. Each address range is specified by an [address,mask] pair and a status indication of either Advertise or DoNotAdvertise (see Section 12.4.3). Moy Standards Track [Page 49] RFC 2328 OSPF Version 2 April 1998 +—-+ |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
  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 area 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.

Moy Standards Track [Page 50] RFC 2328 OSPF Version 2 April 1998

  List of router-LSAs
      A router-LSA is generated by each router in the area.  It
      describes the state of the router's interfaces to the area.
  List of network-LSAs
      One network-LSA is generated for each transit broadcast and NBMA
      network in the area.  A network-LSA describes the set of routers
      currently connected to the network.
  List of summary-LSAs
      Summary-LSAs originate from the area's area border routers.
      They describe routes to destinations internal to the Autonomous
      System, yet external to the area (i.e., inter-area
      destinations).
  Shortest-path tree
      The shortest-path tree for the area, with this router itself as
      root.  Derived from the collected router-LSAs and network-LSAs
      by the Dijkstra algorithm (see Section 16.1).
  TransitCapability
      This parameter indicates whether the area can carry data traffic
      that 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, where TransitCapability is set to TRUE
      if and only if there are one or more fully adjacent virtual
      links using the area as Transit area), and is used as an input
      to a subsequent step of the routing table build process (see
      Section 16.3). When an area's TransitCapability is set to TRUE,
      the area is said to be a "transit area".
  ExternalRoutingCapability
      Whether AS-external-LSAs will be flooded into/throughout the
      area.  This is a configurable parameter.  If AS-external-LSAs
      are excluded from the area, the area is called a "stub". Within
      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.

Moy Standards Track [Page 51] RFC 2328 OSPF Version 2 April 1998

  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-LSA that the router
      should advertise into the area. See Section 12.4.3 for more
      information.
  Unless otherwise specified, the remaining sections of this document
  refer to the operation of the OSPF protocol within 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 broadcast and NBMA networks,
      the Hello Protocol elects a Designated Router for the network.
      The Hello Protocol works differently on broadcast networks, NBMA
      networks and Point-to-MultiPoint 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 NBMA networks some configuration information may be necessary
      for the operation of the Hello Protocol.  Each router that may
      potentially become Designated Router has a list of all other

Moy Standards Track [Page 52] RFC 2328 OSPF Version 2 April 1998

      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 NBMA 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.
      On Point-to-MultiPoint networks, a router sends Hello Packets to
      all neighbors with which it can communicate directly. These
      neighbors may be discovered dynamically through a protocol such
      as Inverse ARP (see [Ref14]), or they may be configured.
      After a neighbor has been discovered, bidirectional
      communication ensured, and (if on a broadcast or NBMA 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). If an adjacency is to be formed, the first step
      is to synchronize the neighbors' link-state 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' link-state 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 LSAs belonging to the router's
      database.  When the neighbor sees an LSA that is more recent
      than its own database copy, it makes a note that this newer LSA
      should be requested.
      This sending and receiving of Database Description packets is
      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

Moy Standards Track [Page 53] RFC 2328 OSPF Version 2 April 1998

      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 per-interface 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 LSAs for which the neighbor has more up-to-date
      instances.  These LSAs 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' router-LSAs.
      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 broadcast and NBMA network has a Designated Router.  The
      Designated Router performs two main functions for the routing
      protocol:
      o   The Designated Router originates a network-LSA on behalf of
          the network.  This LSA lists the set of routers (including
          the Designated Router itself) currently attached to the
          network.  The Link State ID for this LSA (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 network's subnet/network mask.

Moy Standards Track [Page 54] RFC 2328 OSPF Version 2 April 1998

      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.  Transit 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 LSAs.
      Until the link-state 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.

Moy Standards Track [Page 55] RFC 2328 OSPF Version 2 April 1998

  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 broadcast
      and NBMA network.  The Backup Designated Router is also adjacent
      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 link-state 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 LSAs (which
      announce the new Designated Router).
      The Backup Designated Router does not generate a network-LSA 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.

Moy Standards Track [Page 56] RFC 2328 OSPF Version 2 April 1998

      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 a Designated
      Router is elected for the network.  On physical point-to-point
      networks, Point-to-MultiPoint networks and virtual links,
      neighboring routers become adjacent whenever they can
      communicate directly.  In contrast, on broadcast and NBMA
      networks only the Designated Router and the Backup Designated
      Router become adjacent to all other routers attached to the
      network.
        +---+            +---+
        |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 Standards Track [Page 57] RFC 2328 OSPF Version 2 April 1998

      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 link-state
  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
  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 provide details on how to fill in and verify this
  standard header.  Then, for each packet type, the section giving
  more details on that particular packet type's processing is listed.
  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.

Moy Standards Track [Page 58] RFC 2328 OSPF Version 2 April 1998

      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 is calculated as part of the
          appropriate authentication procedure; for some OSPF
          authentication types, the checksum calculation is omitted.
          See Section D.4 for details.
      AuType and Authentication
          Each OSPF packet exchange is authenticated.  Authentication
          types are assigned by the protocol and are documented in
          Appendix D.  A different authentication procedure can be
          used for each IP network/subnet.  Autype indicates the type
          of authentication procedure in use. The 64-bit
          authentication field is then for use by the chosen
          authentication procedure.  This procedure should be the last
          called when forming the packet to be sent. See Section D.4
          for details.
      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

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      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
      directly to the neighbor. On multi-access networks, this means
      that retransmissions should be sent to the neighbor's IP
      address.
      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.
           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.

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  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
      interface.  If they do not, the packet should be discarded:
      o   The version number field must specify protocol version 2.
      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:

Moy Standards Track [Page 61] RFC 2328 OSPF Version 2 April 1998

          (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 is required to
              be on the same network as the receiving interface.  This
              can be verified 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.
      o   The packet must be authenticated.  The authentication
          procedure is indicated by the setting of AuType (see
          Appendix D).  The authentication procedure may use one or
          more Authentication keys, which can be configured on a per-
          interface basis.  The authentication procedure may also
          verify the checksum field in the OSPF packet header (which,
          when used, is set to the standard IP 16-bit one's complement
          checksum of the OSPF packet's contents after excluding the
          64-bit authentication field).  If the authentication
          procedure fails, the packet should be discarded.

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      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 connects to a broadcast
      network, Point-to-MultiPoint network or NBMA network the sender
      is identified by the IP source address found in the packet's IP
      header.  If the receiving interface connects to a point-to-point
      network 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.
  We assume a single OSPF interface to each attached network/subnet,
  although supporting multiple interfaces on a single network is
  considered in Appendix F. Each interface structure has at most one
  IP interface address.

Moy Standards Track [Page 63] RFC 2328 OSPF Version 2 April 1998

  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 LSAs reflect the 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; such items must be the same for all routers
  connected to the network.
  Type
      The OSPF interface type is either point-to-point, broadcast,
      NBMA, Point-to-MultiPoint 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 LSAs.
  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.

Moy Standards Track [Page 64] RFC 2328 OSPF Version 2 April 1998

  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
      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.  LSAs 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.  This list is formed
      by the Hello Protocol.  Adjacencies will be formed to some of

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      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 broadcast and NBMA 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-LSA 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.
  Backup Designated Router
      The Backup Designated Router is also selected on all broadcast
      and NBMA 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-LSA. The cost of an interface must
      be greater than zero.
  RxmtInterval
      The number of seconds between LSA retransmissions, for
      adjacencies belonging to this interface.  Also used when
      retransmitting Database Description and Link State Request
      Packets.
  AuType
      The type of authentication used on the attached network/subnet.
      Authentication types are defined in Appendix D.  All OSPF packet
      exchanges are authenticated.  Different authentication schemes
      may be used on different networks/subnets.

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  Authentication key
      This configured data allows the authentication procedure to
      generate and/or verify OSPF protocol packets.  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 clear password which is
      inserted into the OSPF packet header. If instead Autype
      indicates Cryptographic authentication, then the Authentication
      key is a shared secret which enables the generation/verification
      of message digests which are appended to the OSPF protocol
      packets. When Cryptographic authentication is used, multiple
      simultaneous keys are supported in order to achieve smooth key
      transition (see Section D.3).
  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.
      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

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                                +----+   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
          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
          still be addressed to an interface in Loopback state.  To

Moy Standards Track [Page 68] RFC 2328 OSPF Version 2 April 1998

          facilitate this, such interfaces are advertised in router-
          LSAs 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 broadcast or NBMA 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

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          originate a network-LSA for the network node.  The network-
          LSA 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.
  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.

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      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.
          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

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          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-LSA.  See Section 12.4 for more details.
      Some of the required actions below involve generating events for
      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, Point-to-MultiPoint 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.

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                  Otherwise, the attached network is a broadcast or
                  NBMA network 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.  Additionally, if the
                  network is an NBMA network 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
          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

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      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.
       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

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                  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
          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,

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          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 an NBMA network, 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 broadcast and NBMA networks, Hello Packets
      are also used to elect the Designated Router and Backup
      Designated Router.

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      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
      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.  One optional capability is defined in this
      specification (see Sections 4.5 and A.2).  The E-bit of the
      Options field should be set if and only if the attached area is
      capable of processing AS-external-LSAs (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).
      Unrecognized bits in 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 on
      the network 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 Point-
      to-MultiPoint networks, separate Hello packets are sent to each
      attached neighbor every HelloInterval seconds. Sending of Hello
      packets on NBMA networks is covered in the next section.

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      9.5.1.  Sending Hello packets on NBMA networks
          Static configuration information may be necessary in order
          for the Hello Protocol to function on non-broadcast networks
          (see Sections C.5 and C.6).  On NBMA networks, every
          attached router which is eligible to become Designated
          Router becomes aware of all of its neighbors on the network
          (either through configuration or by some unspecified
          mechanism).  Each neighbor is labelled with the neighbor's
          Designated Router eligibility.
          The interface state must be at least Waiting for any Hello
          Packets to be sent out the NBMA interface.  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 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 an NBMA 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

Moy Standards Track [Page 79] RFC 2328 OSPF Version 2 April 1998

          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.
  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.

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  DD Sequence Number
      The DD Sequence number of the Database Description packet that
      is currently being sent to the neighbor.
  Last received Database Description packet
      The initialize(I), more (M) and master(MS) bits, Options field,
      and DD sequence number contained in the last Database
      Description packet received from the neighbor. Used to determine
      whether the next Database Description packet received from the
      neighbor is a duplicate.
  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-LSAs 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).
  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.

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  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 broadcast and NBMA
      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 broadcast and
      NBMA networks.
  The next set of variables are lists of LSAs.  These lists describe
  subsets of the area link-state database.  This memo defines five
  distinct types of LSAs, all of which may be present in an area
  link-state database: router-LSAs, network-LSAs, and Type 3 and 4
  summary-LSAs (all stored in the area data structure), and AS-
  external-LSAs (stored in the global data structure).
  Link state retransmission list
      The list of LSAs 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 LSAs that make up the area link-state
      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 LSAs that need to be received from this neighbor in
      order to synchronize the two neighbors' link-state 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 appropriate Link State Update
      packets are received.

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  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
      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 LSAs.
      For a more detailed description of neighbor state changes,
      together with the additional actions involved in each change,
      see Section 10.3.
      Down
          This is the initial state of a neighbor conversation.  It
          indicates that there has been no recent information received
          from the neighbor.  On NBMA networks, Hello packets may
          still be sent to "Down" neighbors, although at a reduced
          frequency (see Section 9.5.1).

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                                 +----+
                                 |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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                                +-------+
                                |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
      Attempt
          This state is only valid for neighbors attached to NBMA
          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

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          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 LSAs.
          All adjacencies in Exchange state or greater are used by the
          flooding procedure.  In fact, these 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 LSAs 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-LSAs and
          network-LSAs.

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  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
          An Hello packet has been received from the 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 NBMA
          networks.
      2-WayReceived
          Bidirectional communication has been realized between the
          two neighboring routers.  This is indicated by the router
          seeing itself in the neighbor'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
          10.8.
      BadLSReq
          A Link State Request has been received for an LSA 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

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          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 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 the 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.
      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

Moy Standards Track [Page 88] RFC 2328 OSPF Version 2 April 1998

          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-LSA 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 an NBMA 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

Moy Standards Track [Page 89] RFC 2328 OSPF Version 2 April 1998

          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

Moy Standards Track [Page 90] RFC 2328 OSPF Version 2 April 1998

                  sequence number in the neighbor data structure.  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 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-LSAs, network-LSAs and summary-LSAs contained
                  in the area structure, along with the AS-external-
                  LSAs contained in the global structure.  AS-
                  external-LSAs are omitted from a virtual neighbor's
                  Database summary list.  AS-external-LSAs are omitted
                  from the Database summary list if the area has been
                  configured as a stub (see Section 3.6).  LSAs 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.

Moy Standards Track [Page 91] RFC 2328 OSPF Version 2 April 1998

       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.
                  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 LSAs (which were
                  discovered but not yet received in the Exchange
                  state).  These LSAs 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.

Moy Standards Track [Page 92] RFC 2328 OSPF Version 2 April 1998

       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 LSAs.
       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 LSAs.  Then the
                  router increments the DD sequence number in the
                  neighbor data structure, 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

Moy Standards Track [Page 93] RFC 2328 OSPF Version 2 April 1998

      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
                  LSAs.  Also, the Inactivity Timer is disabled.
       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
                  LSAs.  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
                  LSAs.

Moy Standards Track [Page 94] RFC 2328 OSPF Version 2 April 1998

       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
                  LSAs.
       State(s):  2-Way or greater
          Event:  2-WayReceived
      New state:  No state change.
         Action:  No action required.
       State(s):  Init
          Event:  1-WayReceived
      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,
      Point-to-MultiPoint networks and virtual links always become
      adjacent.  On broadcast and NBMA 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

Moy Standards Track [Page 95] RFC 2328 OSPF Version 2 April 1998

      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 Point-to-MultiPoint
      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
      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

Moy Standards Track [Page 96] RFC 2328 OSPF Version 2 April 1998

      ExternalRoutingCapability.  If AS-external-LSAs 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 connects to a broadcast, Point-to-
      MultiPoint or NBMA network the source is identified by the IP
      source address found in the Hello's IP header.  If the receiving
      interface connects to 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 broadcast,
      Point-to-MultiPoint or NBMA network, set the neighbor
      structure's Neighbor ID equal to the Router ID found in the
      packet's OSPF header.  For these network types, the neighbor
      structure's Router Priority field, Neighbor's Designated Router
      field, and Neighbor's Backup Designated Router field are also
      set equal to the corresponding fields found in the received
      Hello Packet; changes in these fields should be noted for
      possible use in the steps below.  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:

Moy Standards Track [Page 97] RFC 2328 OSPF Version 2 April 1998

      o   Each Hello Packet causes the neighbor state machine to be
          executed with the event HelloReceived.
      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, if a change in the neighbor's Router Priority field
          was noted, the receiving interface's state machine is
          scheduled with the event NeighborChange.
      o   If the neighbor is both declaring itself to be Designated
          Router (Hello Packet's 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.
      o   If the neighbor is declaring itself to be Backup Designated
          Router (Hello Packet's 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.
      On NBMA 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 Standards Track [Page 98] RFC 2328 OSPF Version 2 April 1998

  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).
      Whether the Database Description packet should be accepted, and
      if so, how it should be further processed depends upon the
      neighbor state.
      If a Database Description packet is accepted, the following
      packet fields should be saved in the corresponding neighbor data
      structure under "last received Database Description packet":
      the packet's initialize(I), more (M) and master(MS) bits,
      Options field, and DD sequence number. If these fields are set
      identically in two consecutive Database Description packets
      received from the neighbor, the second Database Description
      packet is considered to be a "duplicate" in the processing
      described below.
      If the Interface MTU field in the Database Description packet
      indicates an IP datagram size that is larger than the router can
      accept on the receiving interface without fragmentation, the
      Database Description packet is rejected.  Otherwise, if the
      neighbor state is:
      Down
          The packet should be rejected.
      Attempt
          The packet should be rejected.
      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.

Moy Standards Track [Page 99] RFC 2328 OSPF Version 2 April 1998

      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 neighbor data structure's 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 neighbor data
              structure's 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.
      Exchange
          Duplicate Database Description packets are discarded by the
          master, and cause the slave to retransmit the last Database
          Description packet that it had sent. Otherwise (the packet
          is not a duplicate):
          o   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.
          o   If the initialize(I) bit is set, generate the neighbor
              event SeqNumberMismatch and stop processing the packet.

Moy Standards Track [Page 100] RFC 2328 OSPF Version 2 April 1998

          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   Database Description packets must be processed in
              sequence, as indicated by the packets' DD sequence
              numbers. If the router is master, the next packet
              received should have DD sequence number equal to the DD
              sequence number in the neighbor data structure. If the
              router is slave, the next packet received should have DD
              sequence number equal to one more than the DD sequence
              number stored in the neighbor data structure. In either
              case, if the packet is the next in sequence it should be
              accepted and its contents processed as specified below.
          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
          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 LSA listed, the LSA'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 an AS-
      external-LSA (LS type = 5) and the neighbor is associated with a

Moy Standards Track [Page 101] RFC 2328 OSPF Version 2 April 1998

      stub area, generate the neighbor event SeqNumberMismatch and
      stop processing the packet.  Otherwise, the router looks up the
      LSA in its database to see whether it also has an instance of
      the LSA.  If it does not, or if the database copy is less recent
      (see Section 13.1), the LSA 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 in the neighbor data
          structure.  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 in the neighbor data structure
          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.
  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 LSAs 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.

Moy Standards Track [Page 102] RFC 2328 OSPF Version 2 April 1998

      Each LSA 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 LSAs
      should NOT be placed on the Link state retransmission list for
      the neighbor.  If an LSA 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 Database Description packet's Interface MTU
      field is set to the size of the largest IP datagram that can be
      sent out the sending interface, without fragmentation.  Common
      MTUs in use in the Internet can be found in Table 7-1 of
      [Ref22]. Interface MTU should be set to 0 in Database
      Description packets sent over virtual links.
      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.  One optional
      capability is defined in this specification (see Sections 4.5
      and A.2). The E-bit should be set if and only if the attached
      network belongs to a non-stub area. Unrecognized bits in 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 LSA in the area's link-state database
      (at the time the neighbor transitions into Exchange state) is
      listed in the neighbor Database summary list.  Each new Database

Moy Standards Track [Page 103] RFC 2328 OSPF Version 2 April 1998

      Description Packet copies its DD sequence number from the
      neighbor data structure and then describes the current top of
      the Database summary list.  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 LSAs that need to be obtained from
      the neighbor.  To request these LSAs, a router sends the
      neighbor the beginning of the Link state request list, packaged
      in a Link State Request packet.

Moy Standards Track [Page 104] RFC 2328 OSPF Version 2 April 1998

      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. LSAs on the Link state request list that have been
      requested, but not yet received, are packaged into Link State
      Request packets for retransmission 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

Moy Standards Track [Page 105] RFC 2328 OSPF Version 2 April 1998

          +---+                                         +---+
          |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

Moy Standards Track [Page 106] RFC 2328 OSPF Version 2 April 1998

                 Figure 14: An adjacency bring-up example
      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
      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.

Moy Standards Track [Page 107] RFC 2328 OSPF Version 2 April 1998

  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
      Destination type is either "network" or "router". Only network
      entries are actually used when forwarding IP data traffic.
      Router routing table entries are used solely as intermediate
      steps in the routing table build process.
      A network is 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 into this category.
      Router entries are kept for area border routers and AS boundary
      routers.  Routing table entries for area border routers are used
      when calculating the inter-area routes (see Section 16.2), and
      when maintaining configured virtual links (see Section 15).
      Routing table entries for AS boundary routers 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 routers, 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 this field indicates the
      optional OSPF capabilities supported by the destination router.
      The only optional capability defined by this specification is
      the ability to process AS-external-LSAs.  For a further
      discussion of OSPF's optional capabilities, see Section 4.5.

Moy Standards Track [Page 108] RFC 2328 OSPF Version 2 April 1998

  The set of paths to use for a destination may vary based on 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 field.
  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
      "router", there may be separate sets of paths (and therefore
      separate routing table entries) associated with each of several
      areas. For example, this will happen when two area border
      routers share multiple areas in common.  For destinations of
      type "network", only the set of paths associated with the best
      area (the one providing the preferred 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 decreasing 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-LSAs.  AS external paths are
      paths to destinations external to the AS.  These are detected
      through the examination of received AS-external-LSAs.
  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

Moy Standards Track [Page 109] RFC 2328 OSPF Version 2 April 1998

      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 LSA
      (router-LSA or network-LSA) that directly references the
      destination.  For example, if the destination is a transit
      network, this is the transit network's network-LSA.  If the
      destination is a stub network, this is the router-LSA for the
      attached router.  The LSA is discovered during the shortest-path
      tree calculation (see Section 16.1).  Multiple LSAs may
      reference the destination, however a tie-breaking scheme always
      reduces the choice to a single LSA. 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 broadcast, Point-to-MultiPoint and NBMA
      networks, the next hop also includes the IP address of the next
      router (if any) in the path towards the destination.
  Advertising router
      Valid only for inter-area and AS external paths.  This field
      indicates the Router ID of the router advertising the summary-
      LSA or AS-external-LSA that led to this path.

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  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.
      Before the lookup begins, "discard" routing table entries should
      be inserted into the routing table for each of the router's
      active area address ranges (see Section 3.5).  (An area range is
      considered "active" if the range contains one or more networks
      reachable by intra-area paths.) The destination of a "discard"
      entry is the set of addresses described by its associated active
      area address range, and the path type of each "discard" entry is
      set to "inter-area".[10]
      Several routing table entries may match the destination address.
      In this case, the "best match" is the routing table entry that
      provides the most specific (longest) match. Another way of
      saying this is to choose the entry that specifies the narrowest
      range of IP addresses.[11] 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. (Note that for any single routing table entry,
      multiple paths may be possible. In these cases, the calculations
      in Sections 16.1, 16.2, and 16.4 always yield the paths having
      the most preferential path-type, as described in Section 11).
      If there is no matching routing table entry, or the best match
      routing table entry is one of the above "discard" routing table
      entries, then the packet's IP destination is considered
      unreachable. Instead of being forwarded, the packet should then
      be discarded and an ICMP destination unreachable message should
      be returned to the packet's source.
  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

Moy Standards Track [Page 111] RFC 2328 OSPF Version 2 April 1998

      outbound interface.  The calculation of Router RT6's routing
      table proceeds as described in Section 2.2.  The resulting
      routing table is shown in Table 12.  Destination types are
      abbreviated: Network as "N", Router as "R".
      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-LSAs 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 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.
      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

Moy Standards Track [Page 112] RFC 2328 OSPF Version 2 April 1998

    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     *
    R      RT5    0      intra-area    6      RT5      *
    R      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).
      are all condensed to a single route when advertised into the
      backbone (by Router RT11).  Note that the cost of this route is
      the maximum 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).

Moy Standards Track [Page 113] RFC 2328 OSPF Version 2 April 1998

      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
      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
 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       *
 R      RT3         1      intra-area    1      *         *
 __________________________________________________________________
 N      Ib          0      intra-area    22     RT5       *
 N      Ia          0      intra-area    27     RT5       *
 R      RT3         0      intra-area    21     RT5       *
 R      RT5         0      intra-area    8      *         *
 R      RT7         0      intra-area    14     RT5       *
 R      RT10        0      intra-area    22     RT5       *
 R      RT11        0      intra-area    25     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    36     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.

Moy Standards Track [Page 114] RFC 2328 OSPF Version 2 April 1998

      in Table 14.

12. Link State Advertisements (LSAs)

  Each router in the Autonomous System originates one or more link
  state advertisements (LSAs).  This memo defines five distinct types
  of LSAs, which are described in Section 4.3.  The collection of LSAs
  forms the link-state database.  Each separate type of LSA has a
  separate function.  Router-LSAs and network-LSAs describe how an
  area's routers and networks are interconnected.  Summary-LSAs
  provide a way of condensing an area's routing information.  AS-
  external-LSAs provide a way of transparently advertising
  externally-derived routing information throughout the Autonomous
  System.
  Each LSA begins with a standard 20-byte header.  This LSA header is
  discussed below.
  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      *
  R      RT3         0      intra-area   1      *        *
  R      RT10        0      intra-area   16     RT3      *
  R      RT11        0      intra-area   19     RT3      *
  ________________________________________________________________
  N      N9-N11,H1   0      inter-area   30     RT3      RT11
                Table 14: Changes resulting from an
                      additional virtual link.

Moy Standards Track [Page 115] RFC 2328 OSPF Version 2 April 1998

  12.1.  The LSA Header
      The LSA header contains the LS type, Link State ID and
      Advertising Router fields.  The combination of these three
      fields uniquely identifies the LSA.
      There may be several instances of an LSA 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 LSA
      header.
      Several of the OSPF packet types list LSAs.  When the instance
      is not important, an LSA 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.
      A detailed explanation of the fields contained in the LSA header
      follows.
      12.1.1.  LS age
          This field is the age of the LSA in seconds.  It should be
          processed as an unsigned 16-bit integer.  It is set to 0
          when the LSA is originated.  It must be incremented by
          InfTransDelay on every hop of the flooding procedure.  LSAs
          are also aged as they are held in each router's database.
          The age of an LSA is never incremented past MaxAge.  LSAs
          having age MaxAge are not used in the routing table
          calculation.  When an LSA's age first reaches MaxAge, it is
          reflooded.  An LSA 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 LSAs,
          consult Section 14.
          The LS age field is examined when a router receives two
          instances of an LSA, both having identical LS sequence
          numbers and LS checksums.  An instance of age MaxAge is then

Moy Standards Track [Page 116] RFC 2328 OSPF Version 2 April 1998

          always accepted as most recent; this allows old LSAs 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.[12] See Section 13.1
          for more details.
      12.1.2.  Options
          The Options field in the LSA header indicates which optional
          capabilities are associated with the LSA.  OSPF's optional
          capabilities are described in Section 4.5.  One optional
          capability is defined by this specification, represented by
          the E-bit found in the Options field.  The unrecognized bits
          in the Options field should be set to zero.
          The E-bit represents OSPF's ExternalRoutingCapability.  This
          bit should be set in all LSAs associated with the backbone,
          and all LSAs associated with non-stub areas (see Section
          3.6).  It should also be set in all AS-external-LSAs.  It
          should be reset in all router-LSAs, network-LSAs and
          summary-LSAs associated with a stub area.  For all LSAs, the
          setting of the E-bit is for informational purposes only; it
          does not affect the routing table calculation.
      12.1.3.  LS type
          The LS type field dictates the format and function of the
          LSA.  LSAs of different types have different names (e.g.,
          router-LSAs or network-LSAs).  All LSA types defined by this
          memo, except the AS-external-LSAs (LS type = 5), are flooded
          throughout a single area only.  AS-external-LSAs are flooded
          throughout the entire Autonomous System, excepting stub
          areas (see Section 3.6).  Each separate LSA 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 LSA.  Depending on the LSA's LS
          type, the Link State ID takes on the values listed in Table

Moy Standards Track [Page 117] RFC 2328 OSPF Version 2 April 1998

          LS Type   LSA description
          ________________________________________________
          1         These are the router-LSAs.
                    They describe the collected
                     states of the router's
                    interfaces. For more information,
                    consult Section 12.4.1.
          ________________________________________________
          2         These are the network-LSAs.
                    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-LSAs.
                    They describe inter-area routes,
                    and enable the condensation of
                    routing information at area
                    borders. Originated by area border
                    routers, the Type 3 summary-LSAs
                    describe routes to networks while the
                    Type 4 summary-LSAs describe routes to
                    AS boundary routers.
          ________________________________________________
          5         These are the AS-external-LSAs.
                    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-LSA.
          Table 15: OSPF link state advertisements (LSAs).
          16.
          Actually, for Type 3 summary-LSAs (LS type = 3) and AS-
          external-LSAs (LS type = 5), the Link State ID may

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          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 LSA's Link State ID.
          additionally have one or more of the destination network's
          "host" bits set. For example, when originating an AS-
          external-LSA for the network 10.0.0.0 with mask of
          255.0.0.0, the 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
          LSAs for two networks having the same address but different
          masks. See Appendix E for details.
          When the LSA 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 LSA.  When the LSA 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-LSA (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 LSA's
          originator.  For router-LSAs, this field is identical to the
          Link State ID field.  Network-LSAs are originated by the

Moy Standards Track [Page 119] RFC 2328 OSPF Version 2 April 1998

          network's Designated Router.  Summary-LSAs originated by
          area border routers.  AS-external-LSAs 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 LSAs.  The space of
          sequence numbers is linearly ordered.  The larger the
          sequence number (when compared as signed 32-bit integers)
          the more recent the LSA.  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; this sequence number
          is referred to as the constant InitialSequenceNumber. A
          router uses InitialSequenceNumber the first time it
          originates any LSA.  Afterwards, the LSA's sequence number
          is incremented each time the router originates a new
          instance of the LSA.  When an attempt is made to increment
          the sequence number past the maximum value of N - 1
          (0x7fffffff; also referred to as MaxSequenceNumber), the
          current instance of the LSA must first be flushed from the
          routing domain.  This is done by prematurely aging the LSA
          (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
          InitialSequenceNumber.
          The router may be forced to promote the sequence number of
          one of its LSAs when a more recent instance of the LSA is
          unexpectedly received during the flooding process.  This
          should be a rare event.  This may indicate that an out-of-
          date LSA, originated by the router itself before its last
          restart/reload, still exists in the Autonomous System.  For
          more information see Section 13.4.

Moy Standards Track [Page 120] RFC 2328 OSPF Version 2 April 1998

      12.1.7.  LS checksum
          This field is the checksum of the complete contents of the
          LSA, excepting the LS age field.  The LS age field is
          excepted so that an LSA'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 [Ref6].  The LSA header also contains the length
          of the LSA 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 LSA.
          This corruption can occur while an LSA 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 considered a checksum
          failure.  In other words, calculation of the checksum is not
          optional.
          The checksum of an LSA 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 LSA 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.[13] 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. All routers belonging to the same area have
      identical link state 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

Moy Standards Track [Page 121] RFC 2328 OSPF Version 2 April 1998

      area link-state 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-LSAs, network-LSAs and
      summary-LSAs (all listed in the area data structure).  In
      addition, external routes (AS-external-LSAs) 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
      LSA's LS type, Link State ID and Advertising Router.[14] There
      will be a single instance (the most up-to-date) of each LSA in
      the database.  The database lookup function is invoked during
      the LSA 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
      originated a particular LSA, and if so, with what LS sequence
      number.
      An LSA 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).  An LSA 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 LSAs (Section 12.4) or c) the LSA ages
      out and is flushed from the routing domain (Section 14).
      Whenever an LSA 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
      For backward compatibility with previous versions of the OSPF
      specification ([Ref9]), TOS-specific information can be included
      in router-LSAs, summary-LSAs and AS-external-LSAs.  The encoding
      of TOS in OSPF LSAs is specified in Table 17. That table relates
      the OSPF encoding to the IP packet header's TOS field (defined
      in [Ref12]).  The OSPF encoding is expressed as a decimal

Moy Standards Track [Page 122] RFC 2328 OSPF Version 2 April 1998

      integer, and the IP packet header's TOS field is expressed in
      the binary TOS values used in [Ref12].
                  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.
  12.4.  Originating LSAs
      Into any given OSPF area, a router will originate several LSAs.
      Each router originates a router-LSA.  If the router is also the
      Designated Router for any of the area's networks, it will
      originate network-LSAs for those networks.
      Area border routers originate a single summary-LSA for each
      known inter-area destination.  AS boundary routers originate a
      single AS-external-LSA 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
      collection of routes.  During the flooding procedure, many LSAs
      can be carried by a single Link State Update packet.

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      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 LSAs into the backbone (one
      router-LSA, and one summary-LSA for each of the networks N1-N4).
      Router RT4 will also originate 8 distinct LSAs into Area 1 (one
      router-LSA and seven summary-LSAs as pictured in Figure 7).  If
      RT4 has been selected as Designated Router for Network N3, it
      will also originate a network-LSA for N3 into Area 1.
      In this same figure, Router RT5 will be originating 3 distinct
      AS-external-LSAs (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 AS-external-LSAs for
      networks N12-N14 will not be flooded into area 3 (see Section
      3.6).  Instead, Router RT11 would originate a default summary-
      LSA 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 an LSA is originated, its LS sequence
      number is incremented, its LS age is set to 0, its LS checksum
      is calculated, and the LSA is added to the link state database
      and flooded out the appropriate interfaces.  See Section 13.2
      for details concerning the installation of the LSA into the link
      state database.  See Section 13.3 for details concerning the
      flooding of newly originated LSAs.
      The ten events that can cause a new instance of an LSA to be
      originated are:
      (1) The LS age field of one of the router's self-originated LSAs
          reaches the value LSRefreshTime. In this case, a new
          instance of the LSA is originated, even though the contents
          of the LSA (apart from the LSA header) will be the same.
          This guarantees periodic originations of all LSAs.  This
          periodic updating of LSAs adds robustness to the link state
          algorithm.  LSAs that solely describe unreachable
          destinations should not be refreshed, but should instead be
          flushed from the routing domain (see Section 14.1).

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      When whatever is being described by an LSA changes, a new LSA is
      originated.  However, two instances of the same LSA 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 an LSA to change.  These events should cause new
      originations if and only if the contents of the new LSA 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-LSA.
      (3) An attached network's Designated Router changes.  A new
          router-LSA should be originated.  Also, if the router itself
          is now the Designated Router, a new network-LSA should be
          produced.  If the router itself is no longer the Designated
          Router, any network-LSA 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-LSA.  Also, if the router is itself
          the Designated Router for the attached network, a new
          network-LSA 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-
          LSA (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-
          LSA (for this route) to be originated in each attached area
          (but NEVER for the backbone).

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      (7) The router becomes newly attached to an area.  The router
          must then originate summary-LSAs 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-LSA into the virtual link's Transit area (see the
          discussion of the router-LSA's bit V in Section 12.4.1), as
          well as originating a new router-LSA 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 BGP) changes.  This will
          cause an AS boundary router to originate a new instance of
          an AS-external-LSA.
      (10)
          A router ceases to be an AS boundary router, perhaps after
          restarting. In this situation the router should flush all
          AS-external-LSAs that it had previously originated.  These
          LSAs can be flushed via the premature aging procedure
          specified in Section 14.1.
      The construction of each type of LSA is explained in detail
      below.  In general, these sections describe the contents of the
      LSA body (i.e., the part coming after the 20-byte LSA header).
      For information concerning the building of the LSA header, see
      Section 12.1.
      12.4.1.  Router-LSAs
          A router originates a router-LSA for each area that it
          belongs to.  Such an LSA describes the collected states of
          the router's links to the area.  The LSA is flooded
          throughout the particular area, and no further.

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                ....................................
                . 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-LSA is shown in Appendix A (Section
          A.4.2).  The first 20 bytes of the LSA consist of the
          generic LSA header that was discussed in Section 12.1.
          router-LSAs have LS type = 1.
          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-LSAs. This
          enables paths to those types of routers to be saved in the
          routing table, for later processing of summary-LSAs and AS-
          external-LSAs.  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-LSA for a stub area (stub
          areas cannot contain AS boundary routers).

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          In addition, the router sets bit V in its router-LSA for
          Area A if and only if the router is the endpoint of one or
          more fully adjacent virtual links having Area A as their
          Transit area. The setting of bit V enables other routers in
          Area A to discover whether the area supports transit traffic
          (see TransitCapability in Section 6).
          The router-LSA 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-LSA.
          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 point-to-point links
          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 stub
          network's IP address mask.  For unnumbered point-to-point
          links, the Link Data field should be set to the unnumbered
          interface's MIB-II [Ref8] ifIndex value.

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          Finally, the cost of using the link for output is specified.
          The output cost of a link is configurable.  With the
          exception of links to stub networks, the output cost must
          always be non-zero.
          To further describe the process of building the list of link
          descriptions, suppose a router wishes to build a router-LSA
          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 LSA, and the next interface
              should be examined.
          o   If the state of the interface is Down, no links are
              added.
          o   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 point-to-point network.  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   Otherwise, the link descriptions added to the router-LSA
              depend on the OSPF interface type. Link descriptions
              used for point-to-point interfaces are specified in
              Section 12.4.1.1, for virtual links in Section 12.4.1.2,
              for broadcast and NBMA interfaces in 12.4.1.3, and for
              Point-to-MultiPoint interfaces in 12.4.1.4.
          After consideration of all the router interfaces, host links
          are added to the router-LSA by examining the list of
          attached hosts belonging to Area A.  A host route is
          represented as a Type 3 link (stub network) whose Link ID is
          the host's IP address, Link Data is the mask of all ones
          (0xffffffff), and cost the host's configured cost (see
          Section C.7).

Moy Standards Track [Page 129] RFC 2328 OSPF Version 2 April 1998

          12.4.1.1.  Describing point-to-point interfaces
              For point-to-point interfaces, one or more link
              descriptions are added to the router-LSA as follows:
              o   If the neighboring router is fully adjacent, add a
                  Type 1 link (point-to-point). The Link ID should be
                  set to the Router ID of the neighboring router. For
                  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 [Ref8]
                  ifIndex value. The cost should be set to the output
                  cost of the point-to-point interface.
              o   In addition, as long as the state of the interface
                  is "Point-to-Point" (and regardless of the
                  neighboring router state), a Type 3 link (stub
                  network) should be added. There are two forms that
                  this stub link can take:
                  Option 1
                      Assuming that the neighboring router's IP
                      address is known, set the Link ID of the Type 3
                      link to the neighbor's IP address, the Link Data
                      to the mask 0xffffffff (indicating a host
                      route), and the cost to the interface's
                      configured output cost.[15]
                  Option 2
                      If a subnet has been assigned to the point-to-
                      point link, set the Link ID of the Type 3 link
                      to the subnet's IP address, the Link Data to the
                      subnet's mask, and the cost to the interface's
                      configured output cost.[16]
          12.4.1.2.  Describing broadcast and NBMA interfaces
              For operational broadcast and NBMA interfaces, a single
              link description is added to the router-LSA as follows:

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              o   If the state of the interface is Waiting, add a Type
                  3 link (stub network) with Link ID set to the IP
                  network number of the attached network, Link Data
                  set to the attached network's address mask, and cost
                  equal to the interface's configured output cost.
              o   Else, there has been a Designated Router elected 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) with Link ID set to the IP
                  interface address of the attached network's
                  Designated Router (which may be the router itself),
                  Link Data set to the router's own IP interface
                  address, and cost equal to the interface's
                  configured output cost.  Otherwise, add a link as if
                  the interface state were Waiting (see above).
          12.4.1.3.  Describing virtual links
              For virtual links, a link description is added to the
              router-LSA only when the virtual neighbor is fully
              adjacent. In this case, add a Type 4 link (virtual link)
              with Link ID set to the Router ID of the virtual
              neighbor, Link Data set to the IP interface address
              associated with the virtual link and cost set to the
              cost calculated for the virtual link during the routing
              table calculation (see Section 15).
          12.4.1.4.  Describing Point-to-MultiPoint interfaces
              For operational Point-to-MultiPoint interfaces, one or
              more link descriptions are added to the router-LSA as
              follows:
              o   A single Type 3 link (stub network) is added with
                  Link ID set to the router's own IP interface
                  address, Link Data set to the mask 0xffffffff
                  (indicating a host route), and cost set to 0.

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              o   For each fully adjacent neighbor associated with the
                  interface, add an additional Type 1 link (point-to-
                  point) with Link ID set to the Router ID of the
                  neighboring router, Link Data set to the IP
                  interface address and cost equal to the interface's
                  configured output cost.
          12.4.1.5.  Examples of router-LSAs
              Consider the router-LSAs 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-LSAs, 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-LSA 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
              an area border router.
      ; RT3's router-LSA for Area 1
      LS age = 0                     ;always true on origination
      Options = (E-bit)              ;
      LS type = 1                    ;indicates router-LSA
      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

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      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
             # TOS metrics = 0
             metric = 1
             Link ID = 192.1.4.0     ;IP Network number
             Link Data = 0xffffff00  ;Network mask
             Type = 3                ;connects to stub network
             # TOS metrics = 0
             metric = 2
                  Next RT3's router-LSA 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 an area border router.
      ; RT3's router-LSA for the backbone
      LS age = 0                     ;always true on origination
      Options = (E-bit)              ;
      LS type = 1                    ;indicates router-LSA
      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
             # TOS metrics = 0
             metric = 8
      12.4.2.  Network-LSAs
          A network-LSA is generated for every transit broadcast or
          NBMA network.  (A transit network is a network having two or
          more attached routers).  The network-LSA describes all the
          routers that are attached to the network.

Moy Standards Track [Page 133] RFC 2328 OSPF Version 2 April 1998

          The Designated Router for the network originates the LSA.
          The Designated Router originates the LSA only if it is fully
          adjacent to at least one other router on the network.  The
          network-LSA is flooded throughout the area that contains the
          transit network, and no further.  The network-LSA 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 includes itself in this
          list.
          The Link State ID for a network-LSA 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-LSA) 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-LSA that
          it had previously originated.  This LSA is no longer used in
          the routing table calculation.  It is flushed by prematurely
          incrementing the LSA'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-LSAs 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-LSAs are
          indicated by having their Link State ID equal to one of the
          router's IP interface addresses and their Advertising Router
          equal to some value other than the router's current Router
          ID (see Section 13.4 for more details).
          12.4.2.1.  Examples of network-LSAs
              Again consider the area configuration in Figure 6.
              Network-LSAs 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-LSA is generated by RT4 on behalf of Network N3
              (see Figure 15 for the address assignments):
      ; Network-LSA for Network N3

Moy Standards Track [Page 134] RFC 2328 OSPF Version 2 April 1998

      LS age = 0                     ;always true on origination
      Options = (E-bit)              ;
      LS type = 2                    ;indicates network-LSA
      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
      12.4.3.  Summary-LSAs
          The destination described by a summary-LSA is either an IP
          network, an AS boundary router or a range of IP addresses.
          Summary-LSAs are flooded throughout a single area only.  The
          destination described is one that is external to the area,
          yet still belongs to the Autonomous System.
          Summary-LSAs 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-LSAs.  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-LSAs.
              If the routing table entry has Path-type of type 1
              external or type 2 external, examine the next routing
              table entry.

Moy Standards Track [Page 135] RFC 2328 OSPF Version 2 April 1998

          o   Else, if the area associated with this set of paths is
              the Area A itself, do not generate a summary-LSA for the
              route.[17]
          o   Else, if the next hops associated with this set of paths
              belong to Area A itself, do not generate a summary-LSA
              for the route.[18] 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-LSA cannot be generated for
              this route.
          o   Else, if the destination of this route is an AS boundary
              router, a summary-LSA should be originated if and only
              if the routing table entry describes the preferred path
              to the AS boundary router (see Step 3 of Section 16.4).
              If so, a Type 4 summary-LSA is originated for 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. Note: these LSAs 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 summary-LSA 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
              E 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-LSAs.  Remember that an area has a
              configured 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 summary-LSA is originated for each range. When
              the range's status indicates Advertise, a Type 3
              summary-LSA is generated with Link State ID equal to the

Moy Standards Track [Page 136] RFC 2328 OSPF Version 2 April 1998

              range's address (if necessary, the Link State ID can
              also have one or more of the range's "host" bits set;
              see Appendix E for details) and cost equal to the
              largest cost of any of the component networks. When the
              range's status indicates DoNotAdvertise, the Type 3
              summary-LSA 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 summary-
              LSA 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 E for details) and metric equal to the
              network's routing table cost.
              If an area is capable of carrying transit traffic (i.e.,
              its TransitCapability is set to TRUE), routing
              information concerning backbone networks should not be
              condensed before being summarized into the area.  Nor
              should the advertisement of backbone networks into
              transit areas be suppressed.  In other words, the
              backbone's configured ranges should be ignored when
              originating summary-LSAs into transit areas.
          If a router advertises a summary-LSA for a destination which
          then becomes unreachable, the router must then flush the LSA
          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 LSA should also be flushed from the
          routing domain.
          12.4.3.1.  Originating summary-LSAs 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-LSAs into the area

Moy Standards Track [Page 137] RFC 2328 OSPF Version 2 April 1998

              according to the Section 12.4.3's algorithm, or can
              choose to originate only a subset of the summary-LSAs,
              possibly under configuration control.  The fewer LSAs
              originated, the smaller the stub area's link state
              database, further reducing the demands on its routers'
              resources. However, omitting LSAs may also lead to sub-
              optimal inter-area routing, although routing will
              continue to function.
              As specified in Section 12.4.3, Type 4 summary-LSAs
              (ASBR-summary-LSAs) are never originated into stub
              areas.
              In a stub area, instead of importing external routes
              each area border router originates a "default summary-
              LSA" into the area. The Link State ID for the default
              summary-LSA 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.
          12.4.3.2.  Examples of summary-LSAs
              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-LSAs.
              Consider in particular Router RT4.  Its routing table
              was calculated as the example in Section 11.3.  RT4
              originates summary-LSAs into both the backbone and Area
              1.  Into the backbone, Router RT4 originates separate
              LSAs for each of the networks N1-N4.  Into Area 1,
              Router RT4 originates separate LSAs for networks N6-N8
              and the AS boundary routers RT5,RT7.  It also condenses
              host routes Ia and Ib into a single summary-LSA.
              Finally, the routes to networks N9,N10,N11 and Host H1
              are advertised by a single summary-LSA.  This
              condensation was originally performed by the router
              RT11.

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              These LSAs are illustrated graphically in Figures 7 and
              8.  Two of the summary-LSAs originated by Router RT4
              follow.  The actual IP addresses for the networks and
              routers in question have been assigned in Figure 15.
      ; Summary-LSA for Network N1,
      ; originated by Router RT4 into the backbone
      LS age = 0                  ;always true on origination
      Options = (E-bit)           ;
      LS type = 3                 ;Type 3 summary-LSA
      Link State ID = 192.1.2.0   ;N1's IP network number
      Advertising Router = 192.1.1.4       ;RT4's ID
      metric = 4
      ; Summary-LSA for AS boundary router RT7
      ; originated by Router RT4 into Area 1
      LS age = 0                  ;always true on origination
      Options = (E-bit)           ;
      LS type = 4                 ;Type 4 summary-LSA
      Link State ID = Router RT7's ID
      Advertising Router = 192.1.1.4       ;RT4's ID
      metric = 14
      12.4.4.  AS-external-LSAs
          AS-external-LSAs describe routes to destinations external to
          the Autonomous System.  Most AS-external-LSAs describe
          routes to specific external destinations; in these cases the
          LSA'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 E for
          details).  However, a default route for the Autonomous
          System can be described in an AS-external-LSA by setting the
          LSA's Link State ID to DefaultDestination (0.0.0.0).  AS-
          external-LSAs are originated by AS boundary routers.  An AS
          boundary router originates a single AS-external-LSA for each
          external route that it has learned, either through another
          routing protocol (such as BGP), or through configuration
          information.

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          AS-external-LSAs are the only type of LSAs that are flooded
          throughout the entire Autonomous System; all other types of
          LSAs are specific to a single area.  However, AS-external-
          LSAs 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.
          If a router advertises an AS-external-LSA for a destination
          which then becomes unreachable, the router must then flush
          the LSA from the routing domain by setting its age to MaxAge
          and reflooding (see Section 14.1).
          12.4.4.1.  Examples of AS-external-LSAs
              Consider once again the AS pictured in Figure 6.  There
              are two AS boundary routers: RT5 and RT7.  Router RT5
              originates three AS-external-LSAs, for networks N12-N14.
              Router RT7 originates two AS-external-LSAs, for networks
              N12 and N15.  Assume that RT7 has learned its route to
              N12 via BGP, and that it wishes to advertise a Type 2
              metric to the AS.  RT7 would then originate the
              following LSA for N12:
      ; AS-external-LSA for Network N12,
      ; originated by Router RT7
      LS age = 0                  ;always true on origination
      Options = (E-bit)           ;
      LS type = 5                 ;AS-external-LSA
      Link State ID = N12's IP network number
      Advertising Router = Router RT7's ID
      bit E = 1                   ;Type 2 metric
      metric = 2
      Forwarding address = 0.0.0.0

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                  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 BGP information with the
                  non-OSPF router RTX.  RTA must then originate AS-
                  external-LSAs for those destinations it has learned
                  from RTX.  By using the AS-external-LSA'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 that all externally-destined packets
                  should by default be forwarded to its BGP peer RTX.
                  The resulting AS-external-LSA 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 = (E-bit)           ;
      LS type = 5                 ;AS-external-LSA
      Link State ID = DefaultDestination  ; default route
      Advertising Router = Router RTA's ID
      bit E = 1                   ;Type 2 metric
      metric = 1
      Forwarding address = RTX's IP address
                  In figure 16, suppose instead that both RTA and RTB
                  exchange BGP information with RTX.  In this case,

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                  RTA and RTB would originate the same set of AS-
                  external-LSAs.  These LSAs, 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 AS-external-LSAs, 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 LSAs
                  (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-LSAs (i.e., same destination, cost and
                  non-zero forwarding address), then the LSA
                  originated by the router having the highest OSPF
                  Router ID is used.  The router having the lower OSPF
                  Router ID can then flush its LSA.  Flushing an LSA
                  is discussed in Section 14.1.
                              +
                              |
                    +---+.....|.BGP
                    |RTA|-----|.....+---+
                    +---+     |-----|RTX|
                              |     +---+
                    +---+     |
                    |RTB|-----|
                    +---+     |
                              |
                    +---+     |
                    |RTC|-----|
                    +---+     |
                              |
                              +
             Figure 16: Forwarding address example

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13. The Flooding Procedure

  Link State Update packets provide the mechanism for flooding LSAs.
  A Link State Update packet may contain several distinct LSAs, and
  floods each LSA one hop further from its point of origination.  To
  make the flooding procedure reliable, each LSA must be acknowledged
  separately.  Acknowledgments are transmitted in Link State
  Acknowledgment packets.  Many separate acknowledgments can also be
  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 LSAs, other than AS-external-LSAs, are associated with
  a specific area.  However, LSAs do not contain an area field.  An
  LSA's area must be deduced from the Link State Update packet header.
  For each LSA contained in a Link State Update packet, the following
  steps are taken:
  (1) Validate the LSA's LS checksum.  If the checksum turns out to be
      invalid, discard the LSA and get the next one from the Link
      State Update packet.
  (2) Examine the LSA's LS type.  If the LS type is unknown, discard
      the LSA and get the next one from the Link State Update Packet.
      This specification defines LS types 1-5 (see Section 4.3).
  (3) Else if this is an AS-external-LSA (LS type = 5), and the area
      has been configured as a stub area, discard the LSA and get the
      next one from the Link State Update Packet.  AS-external-LSAs
      are not flooded into/throughout stub areas (see Section 3.6).
  (4) Else if the LSA's LS age is equal to MaxAge, and there is
      currently no instance of the LSA in the router's link state
      database, and none of router's neighbors are in states Exchange

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      or Loading, then take the following actions: a) Acknowledge the
      receipt of the LSA by sending a Link State Acknowledgment packet
      back to the sending neighbor (see Section 13.5), and b) Discard
      the LSA and examine the next LSA (if any) listed in the Link
      State Update packet.
  (5) Otherwise, find the instance of this LSA that is currently
      contained in the router's link state database.  If there is no
      database copy, or the received LSA is more recent than the
      database copy (see Section 13.1 below for the determination of
      which LSA is more recent) the following steps must be performed:
      (a) If there is already a database copy, and if the database
          copy was received via flooding and installed less than
          MinLSArrival seconds ago, discard the new LSA (without
          acknowledging it) and examine the next LSA (if any) listed
          in the Link State Update packet.
      (b) Otherwise immediately flood the new LSA 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
          LSA was received from a router other than the Backup DR) the
          LSA 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.
      (d) Install the new LSA 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 LSA with the current time (i.e., the time it was
          received).  The flooding procedure cannot overwrite the
          newly installed LSA until MinLSArrival seconds have elapsed.
          The LSA installation process is discussed further in Section
          13.2.
      (e) Possibly acknowledge the receipt of the LSA by sending a
          Link State Acknowledgment packet back out the receiving
          interface.  This is explained below in Section 13.5.

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      (f) If this new LSA indicates that it was originated by the
          receiving router itself (i.e., is considered a self-
          originated LSA), the router must take special action, either
          updating the LSA or in some cases flushing it from the
          routing domain. For a description of how self-originated
          LSAs are detected and subsequently handled, see Section
          13.4.
  (6) Else, if there is an instance of the LSA 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 LSA 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 LSA is listed in the Link state retransmission list
          for the receiving adjacency, the router itself is expecting
          an acknowledgment for this LSA.  The router should treat the
          received LSA as an acknowledgment by removing the LSA 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 LSA by sending a
          Link State Acknowledgment packet back out the receiving
          interface.  This is explained below in Section 13.5.
  (8) Else, the database copy is more recent.  If the database copy
      has LS age equal to MaxAge and LS sequence number equal to
      MaxSequenceNumber, simply discard the received LSA without
      acknowledging it. (In this case, the LSA's LS sequence number is
      wrapping, and the MaxSequenceNumber LSA must be completely
      flushed before any new LSA instance can be introduced).
      Otherwise, as long as the database copy has not been sent in a
      Link State Update within the last MinLSArrival seconds, send the
      database copy back to the sending neighbor, encapsulated within
      a Link State Update Packet. The Link State Update Packet should
      be sent directly to the neighbor. In so doing, do not put the

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      database copy of the LSA on the neighbor's link state
      retransmission list, and do not acknowledge the received (less
      recent) LSA instance.
  13.1.  Determining which LSA is newer
      When a router encounters two instances of an LSA, it must
      determine which is more recent.  This occurred above when
      comparing a received LSA to its database copy.  This comparison
      must also be done during the Database Exchange procedure which
      occurs during adjacency bring-up.
      An LSA is identified by its LS type, Link State ID and
      Advertising Router.  For two instances of the same LSA, the LS
      sequence number, LS age, and LS checksum fields are used to
      determine which instance is more recent:
      o   The LSA 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.

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  13.2.  Installing LSAs in the database
      Installing a new LSA in the database, either as the result of
      flooding or a newly self-originated LSA, may cause the OSPF
      routing table structure to be recalculated.  The contents of the
      new LSA should be compared to the old instance, if present.  If
      there is no difference, there is no need to recalculate the
      routing table. When comparing an LSA to its previous instance,
      the following are all considered to be differences in contents:
          o   The LSA's Options field has changed.
          o   One of the LSA instances has LS age set to MaxAge, and
              the other does not.
          o   The length field in the LSA header has changed.
          o   The body of the LSA (i.e., anything outside the 20-byte
              LSA header) has changed. Note that this excludes changes
              in LS Sequence Number and LS Checksum.
      If the contents are different, the following pieces of the
      routing table must be recalculated, depending on the new LSA's
      LS type field:
      Router-LSAs and network-LSAs
          The entire routing table must be recalculated, starting with
          the shortest path calculations for each area (not just the
          area whose link-state 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.[19]
      Summary-LSAs
          The best route to the destination described by the summary-
          LSA 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-LSAs.

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      AS-external-LSAs
          The best route to the destination described by the AS-
          external-LSA must be recalculated (see Section 16.6).
      Also, any old instance of the LSA must be removed from the
      database when the new LSA 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) LSA 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 LSA to the
      appropriate neighbors' Link state 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 LSA
      that the router itself has just originated (see Section 12.4).
      For these LSAs, this section provides the entirety of the
      flooding procedure (i.e., the processing of Section 13 is not
      performed, since, for example, the LSA has not been received
      from a neighbor and therefore does not need to be acknowledged).
      Depending upon the LSA's LS type, the LSA can be flooded out
      only certain interfaces.  These interfaces, defined by the
      following, are called the eligible interfaces:
      AS-external-LSAs (LS Type = 5)
          AS-external-LSAs 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

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          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 an LSA out a particular interface, if there
      is a high probability that the attached neighbors have already
      received the LSA.  However, in these cases the flooding
      procedure must be absolutely sure that the neighbors eventually
      do receive the LSA, so the LSA 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
          LSA.  The following steps are executed for each neighbor:
          (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 LSA on the list, it indicates that
              the neighboring router has an instance of the LSA
              already.  Compare the new LSA to the neighbor's copy:
              o   If the new LSA is less recent, then examine the next
                  neighbor.
              o   If the two copies are the same instance, then delete
                  the LSA from the Link state request list, and
                  examine the next neighbor.[20]
              o   Else, the new LSA is more recent.  Delete the LSA
                  from the Link state request list.

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          (c) If the new LSA 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 LSA.  Add the new LSA
              to the Link state retransmission list for the adjacency.
              This ensures that the flooding procedure is reliable;
              the LSA will be retransmitted at intervals until an
              acknowledgment is seen from the neighbor.
      (2) The router must now decide whether to flood the new LSA out
          this interface.  If in the previous step, the LSA was NOT
          added to any of the Link state retransmission lists, there
          is no need to flood the LSA out the interface and the next
          interface should be examined.
      (3) If the new LSA 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 LSA already.  Therefore, examine the next
          interface.
      (4) If the new LSA was received on this interface, and the
          interface state is Backup (i.e., the router itself is the
          Backup Designated Router), examine the next interface.  The
          Designated Router will do the flooding on this interface.
          However, if the Designated Router fails the router (i.e.,
          the Backup Designated Router) will end up retransmitting the
          updates.
      (5) If this step is reached, the LSA must be flooded out the
          interface.  Send a Link State Update packet (including the
          new LSA as contents) out the interface.  The LSA's LS age
          must be incremented by InfTransDelay (which must be > 0)
          when it is copied into the outgoing Link State Update packet
          (until the LS age field reaches the 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

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          address AllSPFRouters should be used.  Otherwise, the
          address AllDRouters should be used.
          On non-broadcast 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 LSAs
      It is a common occurrence for a router to receive self-
      originated LSAs via the flooding procedure. A self-originated
      LSA is detected when either 1) the LSA's Advertising Router is
      equal to the router's own Router ID or 2) the LSA is a network-
      LSA and its Link State ID is equal to one of the router's own IP
      interface addresses.
      However, if the received self-originated LSA is newer than the
      last instance that the router actually originated, the router
      must take special action.  The reception of such an LSA
      indicates that there are LSAs in the routing domain that were
      originated by the router before the last time it was restarted.
      In most cases, the router must then advance the LSA's LS
      sequence number one past the received LS sequence number, and
      originate a new instance of the LSA.
      It may be the case the router no longer wishes to originate the
      received LSA. Possible examples include: 1) the LSA is a
      summary-LSA or AS-external-LSA and the router no longer has an
      (advertisable) route to the destination, 2) the LSA is a
      network-LSA but the router is no longer Designated Router for
      the network or 3) the LSA is a network-LSA 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 LSA).  In
      all these cases, instead of updating the LSA, the LSA should be
      flushed from the routing domain by incrementing the received
      LSA's LS age to MaxAge and reflooding (see Section 14.1).

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  13.5.  Sending Link State Acknowledgment packets
      Each newly received LSA 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 which received the LSAs.  The packet can be sent in
      one of two ways: delayed and sent on an interval timer, or sent
      directly to a particular neighbor.  The particular
      acknowledgment strategy used depends on the circumstances
      surrounding the receipt of the LSA.
      Sending delayed acknowledgments accomplishes several things: 1)
      it facilitates the packaging of multiple acknowledgments in a
      single Link State Acknowledgment packet, 2) it enables a single
      Link State Acknowledgment packet to indicate acknowledgments to
      several neighbors at once (through multicasting) and 3) it
      randomizes the Link State Acknowledgment packets sent by the
      various routers attached to a common 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 directly to a particular
      neighbor in response to the receipt of duplicate LSAs. Direct
      acknowledgments are sent immediately when the duplicate is
      received. On multi-access networks, these acknowledgments are
      sent directly to the neighbor's IP address.
      The precise procedure for sending Link State Acknowledgment
      packets is described in Table 19.  The circumstances surrounding
      the receipt of the LSA 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

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                                   Action taken in state
 Circumstances            Backup                All other states
 _________________________________________________________________
 LSA  has                 No  acknowledgment    No  acknowledgment
 been  flooded back       sent.                 sent.
 out receiving  in-
 terface  (see Sec-
 tion 13, step 5b).
 _________________________________________________________________
 LSA   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
 _________________________________________________________________
 LSA 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
 _________________________________________________________________
 LSA is a                 Direct acknowledg-    Direct acknowledg-
 duplicate, and was       ment sent.            ment sent.
 not treated as  an
 implied       ack-
 nowledgment.
 _________________________________________________________________
 LSA's LS                 Direct acknowledg-    Direct acknowledg-
 age is equal to          ment sent.            ment sent.
 MaxAge, and there is
 no current instance
 of the LSA
 in the link state
 database, and none
 of router's neighbors
 are in states Exchange

Moy Standards Track [Page 153] RFC 2328 OSPF Version 2 April 1998

 or Loading (see
 Section 13, step 4).
           Table 19: Sending link state acknowledgements.
      on the state of the interface.  If the interface state is DR or
      Backup, the destination AllSPFRouters is used.  In all 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 LSA to
      Network N3, it is received by routers RT1, RT2, and RT3.  These
      routers will not flood the LSA back onto net N3, but they still
      must ensure that their link-state 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 LSAs (see Section 13.3, step 4).
  13.6.  Retransmitting LSAs
      LSAs flooded out an adjacency are placed on the adjacency's Link
      state retransmission list.  In order to ensure that flooding is
      reliable, these LSAs are retransmitted until they are
      acknowledged.  The length of time between retransmissions is a
      configurable per-interface value, RxmtInterval.  If this is set

Moy Standards Track [Page 154] RFC 2328 OSPF Version 2 April 1998

      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 affected.
      Several retransmitted LSAs may fit into a single Link State
      Update packet.  When LSAs are to be retransmitted, only the
      number fitting in a single Link State Update packet should be
      sent.  Another packet of retransmissions can be sent whenever
      some of the LSAs are acknowledged, or on the next firing of the
      retransmission timer.
      Link State Update Packets carrying retransmissions are always
      sent directly to the neighbor. On multi-access networks, this
      means that retransmissions are sent directly to the neighbor's
      IP address.  Each LSA's LS age must be incremented by
      InfTransDelay (which must be > 0) when it is copied into the
      outgoing Link State Update packet (until the LS age field
      reaches the maximum value of MaxAge).
      If an 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 LSA acknowledged have an instance on the Link state
          retransmission list for the neighbor?  If not, examine the
          next acknowledgment.  Otherwise:

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      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.

14. Aging The Link State Database

  Each LSA has an LS age field.  The LS age is expressed in seconds.
  An LSA'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 LSA's LS age is
  incremented by InfTransDelay.
  An LSA's LS age is never incremented past the value MaxAge.  LSAs
  having age MaxAge are not used in the routing table calculation.  As
  a router ages its link state database, an LSA's LS age may reach
  MaxAge.[21] At this time, the router must attempt to flush the LSA
  from the routing domain.  This is done simply by reflooding the
  MaxAge LSA just as if it was a newly originated LSA (see Section
  13.3).
  When creating a Database summary list for a newly forming adjacency,
  any MaxAge LSAs 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 LSA 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 LSA'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.

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  14.1.  Premature aging of LSAs
      An LSA can be flushed from the routing domain by setting its LS
      age to MaxAge, while leaving its LS sequence number alone, and
      then reflooding the LSA.  This procedure follows the same course
      as flushing an LSA whose LS age has naturally reached the value
      MaxAge (see Section 14).  In particular, the MaxAge LSA 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 LSA's LS age to
      MaxAge "premature aging".
      Premature aging is used when it is time for a self-originated
      LSA's sequence number field to wrap.  At this point, the current
      LSA instance (having LS sequence number MaxSequenceNumber) must
      be prematurely aged and flushed from the routing domain before a
      new instance with sequence number equal to InitialSequenceNumber
      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 AS-
      external-LSA from the routing domain via premature aging. This
      procedure is preferable to the alternative, which is to
      originate a new LSA for the destination specifying a metric of
      LSInfinity.  Premature aging is also be used when unexpectedly
      receiving self-originated LSAs during the flooding procedure
      (see Section 13.4).
      A router may only prematurely age its own self-originated LSAs.
      The router may not prematurely age LSAs that have been
      originated by other routers. An LSA is considered self-
      originated when either 1) the LSA's Advertising Router is equal
      to the router's own Router ID or 2) the LSA is a network-LSA and
      its Link State ID is equal to one of the router's own IP
      interface addresses.

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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 and 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-LSAs, and OSPF packets
  pertaining to the backbone area will flow over the 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).  This is called the virtual link's
  corresponding routing table entry.  The InterfaceUp event occurs for
  a virtual link when its corresponding routing table entry becomes
  reachable.  Conversely, the InterfaceDown event occurs when its
  routing table entry becomes unreachable.  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-LSA) 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-LSAs are NEVER flooded over virtual adjacencies.
      This would be duplication of effort, since the same AS-

Moy Standards Track [Page 158] RFC 2328 OSPF Version 2 April 1998

      external-LSAs are already flooded throughout the virtual link's
      Transit area.  For this same reason, AS-external-LSAs 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-LSA 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.
  o   In each endpoint's router-LSA 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.
  o   A non-backbone area can carry transit data traffic (i.e., is
      considered a "transit area") if and only if it serves as the
      Transit area for one or more fully adjacent virtual links (see
      TransitCapability in Sections 6 and 16.1). Such an area requires
      special treatment when summarizing backbone networks into it
      (see Section 12.4.3), and during the routing calculation (see
      Section 16.3).
  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

Moy Standards Track [Page 159] RFC 2328 OSPF Version 2 April 1998

      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-LSA originated by a certain router).
  This access is performed by the lookup function discussed in Section
  12.2.  The lookup process may return an LSA whose LS age is equal to
  MaxAge.  Such an LSA 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.
  (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-LSAs.  If the router is attached to multiple areas
      (i.e., it is an area border router), only backbone summary-LSAs
      are examined.

Moy Standards Track [Page 160] RFC 2328 OSPF Version 2 April 1998

  (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-LSAs 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-LSAs.  The locations of the AS
      boundary routers (which originate the AS-external-LSAs) have
      been determined in steps 2-4.
  Steps 2-5 are explained in further detail below.
  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-LSAs (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.[22] 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
      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:

Moy Standards Track [Page 161] RFC 2328 OSPF Version 2 April 1998

      Vertex (node) ID
          A 32-bit number which together with the vertex type (router
          or network) uniquely identifies the vertex.  For router
          vertices the Vertex ID is the router's OSPF Router ID.  For
          network vertices, it is the IP address of the network's
          Designated Router.
      An LSA
          Each transit vertex has an associated LSA.  For router
          vertices, this is a router-LSA.  For transit networks, this
          is a network-LSA (which is actually originated by the
          network's Designated Router).  In any case, the LSA'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 broadcast,
          Point-to-MultiPoint and NBMA 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-LSAs and
          network-LSAs).  One path is said to be "shorter" than
          another if it has a smaller link state cost.
      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

Moy Standards Track [Page 162] RFC 2328 OSPF Version 2 April 1998

      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 LSA 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-LSA, and bit V of the router-LSA (see
          Section A.4.2) is set, set Area A's TransitCapability to
          TRUE.  In any case, each link described by the LSA 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 LSA.  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 LSA (router-LSA or
              network-LSA) in Area A's link state database.  If the
              LSA 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 LSA.[23]
          (c) If vertex W is already on the shortest-path tree,
              examine the next link in the LSA.
          (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:

Moy Standards Track [Page 163] RFC 2328 OSPF Version 2 April 1998

              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.

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          If the newly added vertex is an area border router or AS
          boundary router, a routing table entry is added whose
          destination type is "router".  The Options field found in
          the associated router-LSA is copied into the routing table
          entry's Optional capabilities field. Call the newly added
          vertex Router X.  If Router X is the endpoint of one of the
          calculating router's virtual links, and the virtual link
          uses Area A as 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
          Router X, and the virtual neighbor's IP address is set to
          Router X's interface address (contained in Router X's
          router-LSA) that points back to the root of the shortest-
          path tree; equivalently, this is the interface that points
          back to Router X's parent vertex on the shortest-path tree
          (similar to the calculation in Section 16.1.1).
          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-LSA).  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
          added vertex' LSA.
          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' LSA.
      (5) Iterate the algorithm by returning to Step 2.

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      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-LSA is found in the
      link state database.  Each stub network link appearing in the
      LSA 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 LSA.
      (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-LSA whose
          Link State ID is smaller than V's Router ID, reset the Link
          State Origin to V's router-LSA.
          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
          routing table entry's Link State Origin to V's router-LSA.
          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-LSAs is exhausted, the second stage is completed.  At

Moy Standards Track [Page 166] RFC 2328 OSPF Version 2 April 1998

      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 order 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 [Ref1].
      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 IP address of 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
          always a transit vertex (i.e., always a router or a transit
          network).

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          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 outgoing interface in this case is simply the OSPF
          interface connecting to the destination network/router. If
          the destination is a router which connects to the
          calculating router via a Point-to-MultiPoint network, the
          destination's next hop IP address(es) can be determined by
          examining the destination's router-LSA: each link pointing
          back to the calculating router and having a Link Data field
          belonging to the Point-to-MultiPoint network provides an IP
          address of the next hop router. If the destination is a
          directly connected network, or a router which connects to
          the calculating router via a point-to-point interface, no
          next hop IP address is required. If the destination is a
          router connected to the calculating router via 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-LSA.  For each link in
          the router-LSA 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-LSAs.
      If the router has active attachments to multiple areas, only
      backbone summary-LSAs are examined.  Routers attached to a
      single area examine that area's summary-LSAs.  In either case,
      the summary-LSAs examined below are all part of a single area's
      link state database (call it Area A).

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      Summary-LSAs are originated by the area border routers.  Each
      summary-LSA in Area A is considered in turn.  Remember that the
      destination described by a summary-LSA is either a network (Type
      3 summary-LSAs) or an AS boundary router (Type 4 summary-LSAs).
      For each summary-LSA:
      (1) If the cost specified by the LSA is LSInfinity, or if the
          LSA's LS age is equal to MaxAge, then examine the the next
          LSA.
      (2) If the LSA was originated by the calculating router itself,
          examine the next LSA.
      (3) If it is a Type 3 summary-LSA, and the collection of
          destinations described by the summary-LSA equals one of the
          router's configured area address ranges (see Section 3.5),
          and the particular area address range is active, then the
          summary-LSA should be ignored.  "Active" means that there
          are one or more reachable (by intra-area paths) networks
          contained in the area range.
      (4) Else, call the destination described by the LSA N (for Type
          3 summary-LSAs, N's address is obtained by masking the LSA's
          Link State ID with the network/subnet mask contained in the
          body of the LSA), and the area border originating the LSA
          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
          LSA and consider the next in the list.  Else, this LSA
          describes an inter-area path to destination N, whose cost is
          the distance to BR plus the cost specified in the LSA. Call
          the cost of this inter-area path IAC.
      (5) Next, look up the routing table entry for the destination N.
          (If N is an AS boundary router, look up the "router" routing
          table entry associated with Area A).  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.

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      (6) Else, if the paths present in the table are intra-area
          paths, do nothing with the LSA (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.
  16.3.  Examining transit areas' summary-LSAs
      This step is only performed by area border routers attached to
      one or more non-backbone areas that are capable of carrying
      transit traffic (i.e., "transit areas", or those areas whose
      TransitCapability parameter has been set to TRUE in Step 2 of
      the Dijkstra algorithm (see Section 16.1).
      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 also determines the actual next hop(s) for those
      destinations whose next hop was calculated as a virtual link in
      Sections 16.1 and 16.2.  After completion of the calculation
      below, any paths calculated in Sections 16.1 and 16.2 that still
      have unresolved virtual next hops should be discarded.
      The calculation proceeds as follows. All the transit areas'
      summary-LSAs are examined in turn.  Each such summary-LSA
      describes a route through a transit area Area A to a Network N
      (N's address is obtained by masking the LSA's Link State ID with
      the network/subnet mask contained in the body of the LSA) or in
      the case of a Type 4 summary-LSA, to an AS boundary router N.
      Suppose also that the summary-LSA was originated by an area
      border router BR.
      (1) If the cost advertised by the summary-LSA is LSInfinity, or
          if the LSA's LS age is equal to MaxAge, then examine the
          next LSA.

Moy Standards Track [Page 170] RFC 2328 OSPF Version 2 April 1998

      (2) If the summary-LSA was originated by the calculating router
          itself, examine the next LSA.
      (3) Look up the routing table entry for N. (If N is an AS
          boundary router, look up the "router" routing table entry
          associated with the backbone area). 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 LSA. 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 LSA. 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 LSA. Call this cost IAC.
      (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.  The
      calculation installs any better cost found into the routing
      table entry, from which it may be readvertised in summary-LSAs
      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

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                    ........................
                    . 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
      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-LSAs 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-LSAs by the above calculation, Router
      RT1 will also forward Network N1 traffic towards RT5. Note that
      in this example the virtual link enables transit data traffic to
      be forwarded through Area 1, but the actual path the transit
      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.

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  16.4.  Calculating AS external routes
      AS external routes are calculated by examining AS-external-LSAs.
      Each of the AS-external-LSAs is considered in turn.  Most AS-
      external-LSAs describe routes to specific IP destinations.  An
      AS-external-LSA can also describe a default route for the
      Autonomous System (Destination ID = DefaultDestination,
      network/subnet mask = 0x00000000).  For each AS-external-LSA:
      (1) If the cost specified by the LSA is LSInfinity, or if the
          LSA's LS age is equal to MaxAge, then examine the next LSA.
      (2) If the LSA was originated by the calculating router itself,
          examine the next LSA.
      (3) Call the destination described by the LSA N.  N's address is
          obtained by masking the LSA's Link State ID with the
          network/subnet mask contained in the body of the LSA.  Look
          up the routing table entries (potentially one per attached
          area) for the AS boundary router (ASBR) that originated the
          LSA. If no entries exist for router ASBR (i.e., ASBR is
          unreachable), do nothing with this LSA and consider the next
          in the list.
          Else, this LSA describes an AS external path to destination
          N.  Examine the forwarding address specified in the AS-
          external-LSA.  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. Among the multiple routing table
          entries for the ASBR, select the preferred entry as follows.
          If RFC1583Compatibility is set to "disabled", prune the set
          of routing table entries for the ASBR as described in
          Section 16.4.1. In any case, among the remaining routing
          table entries, select the routing table entry with the least
          cost; when there are multiple least cost routing table
          entries the entry whose associated area has the largest OSPF
          Area ID (when considered as an unsigned 32-bit integer) is
          chosen.

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          If the forwarding address is non-zero, look up the
          forwarding address in the routing table.[24] The matching
          routing table entry must specify an intra-area or inter-area
          path; if no such path exists, do nothing with the LSA and
          consider the next in the list.
      (4) Let X be the cost specified by the preferred routing table
          entry for the ASBR/forwarding address, and Y the cost
          specified in the LSA.  X is in terms of the link state
          metric, and Y is a type 1 or 2 external metric.
      (5) 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.
      (6) Compare the AS external path described by the LSA with the
          existing paths in N's routing table entry, as follows. If
          the new path is preferred, it replaces the present paths in
          N's routing table entry.  If the new path is of equal
          preference, it is added to N's routing table entry's list of
          paths.
          (a) Intra-area and inter-area paths are always preferred
              over AS external paths.
          (b) Type 1 external paths are always preferred over type 2
              external paths. When all paths are type 2 external
              paths, the paths with the smallest advertised type 2
              metric are always preferred.
          (c) If the new AS external path is still indistinguishable
              from the current paths in the N's routing table entry,
              and RFC1583Compatibility is set to "disabled", select
              the preferred paths based on the intra-AS paths to the
              ASBR/forwarding addresses, as specified in Section
              16.4.1.

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          (d) If the new AS external path is still indistinguishable
              from the current paths in the N's routing table entry,
              select the preferred path based on a least cost
              comparison.  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 advertising equal type 2 metrics are
              compared by looking at the distance to the forwarding
              addresses.
      16.4.1.  External path preferences
          When multiple intra-AS paths are available to
          ASBRs/forwarding addresses, the following rules indicate
          which paths are preferred. These rules apply when the same
          ASBR is reachable through multiple areas, or when trying to
          decide which of several AS-external-LSAs should be
          preferred. In the former case the paths all terminate at the
          same ASBR, while in the latter the paths terminate at
          separate ASBRs/forwarding addresses. In either case, each
          path is represented by a separate routing table entry as
          defined in Section 11.
          This section only applies when RFC1583Compatibility is set
          to "disabled".
          The path preference rules, stated from highest to lowest
          preference, are as follows. Note that as a result of these
          rules, there may still be multiple paths of the highest
          preference. In this case, the path to use must be determined
          based on cost, as described in Section 16.4.
          o   Intra-area paths using non-backbone areas are always the
              most preferred.
          o   The other paths, intra-area backbone paths and inter-
              area paths, are of equal preference.
  16.5.  Incremental updates -- summary-LSAs
      When a new summary-LSA is received, it is not necessary to
      recalculate the entire routing table.  Call the destination

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      described by the summary-LSA N (N's address is obtained by
      masking the LSA's Link State ID with the network/subnet mask
      contained in the body of the LSA), and let Area A be the area to
      which the LSA 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-LSAs 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-LSA) or to any forwarding addresses, all AS-
          external-LSAs 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-LSA) or to any
          forwarding addresses has changed, all AS-external-LSAs will

Moy Standards Track [Page 176] RFC 2328 OSPF Version 2 April 1998

          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.
  16.6.  Incremental updates -- AS-external-LSAs
      When a new AS-external-LSA is received, it is not necessary to
      recalculate the entire routing table.  Call the destination
      described by the AS-external-LSA N.  N's address is obtained by
      masking the LSA's Link State ID with the network/subnet mask
      contained in the body of the LSA. 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-LSAs 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-LSAs 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 LSA 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.

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          If the entry indicates that the area border router is newly
          reachable, the corresponding virtual link is now
          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, 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-LSA 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 may specify 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.

Moy Standards Track [Page 178] RFC 2328 OSPF Version 2 April 1998

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-LSA 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

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  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
  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]"Discard" entries are necessary to ensure that route
  summarization at area boundaries will not cause packet looping.
  [11]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.
  [12]MaxAgeDiff is an architectural constant.  It indicates the
  maximum dispersion of ages, in seconds, that can occur for a single
  LSA instance as it is flooded throughout the routing domain.  If two
  LSAs differ by more than this, they are assumed to be different
  instances of the same LSA.  This can occur when a router restarts
  and loses track of the LSA's previous LS sequence number.  See
  Section 13.4 for more details.
  [13]When two LSAs have different LS checksums, they are assumed to
  be separate instances.  This can occur when a router restarts, and
  loses track of the LSA's previous LS sequence number.  In the case
  where the two LSAs have the same LS sequence number, it is not
  possible to determine which LSA is actually newer.  However, if the
  wrong LSA is accepted as newer, the originating router will simply
  originate another instance.  See Section 13.4 for further details.
  [14]There is one instance where a lookup must be done based on
  partial information.  This is during the routing table calculation,
  when a network-LSA must be found based solely on its Link State ID.

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  The lookup in this case is still well defined, since no two
  network-LSAs can have the same Link State ID.
  [15]This is the way RFC 1583 specified point-to-point
  representation.  It has three advantages: a) it does not require
  allocating a subnet to the point-to-point link, b) it tends to bias
  the routing so that packets destined for the point-to-point
  interface will actually be received over the interface (which is
  useful for diagnostic purposes) and c) it allows network
  bootstrapping of a neighbor, without requiring that the bootstrap
  program contain an OSPF implementation.
  [16]This is the more traditional point-to-point representation used
  by protocols such as RIP.
  [17]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.
  [18]This clause is only invoked when a non-backbone Area A supports
  transit data traffic (i.e., has TransitCapability set to TRUE).  For
  example, in the area configuration of Figure 6, Area 2 can support
  transit traffic due to the configured virtual link between Routers
  RT10 and RT11. As a result, Router RT11 need only originate a single
  summary-LSA into Area 2 (having the collapsed destination N9-
  N11,H1), since all of Router RT11's other eligible routes have next
  hops belonging to Area 2 itself (and as such only need be advertised
  by other area border routers; in this case, Routers RT10 and RT7).
  [19]By keeping more information in the routing table, it is possible
  for an implementation to recalculate the shortest path tree for only
  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 [Ref1].  However, these algorithms are beyond the
  scope of this specification.
  [20]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.
  [21]It should be a relatively rare occurrence for an LSA's LS age to
  reach MaxAge in this fashion.  Usually, the LSA will be replaced by

Moy Standards Track [Page 181] RFC 2328 OSPF Version 2 April 1998

  a more recent instance before it ages out.
  [22]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.
  [23]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.
  [24]When the forwarding address is non-zero, it should point to a
  router belonging to another Autonomous System.  See Section 12.4.4
  for more details.

Moy Standards Track [Page 182] RFC 2328 OSPF Version 2 April 1998

References

  [Ref1]  McQuillan, J., I. Richer and E. Rosen, "ARPANET Routing
          Algorithm Improvements", BBN Technical Report 3803, April
          1978.
  [Ref2]  Digital Equipment Corporation, "Information processing
          systems -- Data communications -- Intermediate System to
          Intermediate System Intra-Domain Routing Protocol", October
          1987.
  [Ref3]  McQuillan, J., et.al., "The New Routing Algorithm for the
          ARPANET", IEEE Transactions on Communications, May 1980.
  [Ref4]  Perlman, R., "Fault-Tolerant Broadcast of Routing
          Information", Computer Networks, December 1983.
  [Ref5]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
          1981.
  [Ref6]  McKenzie, A., "ISO Transport Protocol specification ISO DP
          8073", RFC 905, April 1984.
  [Ref7]  Deering, S., "Host extensions for IP multicasting", STD 5,
          RFC 1112, May 1988.
  [Ref8]  McCloghrie, K., and M. Rose, "Management Information Base
          for network management of TCP/IP-based internets: MIB-II",
          STD 17, RFC 1213, March 1991.
  [Ref9]  Moy, J., "OSPF Version 2", RFC 1583, March 1994.
  [Ref10] Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
          Inter-Domain Routing (CIDR): an Address Assignment and
          Aggregation Strategy", RFC1519, September 1993.
  [Ref11] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
          1700, October 1994.
  [Ref12] Almquist, P., "Type of Service in the Internet Protocol
          Suite", RFC 1349, July 1992.

Moy Standards Track [Page 183] RFC 2328 OSPF Version 2 April 1998

  [Ref13] Leiner, B., et.al., "The DARPA Internet Protocol Suite", DDN
          Protocol Handbook, April 1985.
  [Ref14] Bradley, T., and C. Brown, "Inverse Address Resolution
          Protocol", RFC 1293, January 1992.
  [Ref15] deSouza, O., and M. Rodrigues, "Guidelines for Running OSPF
          Over Frame Relay Networks", RFC 1586, March 1994.
  [Ref16] Bellovin, S., "Security Problems in the TCP/IP Protocol
          Suite", ACM Computer Communications Review, Volume 19,
          Number 2, pp. 32-38, April 1989.
  [Ref17] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
          April 1992.
  [Ref18] Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
          1994.
  [Ref19] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC 1587,
          March 1994.
  [Ref20] Ferguson, D., "The OSPF External Attributes LSA", work in
          progress.
  [Ref21] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
          1793, April 1995.
  [Ref22] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
          November 1990.
  [Ref23] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-
          4)", RFC 1771, March 1995.
  [Ref24] Hinden, R., "Internet Routing Protocol Standardization
          Criteria", BBN, October 1991.
  [Ref25] Moy, J., "OSPF Version 2", RFC 2178, July 1997.
  [Ref26] Rosen, E., "Vulnerabilities of Network Control Protocols: An
          Example", Computer Communication Review, July 1981.

Moy Standards Track [Page 184] RFC 2328 OSPF Version 2 April 1998

A. OSPF data formats

  This appendix describes the format of OSPF protocol packets and OSPF
  LSAs.  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 LSAs.
  OSPF packet formats are detailed in Section A.3.  A description of
  OSPF LSAs 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. If necessary, the length of OSPF packets can be up
  to 65,535 bytes (including the IP header).  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 OSPF
  packets sent over virtual links to 576 bytes unless Path MTU
  Discovery is being performed (see [Ref22]).
  The other important features of OSPF's IP encapsulation are:
  o   Use of IP multicast.  Some OSPF messages are multicast, when
      sent over broadcast 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.

Moy Standards Track [Page 185] RFC 2328 OSPF Version 2 April 1998

      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
          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 [Ref11].
  o   All OSPF routing protocol packets are sent using the normal
      service TOS value of binary 0000 defined in [Ref12].
  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 [Ref5] may help implement this objective.

Moy Standards Track [Page 186] RFC 2328 OSPF Version 2 April 1998

A.2 The Options field

  The OSPF Options field is present in OSPF Hello packets, Database
  Description packets and all LSAs.  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 LSAs to a neighbor because
  of its reduced functionality.  Lastly, listing capabilities in LSAs
  allows routers to forward traffic around reduced functionality
  routers, by excluding them from parts of the routing table
  calculation.
  Five bits of the OSPF Options field have been assigned, although
  only one (the E-bit) is described completely by this memo. Each bit
  is described briefly below. Routers should reset (i.e.  clear)
  unrecognized bits in the Options field when sending Hello packets or
  Database Description packets and when originating LSAs. Conversely,
  routers encountering unrecognized Option bits in received Hello
  Packets, Database Description packets or LSAs should ignore the
  capability and process the packet/LSA normally.
                     +------------------------------------+
                     | * | * | DC | EA | N/P | MC | E | * |
                     +------------------------------------+
                           The Options field
  E-bit
      This bit describes the way AS-external-LSAs are flooded, as
      described in Sections 3.6, 9.5, 10.8 and 12.1.2 of this memo.
  MC-bit
      This bit describes whether IP multicast datagrams are forwarded
      according to the specifications in [Ref18].

Moy Standards Track [Page 187] RFC 2328 OSPF Version 2 April 1998

  N/P-bit
      This bit describes the handling of Type-7 LSAs, as specified in
      [Ref19].
  EA-bit
      This bit describes the router's willingness to receive and
      forward External-Attributes-LSAs, as specified in [Ref20].
  DC-bit
      This bit describes the router's handling of demand circuits, as
      specified in [Ref21].

Moy Standards Track [Page 188] RFC 2328 OSPF Version 2 April 1998

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 LSAs.  For example, Link State Update packets implement the
  flooding of LSAs throughout the OSPF routing domain.  Because of
  this, OSPF protocol packets cannot be parsed unless the format of
  LSAs is also understood.  The format of LSAs 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 Standards Track [Page 189] RFC 2328 OSPF Version 2 April 1998

A.3.1 The OSPF packet header

  Every OSPF packet starts with a standard 24 byte header.  This
  header contains all the information necessary 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. See Sections A.3.2 through
      A.3.6 for details.

Moy Standards Track [Page 190] RFC 2328 OSPF Version 2 April 1998

                        Type   Description
                        ________________________________
                        1      Hello
                        2      Database Description
                        3      Link State Request
                        4      Link State Update
                        5      Link State Acknowledgment
  Packet length
      The length of the OSPF protocol packet in bytes.  This length
      includes the standard OSPF header.
  Router ID
      The Router ID of the packet's source.
  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.  The
      checksum is considered to be part of the packet authentication
      procedure; for some authentication types the checksum
      calculation is omitted.
  AuType
      Identifies the authentication procedure 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.

Moy Standards Track [Page 191] RFC 2328 OSPF Version 2 April 1998

  Authentication
      A 64-bit field for use by the authentication scheme. See
      Appendix D for details.

Moy Standards Track [Page 192] RFC 2328 OSPF Version 2 April 1998

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                    |

Moy Standards Track [Page 193] RFC 2328 OSPF Version 2 April 1998

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Neighbor                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
  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 sending 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 sending 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 Standards Track [Page 194] RFC 2328 OSPF Version 2 April 1998

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 link-state 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 the
  master, the other the 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                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Interface MTU         |    Options    |0|0|0|0|0|I|M|MS
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     DD sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                      An LSA Header                          -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

Moy Standards Track [Page 195] RFC 2328 OSPF Version 2 April 1998

  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
  a piece of the link-state 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.
  Interface MTU
      The size in bytes of the largest IP datagram that can be sent
      out the associated interface, without fragmentation.  The MTUs
      of common Internet link types can be found in Table 7-1 of
      [Ref22]. Interface MTU should be set to 0 in Database
      Description packets sent over virtual links.
  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
  link-state database's pieces.  Each LSA in the database is described
  by its LSA header.  The LSA header is documented in Section A.4.1.
  It contains all the information required to uniquely identify both
  the LSA and the LSA's current instance.

Moy Standards Track [Page 196] RFC 2328 OSPF Version 2 April 1998

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 link-state 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.
  A router that sends a Link State Request packet has in mind the
  precise instance of the database pieces it is requesting. Each
  instance is defined by its 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                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

Moy Standards Track [Page 197] RFC 2328 OSPF Version 2 April 1998

  Each LSA requested is specified by its LS type, Link State ID, and
  Advertising Router.  This uniquely identifies the LSA, but not its
  instance.  Link State Request packets are understood to be requests
  for the most recent instance (whatever that might be).

Moy Standards Track [Page 198] RFC 2328 OSPF Version 2 April 1998

A.3.5 The Link State Update packet

  Link State Update packets are OSPF packet type 4.  These packets
  implement the flooding of LSAs.  Each Link State Update packet
  carries a collection of LSAs one hop further from their origin.
  Several LSAs 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 LSAs are acknowledged in Link State
  Acknowledgment packets.  If retransmission of certain LSAs is
  necessary, the retransmitted LSAs are always sent directly to the
  neighbor.  For more information on the reliable flooding of LSAs,
  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                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            # LSAs                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                            +-+
     |                             LSAs                              |
     +-                                                            +-+
     |                              ...                              |

Moy Standards Track [Page 199] RFC 2328 OSPF Version 2 April 1998

  # LSAs
      The number of LSAs included in this update.
  The body of the Link State Update packet consists of a list of LSAs.
  Each LSA begins with a common 20 byte header, described in Section
  A.4.1. Detailed formats of the different types of LSAs are described
  in Section A.4.

Moy Standards Track [Page 200] RFC 2328 OSPF Version 2 April 1998

A.3.6 The Link State Acknowledgment packet

  Link State Acknowledgment Packets are OSPF packet type 5.  To make
  the flooding of LSAs reliable, flooded LSAs are explicitly
  acknowledged.  This acknowledgment is accomplished through the
  sending and receiving of Link State Acknowledgment packets.
  Multiple LSAs can be acknowledged in a single Link State
  Acknowledgment packet.
  Depending on the state of the sending interface and the sender of
  the corresponding Link State Update packet, 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 LSA 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                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                         An LSA Header                       -+
     |                                                               |
     +-                                                             -+

Moy Standards Track [Page 201] RFC 2328 OSPF Version 2 April 1998

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

Moy Standards Track [Page 202] RFC 2328 OSPF Version 2 April 1998

A.4 LSA formats

  This memo defines five distinct types of LSAs.  Each LSA begins with
  a standard 20 byte LSA header.  This header is explained in Section
  A.4.1.  Succeeding sections then diagram the separate LSA types.
  Each LSA describes a piece of the OSPF routing domain.  Every router
  originates a router-LSA.  In addition, whenever the router is
  elected Designated Router, it originates a network-LSA.  Other types
  of LSAs may also be originated (see Section 12.4).  All LSAs are
  then flooded throughout the OSPF routing domain.  The flooding
  algorithm is reliable, ensuring that all routers have the same
  collection of LSAs.  (See Section 13 for more information concerning
  the flooding algorithm).  This collection of LSAs is called the
  link-state 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 Standards Track [Page 203] RFC 2328 OSPF Version 2 April 1998

A.4.1 The LSA header

  All LSAs begin with a common 20 byte header.  This header contains
  enough information to uniquely identify the LSA (LS type, Link State
  ID, and Advertising Router).  Multiple instances of the LSA 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 LSA 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 LSA 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 LSA.  Each LSA type has a separate advertisement
      format.  The LSA types defined in this memo are as follows (see
      Section 12.1.3 for further explanation):

Moy Standards Track [Page 204] RFC 2328 OSPF Version 2 April 1998

                      LS Type   Description
                      ___________________________________
                      1         Router-LSAs
                      2         Network-LSAs
                      3         Summary-LSAs (IP network)
                      4         Summary-LSAs (ASBR)
                      5         AS-external-LSAs
  Link State ID
      This field identifies the portion of the internet environment
      that is being described by the LSA.  The contents of this field
      depend on the LSA's LS type.  For example, in network-LSAs 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 LSA.  For
      example, in network-LSAs this field is equal to the Router ID of
      the network's Designated Router.
  LS sequence number
      Detects old or duplicate LSAs.  Successive instances of an LSA
      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 LSA,
      including the LSA header but excluding the LS age field. See
      Section 12.1.7 for more details.
  length
      The length in bytes of the LSA.  This includes the 20 byte LSA
      header.

Moy Standards Track [Page 205] RFC 2328 OSPF Version 2 April 1998

A.4.2 Router-LSAs

  Router-LSAs are the Type 1 LSAs.  Each router in an area originates
  a router-LSA.  The LSA 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-LSA.  For details
  concerning the construction of router-LSAs, 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     |            metric             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      TOS      |        0      |          TOS  metric          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Link ID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Link Data                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

Moy Standards Track [Page 206] RFC 2328 OSPF Version 2 April 1998

  In router-LSAs, the Link State ID field is set to the router's OSPF
  Router ID. Router-LSAs are flooded throughout a single area only.
  bit V
      When set, the router is an endpoint of one or more fully
      adjacent virtual links having the described area as 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 in this LSA.  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 Link Data field.  For links to stub networks this
  field specifies the network's IP address mask.  For other link types
  the Link Data field specifies the router interface's IP address.
  Type
      A quick description of the router link.  One of the following.
      Note that host routes are classified as links to stub networks
      with network mask of 0xffffffff.

Moy Standards Track [Page 207] RFC 2328 OSPF Version 2 April 1998

               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
  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 an LSA (i.e., another router or a transit
      network) the Link ID is equal to the neighboring LSA's Link
      State ID.  This provides the key for looking up the neighboring
      LSA in the link state database during the routing table
      calculation. 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
      Value again depends on the link's Type field. For connections to
      stub networks, Link Data specifies the network's IP address
      mask. For unnumbered point-to-point connections, it specifies
      the interface's MIB-II [Ref8] ifIndex value. For the other link
      types it specifies the router interface's IP 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.

Moy Standards Track [Page 208] RFC 2328 OSPF Version 2 April 1998

  # TOS
      The number of different TOS metrics given for this link, not
      counting the required link metric (referred to as the TOS 0
      metric in [Ref9]).  For example, if no additional TOS metrics
      are given, this field is set to 0.
  metric
      The cost of using this router link.
  Additional TOS-specific information may also be included, for
  backward compatibility with previous versions of the OSPF
  specification ([Ref9]). Within each link, and for each desired TOS,
  TOS TOS-specific link information may be encoded as follows:
  TOS IP Type of Service that this metric refers to.  The encoding of
      TOS in OSPF LSAs is described in Section 12.3.
  TOS metric
      TOS-specific metric information.

Moy Standards Track [Page 209] RFC 2328 OSPF Version 2 April 1998

A.4.3 Network-LSAs

  Network-LSAs are the Type 2 LSAs.  A network-LSA is originated for
  each broadcast and NBMA network in the area which supports two or
  more routers.  The network-LSA is originated by the network's
  Designated Router.  The LSA describes all routers attached to the
  network, including the Designated Router itself.  The LSA'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.  This
  is why metric fields need not be specified in the network-LSA.  For
  details concerning the construction of network-LSAs, 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.

Moy Standards Track [Page 210] RFC 2328 OSPF Version 2 April 1998

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

Moy Standards Track [Page 211] RFC 2328 OSPF Version 2 April 1998

A.4.4 Summary-LSAs

  Summary-LSAs are the Type 3 and 4 LSAs.  These LSAs are originated
  by area border routers. Summary-LSAs describe inter-area
  destinations.  For details concerning the construction of summary-
  LSAs, see Section 12.4.3.
  Type 3 summary-LSAs are used when the destination is an IP network.
  In this case the LSA'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 E for details). When the
  destination is an AS boundary router, a Type 4 summary-LSA 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 summary-LSAs 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                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      0        |                  metric                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TOS       |                TOS  metric                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

Moy Standards Track [Page 212] RFC 2328 OSPF Version 2 April 1998

  For stub areas, Type 3 summary-LSAs 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 summary-LSA's Link State ID
  is always set to DefaultDestination (0.0.0.0) and the Network Mask
  is set to 0.0.0.0.
  Network Mask
      For Type 3 summary-LSAs, 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
      summary-LSAs.
  metric
      The cost of this route.  Expressed in the same units as the
      interface costs in the router-LSAs.
  Additional TOS-specific information may also be included, for
  backward compatibility with previous versions of the OSPF
  specification ([Ref9]). For each desired TOS, TOS-specific
  information is encoded as follows:
  TOS IP Type of Service that this metric refers to.  The encoding of
      TOS in OSPF LSAs is described in Section 12.3.
  TOS metric
      TOS-specific metric information.

Moy Standards Track [Page 213] RFC 2328 OSPF Version 2 April 1998

A.4.5 AS-external-LSAs

  AS-external-LSAs are the Type 5 LSAs.  These LSAs are originated by
  AS boundary routers, and describe destinations external to the AS.
  For details concerning the construction of AS-external-LSAs, see
  Section 12.4.3.
  AS-external-LSAs usually describe a particular external destination.
  For these LSAs 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 E for details).  AS-
  external-LSAs 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|     0       |                  metric                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Forwarding address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      External Route Tag                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |E|    TOS      |                TOS  metric                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Forwarding address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Moy Standards Track [Page 214] RFC 2328 OSPF Version 2 April 1998

     |                      External Route Tag                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
  Network Mask
      The IP address mask for the advertised destination.  For
      example, when advertising a class A network the mask 0xff000000
      would be used.
  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 it is expressed in the same units as the link state metric
      (i.e., the same units as interface cost).
  metric
      The cost of this route.  Interpretation depends on the external
      type indication (bit E above).
  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 LSA's originator (i.e.,
      the responsible AS boundary router).
  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.
  Additional TOS-specific information may also be included, for
  backward compatibility with previous versions of the OSPF
  specification ([Ref9]). For each desired TOS, TOS-specific
  information is encoded as follows:
  TOS The Type of Service that the following fields concern.  The
      encoding of TOS in OSPF LSAs is described in Section 12.3.

Moy Standards Track [Page 215] RFC 2328 OSPF Version 2 April 1998

  bit E
      For backward-compatibility with [Ref9].
  TOS metric
      TOS-specific metric information.
  Forwarding address
      For backward-compatibility with [Ref9].
  External Route Tag
      For backward-compatibility with [Ref9].

Moy Standards Track [Page 216] RFC 2328 OSPF Version 2 April 1998

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
      LSA.  If the LS age field of one of the router's self-originated
      LSAs reaches the value LSRefreshTime, a new instance of the LSA
      is originated, even though the contents of the LSA (apart from
      the LSA header) will be the same.  The value of LSRefreshTime is
      set to 30 minutes.
  MinLSInterval
      The minimum time between distinct originations of any particular
      LSA.  The value of MinLSInterval is set to 5 seconds.
  MinLSArrival
      For any particular LSA, the minimum time that must elapse
      between reception of new LSA instances during flooding. LSA
      instances received at higher frequencies are discarded. The
      value of MinLSArrival is set to 1 second.
  MaxAge
      The maximum age that an LSA can attain. When an LSA's LS age
      field reaches MaxAge, it is reflooded in an attempt to flush the
      LSA from the routing domain (See Section 14). LSAs of age MaxAge
      are not used in the routing table calculation.  The value of
      MaxAge is set to 1 hour.
  CheckAge
      When the age of an LSA in the link state database hits a
      multiple of CheckAge, the LSA's checksum is verified.  An
      incorrect checksum at this time indicates a serious error.  The
      value of CheckAge is set to 5 minutes.

Moy Standards Track [Page 217] RFC 2328 OSPF Version 2 April 1998

  MaxAgeDiff
      The maximum time dispersion that can occur, as an LSA is flooded
      throughout the AS.  Most of this time is accounted for by the
      LSAs sitting on router output queues (and therefore not aging)
      during the flooding process.  The value of MaxAgeDiff is set to
      15 minutes.
  LSInfinity
      The metric value indicating that the destination described by an
      LSA is unreachable. Used in summary-LSAs and AS-external-LSAs 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-
      LSAs and in stub areas' type 3 summary-LSAs.  Its value is the
      IP address 0.0.0.0. Its associated Network Mask is also always
      0.0.0.0.
  InitialSequenceNumber
      The value used for LS Sequence Number when originating the first
      instance of any LSA. Its value is the signed 32-bit integer
      0x80000001.
  MaxSequenceNumber
      The maximum value that LS Sequence Number can attain.  Its value
      is the signed 32-bit integer 0x7fffffff.

Moy Standards Track [Page 218] RFC 2328 OSPF Version 2 April 1998

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 LSAs from the routing domain (see Section
          14.1), or they will persist for up to MaxAge minutes.
      RFC1583Compatibility
          Controls the preference rules used in Section 16.4 when
          choosing among multiple AS-external-LSAs advertising the
          same destination. When set to "enabled", the preference
          rules remain those specified by RFC 1583 ([Ref9]). When set
          to "disabled", the preference rules are those stated in

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          Section 16.4.1, which prevent routing loops when AS-
          external-LSAs for the same destination have been originated
          from different areas. Set to "enabled" by default.
          In order to minimize the chance of routing loops, all OSPF
          routers in an OSPF routing domain should have
          RFC1583Compatibility set identically. When there are routers
          present that have not been updated with the functionality
          specified in Section 16.4.1 of this memo, all routers should
          have RFC1583Compatibility set to "enabled". Otherwise, all
          routers should have RFC1583Compatibility set to "disabled",
          preventing all routing loops.
  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
      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.

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          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-LSA) 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.
      ExternalRoutingCapability
          Whether AS-external-LSAs will be flooded into/throughout the
          area.  If AS-external-LSAs are 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-
          LSA that the router should advertise into the area.
  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:

Moy Standards Track [Page 221] RFC 2328 OSPF Version 2 April 1998

      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 point-to-point networks.  Such a
          point-to-point network is called "unnumbered".
      IP interface mask
          Also referred to as the subnet/network 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 OSPF area to which the attached network belongs.
      Interface output cost
          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-LSA. The interface
          output cost must always be greater than 0.
      RxmtInterval
          The number of seconds between LSA 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.  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.  LSAs 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

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          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 broadcast and NBMA 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; however, 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
          network.
      AuType
          Identifies the authentication procedure to be used on the
          attached network.  This value must be the same for all
          routers attached to the network.  See Appendix D for a
          discussion of the defined authentication types.
      Authentication key
          This configured data allows the authentication procedure to
          verify OSPF protocol packets received over the interface.
          For example, if the AuType indicates simple password, the

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          Authentication key would be a clear 64-bit password.
          Authentication keys associated with the other OSPF
          authentication types are discussed in Appendix D.
  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-LSAs (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 NBMA network parameters
      OSPF treats an NBMA 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 network.  This
      Designated Router then originates a network-LSA, which lists all
      routers attached to the NBMA network.

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      However, due to the lack of broadcast capabilities, it may be
      necessary to use configuration parameters in the Designated
      Router selection.  These parameters will only need to 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), and then only if no
      automatic procedure for discovering neighbors exists:
      List of all other attached routers
          The list of all other routers attached to the NBMA 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 NBMA 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 Point-to-MultiPoint network parameters
      On Point-to-MultiPoint networks, it may be necessary to
      configure the set of neighbors that are directly reachable over
      the Point-to-MultiPoint network. Each neighbor is identified by
      its IP address on the Point-to-MultiPoint network. Designated
      Routers are not elected on Point-to-MultiPoint networks, so the
      Designated Router eligibility of configured neighbors is
      undefined.
      Alternatively, neighbors on Point-to-MultiPoint networks may be
      dynamically discovered by lower-level protocols such as Inverse
      ARP ([Ref14]).

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  C.7 Host route parameters
      Host routes are advertised in router-LSAs 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.  However, since the host probably has
          only a single connection to the internet, the actual
          configured cost in many cases is unimportant (i.e., will
          have no effect on routing).
      Area ID
          The OSPF area to which the host belongs.

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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-interface (or
  equivalently, on a per-network/subnet) basis.  Additional
  authentication data is also configurable on a per-interface basis.
  Authentication types 0, 1 and 2 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            Null authentication
                1            Simple password
                2            Cryptographic authentication
                All others   Reserved for assignment by the
                             IANA (iana@ISI.EDU)
                    Table 20: OSPF authentication types.
  D.1 Null authentication
      Use of this authentication type means that routing exchanges
      over the network/subnet are not authenticated.  The 64-bit
      authentication field in the OSPF header can contain anything; it
      is not examined on packet reception. When employing Null
      authentication, the entire contents of each OSPF packet (other
      than the 64-bit authentication field) are checksummed in order
      to detect data corruption.

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  D.2 Simple password authentication
      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. In addition, the entire contents of each OSPF
      packet (other than the 64-bit authentication field) are
      checksummed in order to detect data corruption.
      Simple password authentication guards against routers
      inadvertently joining the routing domain; each router must first
      be configured with its attached networks' passwords before it
      can participate in routing.  However, simple password
      authentication is vulnerable to passive attacks currently
      widespread in the Internet (see [Ref16]). Anyone with physical
      access to the network can learn the password and compromise the
      security of the OSPF routing domain.
  D.3 Cryptographic authentication
      Using this authentication type, a shared secret key is
      configured in all routers attached to a common network/subnet.
      For each OSPF protocol packet, the key is used to
      generate/verify a "message digest" that is appended to the end
      of the OSPF packet. The message digest is a one-way function of
      the OSPF protocol packet and the secret key. Since the secret
      key is never sent over the network in the clear, protection is
      provided against passive attacks.
      The algorithms used to generate and verify the message digest
      are specified implicitly by the secret key. This specification
      completely defines the use of OSPF Cryptographic authentication
      when the MD5 algorithm is used.
      In addition, a non-decreasing sequence number is included in
      each OSPF protocol packet to protect against replay attacks.
      This provides long term protection; however, it is still
      possible to replay an OSPF packet until the sequence number
      changes. To implement this feature, each neighbor data structure
      contains a new field called the "cryptographic sequence number".
      This field is initialized to zero, and is also set to zero

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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              0                |    Key ID     | Auth Data Len |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Cryptographic sequence number                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 18: Usage of the Authentication field
                 in the OSPF packet header when Cryptographic
                        Authentication is employed
      whenever the neighbor's state transitions to "Down". Whenever an
      OSPF packet is accepted as authentic, the cryptographic sequence
      number is set to the received packet's sequence number.
      This specification does not provide a rollover procedure for the
      cryptographic sequence number. When the cryptographic sequence
      number that the router is sending hits the maximum value, the
      router should reset the cryptographic sequence number that it is
      sending back to 0. After this is done, the router's neighbors
      will reject the router's OSPF packets for a period of
      RouterDeadInterval, and then the router will be forced to
      reestablish all adjacencies over the interface. However, it is
      expected that many implementations will use "seconds since
      reboot" (or "seconds since 1960", etc.) as the cryptographic
      sequence number. Such a choice will essentially prevent
      rollover, since the cryptographic sequence number field is 32
      bits in length.
      The OSPF Cryptographic authentication option does not provide
      confidentiality.
      When cryptographic authentication is used, the 64-bit
      Authentication field in the standard OSPF packet header is
      redefined as shown in Figure 18. The new field definitions are
      as follows:

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      Key ID
          This field identifies the algorithm and secret key used to
          create the message digest appended to the OSPF packet. Key
          Identifiers are unique per-interface (or equivalently, per-
          subnet).
      Auth Data Len
          The length in bytes of the message digest appended to the
          OSPF packet.
      Cryptographic sequence number
          An unsigned 32-bit non-decreasing sequence number. Used to
          guard against replay attacks.
      The message digest appended to the OSPF packet is not actually
      considered part of the OSPF protocol packet: the message digest
      is not included in the OSPF header's packet length, although it
      is included in the packet's IP header length field.
      Each key is identified by the combination of interface and Key
      ID. An interface may have multiple keys active at any one time.
      This enables smooth transition from one key to another. Each key
      has four time constants associated with it. These time constants
      can be expressed in terms of a time-of-day clock, or in terms of
      a router's local clock (e.g., number of seconds since last
      reboot):
      KeyStartAccept
          The time that the router will start accepting packets that
          have been created with the given key.
      KeyStartGenerate
          The time that the router will start using the key for packet
          generation.
      KeyStopGenerate
          The time that the router will stop using the key for packet
          generation.
      KeyStopAccept
          The time that the router will stop accepting packets that
          have been created with the given key.

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      In order to achieve smooth key transition, KeyStartAccept should
      be less than KeyStartGenerate and KeyStopGenerate should be less
      than KeyStopAccept. If KeyStopGenerate and KeyStopAccept are
      left unspecified, the key's lifetime is infinite. When a new key
      replaces an old, the KeyStartGenerate time for the new key must
      be less than or equal to the KeyStopGenerate time of the old
      key.
      Key storage should persist across a system restart, warm or
      cold, to avoid operational issues. In the event that the last
      key associated with an interface expires, it is unacceptable to
      revert to an unauthenticated condition, and not advisable to
      disrupt routing.  Therefore, the router should send a "last
      authentication key expiration" notification to the network
      manager and treat the key as having an infinite lifetime until
      the lifetime is extended, the key is deleted by network
      management, or a new key is configured.
  D.4 Message generation
      After building the contents of an OSPF packet, the
      authentication procedure indicated by the sending interface's
      Autype value is called before the packet is sent. The
      authentication procedure modifies the OSPF packet as follows.
      D.4.1 Generating Null authentication
          When using Null authentication, the packet is modified as
          follows:
          (1) The Autype field in the standard OSPF header is set to
              0.
          (2) The checksum field in the standard OSPF header is set to
              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

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              packet's length is not an integral number of 16-bit
              words, the packet is padded with a byte of zero before
              checksumming.
      D.4.2 Generating Simple password authentication
          When using Simple password authentication, the packet is
          modified as follows:
          (1) The Autype field in the standard OSPF header is set to
              1.
          (2) The checksum field in the standard OSPF header is set to
              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.
          (3) The 64-bit authentication field in the OSPF packet
              header is set to the 64-bit password (i.e.,
              authentication key) that has been configured for the
              interface.
      D.4.3 Generating Cryptographic authentication
          When using Cryptographic authentication, there may be
          multiple keys configured for the interface. In this case,
          among the keys that are valid for message generation (i.e,
          that have KeyStartGenerate <= current time <
          KeyStopGenerate) choose the one with the most recent
          KeyStartGenerate time. Using this key, modify the packet as
          follows:
          (1) The Autype field in the standard OSPF header is set to
              2.

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          (2) The checksum field in the standard OSPF header is not
              calculated, but is instead set to 0.
          (3) The Key ID (see Figure 18) is set to the chosen key's
              Key ID.
          (4) The Auth Data Len field is set to the length in bytes of
              the message digest that will be appended to the OSPF
              packet. When using MD5 as the authentication algorithm,
              Auth Data Len will be 16.
          (5) The 32-bit Cryptographic sequence number (see Figure 18)
              is set to a non-decreasing value (i.e., a value at least
              as large as the last value sent out the interface). The
              precise values to use in the cryptographic sequence
              number field are implementation-specific. For example,
              it may be based on a simple counter, or be based on the
              system's clock.
          (6) The message digest is then calculated and appended to
              the OSPF packet.  The authentication algorithm to be
              used in calculating the digest is indicated by the key
              itself.  Input to the authentication algorithm consists
              of the OSPF packet and the secret key. When using MD5 as
              the authentication algorithm, the message digest
              calculation proceeds as follows:
              (a) The 16 byte MD5 key is appended to the OSPF packet.
              (b) Trailing pad and length fields are added, as
                  specified in [Ref17].
              (c) The MD5 authentication algorithm is run over the
                  concatenation of the OSPF packet, secret key, pad
                  and length fields, producing a 16 byte message
                  digest (see [Ref17]).
              (d) The MD5 digest is written over the OSPF key (i.e.,
                  appended to the original OSPF packet). The digest is
                  not counted in the OSPF packet's length field, but

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                  is included in the packet's IP length field. Any
                  trailing pad or length fields beyond the digest are
                  not counted or transmitted.
  D.5 Message verification
      When an OSPF packet has been received on an interface, it must
      be authenticated. The authentication procedure is indicated by
      the setting of Autype in the standard OSPF packet header, which
      matches the setting of Autype for the receiving OSPF interface.
      If an OSPF protocol packet is accepted as authentic, processing
      of the packet continues as specified in Section 8.2. Packets
      which fail authentication are discarded.
      D.5.1 Verifying Null authentication
          When using Null authentication, the checksum field in the
          OSPF header must be verified. It must be set to 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.)
      D.5.2 Verifying Simple password authentication
          When using Simple password authentication, the received OSPF
          packet is authenticated as follows:
          (1) The checksum field in the OSPF header must be verified.
              It must be set to 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.)
          (2) The 64-bit authentication field in the OSPF packet
              header must be equal to the 64-bit password (i.e.,
              authentication key) that has been configured for the
              interface.

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      D.5.3 Verifying Cryptographic authentication
          When using Cryptographic authentication, the received OSPF
          packet is authenticated as follows:
          (1) Locate the receiving interface's configured key having
              Key ID equal to that specified in the received OSPF
              packet (see Figure 18). If the key is not found, or if
              the key is not valid for reception (i.e., current time <
              KeyStartAccept or current time >= KeyStopAccept), the
              OSPF packet is discarded.
          (2) If the cryptographic sequence number found in the OSPF
              header (see Figure 18) is less than the cryptographic
              sequence number recorded in the sending neighbor's data
              structure, the OSPF packet is discarded.
          (3) Verify the appended message digest in the following
              steps:
              (a) The received digest is set aside.
              (b) A new digest is calculated, as specified in Step 6
                  of Section D.4.3.
              (c) The calculated and received digests are compared. If
                  they do not match, the OSPF packet is discarded. If
                  they do match, the OSPF protocol packet is accepted
                  as authentic, and the "cryptographic sequence
                  number" in the neighbor's data structure is set to
                  the sequence number found in the packet's OSPF
                  header.

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E. An algorithm for assigning Link State IDs

  The Link State ID in AS-external-LSAs and summary-LSAs is usually
  set to the described network's IP address. However, if necessary one
  or more of the network's host bits may be set in the Link State ID.
  This allows the router to originate separate LSAs for networks
  having the same address, yet different masks. Such networks can
  occur in the presence of supernetting and subnet 0s (see [Ref10]).
  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 LSAs 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 whenever
  possible; this maximizes interoperability with OSPF implementations
  predating RFC 1583.
  The algorithm below is stated for AS-external-LSAs.  This is only
  for clarity; the exact same algorithm can be used for summary-LSAs.
  Suppose that the router wishes to originate an AS-external-LSA for a
  network having address NA and mask NM1. The following steps are then
  used to determine the LSA's Link State ID:
  (1) Determine whether the router is already originating an AS-
      external-LSA with Link State ID equal to NA (in such an LSA the
      router itself will be listed as the LSA's Advertising Router).
      If not, the Link State ID is set equal to NA and the algorithm
      terminates. Otherwise,
  (2) Obtain the network mask from the body of the already existing
      AS-external-LSA. 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 LSA 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
          LSA (having Link State ID of NA) to reference the new
          network [NA,NM1] by incrementing the sequence number,

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          changing the mask in the body to NM1 and inserting the cost
          of the new network. Then originate a new LSA for the 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-LSA, 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 LSAs 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-LSA 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-LSA for
      [10.0.0.0,255.255.0.0]:
      (a) The LSA 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-LSA for
      [10.0.0.0,255.0.0.0]:

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

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F. Multiple interfaces to the same network/subnet

  There are at least two ways to support multiple physical interfaces
  to the same IP subnet. Both methods will interoperate with
  implementations of RFC 1583 (and of course this memo). The two
  methods are sketched briefly below. An assumption has been made that
  each interface has been assigned a separate IP address (otherwise,
  support for multiple interfaces is more of a link-level or ARP issue
  than an OSPF issue).
  Method 1:
      Run the entire OSPF functionality over both interfaces, sending
      and receiving hellos, flooding, supporting separate interface
      and neighbor FSMs for each interface, etc. When doing this all
      other routers on the subnet will treat the two interfaces as
      separate neighbors, since neighbors are identified (on broadcast
      and NBMA networks) by their IP address.
      Method 1 has the following disadvantages:
      (1) You increase the total number of neighbors and adjacencies.
      (2) You lose the bidirectionality test on both interfaces, since
          bidirectionality is based on Router ID.
      (3) You have to consider both interfaces together during the
          Designated Router election, since if you declare both to be
          DR simultaneously you can confuse the tie-breaker (which is
          Router ID).
  Method 2:
      Run OSPF over only one interface (call it the primary
      interface), but include both the primary and secondary
      interfaces in your Router-LSA.
      Method 2 has the following disadvantages:
      (1) You lose the bidirectionality test on the secondary
          interface.
      (2) When the primary interface fails, you need to promote the
          secondary interface to primary status.

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G. Differences from RFC 2178

  This section documents the differences between this memo and RFC
  2178.  All differences are backward-compatible. Implementations of
  this memo and of RFCs 2178, 1583, and 1247 will interoperate.
  G.1 Flooding modifications
      Three changes have been made to the flooding procedure in
      Section 13.
      The first change is to step 4 in Section 13. Now MaxAge LSAs are
      acknowledged and then discarded only when both a) there is no
      database copy of the LSA and b) none of router's neighbors are
      in states Exchange or Loading. In all other cases, the MaxAge
      LSA is processed like any other LSA, installing the LSA in the
      database and flooding it out the appropriate interfaces when the
      LSA is more recent than the database copy (Step 5 of Section
      13). This change also affects the contents of Table 19.
      The second change is to step 5a in Section 13. The MinLSArrival
      check is meant only for LSAs received during flooding, and
      should not be performed on those LSAs that the router itself
      originates.
      The third change is to step 8 in Section 13. Confusion between
      routers as to which LSA instance is more recent can cause a
      disastrous amount of flooding in a link-state protocol (see
      [Ref26]). OSPF guards against this problem in two ways: a) the
      LS age field is used like a TTL field in flooding, to eventually
      remove looping LSAs from the network (see Section 13.3), and b)
      routers refuse to accept LSA updates more frequently than once
      every MinLSArrival seconds (see Section 13).  However, there is
      still one case in RFC 2178 where disagreements regarding which
      LSA is more recent can cause a lot of flooding traffic:
      responding to old LSAs by reflooding the database copy.  For
      this reason, Step 8 of Section 13 has been amended to only
      respond with the database copy when that copy has not been sent
      in any Link State Update within the last MinLSArrival seconds.

Moy Standards Track [Page 240] RFC 2328 OSPF Version 2 April 1998

  G.2 Changes to external path preferences
      There is still the possibility of a routing loop in RFC 2178
      when both a) virtual links are in use and b) the same external
      route is being imported by multiple ASBRs, each of which is in a
      separate area. To fix this problem, Section 16.4.1 has been
      revised. To choose the correct ASBR/forwarding address, intra-
      area paths through non-backbone areas are always preferred.
      However, intra-area paths through the backbone area (Area 0) and
      inter-area paths are now of equal preference, and must be
      compared solely based on cost.
      The reasoning behind this change is as follows. When virtual
      links are in use, an intra-area backbone path for one router can
      turn into an inter-area path in a router several hops closer to
      the destination. Hence, intra-area backbone paths and inter-area
      paths must be of equal preference. We can safely compare their
      costs, preferring the path with the smallest cost, due to the
      calculations in Section 16.3.
      Thanks to Michael Briggs and Jeremy McCooey of the UNH
      InterOperability Lab for pointing out this problem.
  G.3 Incomplete resolution of virtual next hops
      One of the functions of the calculation in Section 16.3 is to
      determine the actual next hop(s) for those destinations whose
      next hop was calculated as a virtual link in Sections 16.1 and
      16.2.  After completion of the calculation in Section 16.3, any
      paths calculated in Sections 16.1 and 16.2 that still have
      unresolved virtual next hops should be discarded.
  G.4 Routing table lookup
      The routing table lookup algorithm in Section 11.1 has been
      modified to reflect current practice. The "best match" routing
      table entry is now always selected to be the one providing the
      most specific (longest) match. Suppose for example a router is
      forwarding packets to the destination 192.9.1.1. A routing table
      entry for 192.9.1/24 will always be a better match than the
      routing table entry for 192.9/16, regardless of the routing
      table entries' path-types. Note however that when multiple paths

Moy Standards Track [Page 241] RFC 2328 OSPF Version 2 April 1998

      are available for a given routing table entry, the calculations
      in Sections 16.1, 16.2, and 16.4 always yield the paths having
      the most preferential path-type. (Intra-area paths are the most
      preferred, followed in order by inter-area, type 1 external and
      type 2 external paths; see Section 11).

Moy Standards Track [Page 242] RFC 2328 OSPF Version 2 April 1998

Security Considerations

  All OSPF protocol exchanges are authenticated. OSPF supports
  multiple types of authentication; the type of authentication in use
  can be configured on a per network segment basis. One of OSPF's
  authentication types, namely the Cryptographic authentication
  option, is believed to be secure against passive attacks and provide
  significant protection against active attacks. When using the
  Cryptographic authentication option, each router appends a "message
  digest" to its transmitted OSPF packets. Receivers then use the
  shared secret key and received digest to verify that each received
  OSPF packet is authentic.
  The quality of the security provided by the Cryptographic
  authentication option depends completely on the strength of the
  message digest algorithm (MD5 is currently the only message digest
  algorithm specified), the strength of the key being used, and the
  correct implementation of the security mechanism in all
  communicating OSPF implementations.  It also requires that all
  parties maintain the secrecy of the shared secret key.
  None of the OSPF authentication types provide confidentiality. Nor
  do they protect against traffic analysis. Key management is also not
  addressed by this memo.
  For more information, see Sections 8.1, 8.2, and Appendix D.

Author's Address

  John Moy
  Ascend Communications, Inc.
  1 Robbins Road
  Westford, MA 01886
  Phone: 978-952-1367
  Fax:   978-392-2075
  EMail: jmoy@casc.com

Moy Standards Track [Page 243] RFC 2328 OSPF Version 2 April 1998

Full Copyright Statement

  Copyright (C) The Internet Society (1998).  All Rights Reserved.
  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
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  kind, provided that the above copyright notice and this paragraph
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  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
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  The limited permissions granted above are perpetual and will not be
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  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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Moy Standards Track [Page 244]

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