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Network Working Group J. Moy Request for Comments: 2178 Cascade Communications Corp. Obsoletes: 1583 July 1997 Category: Standards Track

                           OSPF Version 2

Status of this Memo

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

Abstract

 This memo documents version 2 of the OSPF protocol. OSPF is a link-
 state routing protocol.  It is designed to be run internal to a
 single Autonomous System.  Each OSPF router maintains an identical
 database describing the Autonomous System's topology.  From this
 database, a routing table is calculated by constructing a shortest-
 path tree.
 OSPF recalculates routes quickly in the face of topological changes,
 utilizing a minimum of routing protocol traffic.  OSPF provides
 support for equal-cost multipath.  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 1583 are explained in
 Appendix G. All differences are backward-compatible in nature.
 Implementations of this memo and of RFC 1583 will interoperate.
 Please send comments to ospf@gated.cornell.edu.

Table of Contents

  1        Introduction ........................................... 5
  1.1      Protocol Overview ...................................... 5
  1.2      Definitions of commonly used terms ..................... 6
  1.3      Brief history of link-state routing technology ........  9
  1.4      Organization of this document ......................... 10
  1.5      Acknowledgments ....................................... 11
  2        The link-state database: organization and calculations  11
  2.1      Representation of routers and networks ................ 11

Moy Standards Track [Page 1] RFC 2178 OSPF Version 2 July 1997

  2.1.1    Representation of non-broadcast networks .............. 13
  2.1.2    An example link-state database ........................ 14
  2.2      The shortest-path tree ................................ 18
  2.3      Use of external routing information ................... 20
  2.4      Equal-cost multipath .................................. 22
  3        Splitting the AS into Areas ........................... 22
  3.1      The backbone of the Autonomous System ................. 23
  3.2      Inter-area routing .................................... 23
  3.3      Classification of routers ............................. 24
  3.4      A sample area configuration ........................... 25
  3.5      IP subnetting support ................................. 31
  3.6      Supporting stub areas ................................. 32
  3.7      Partitions of areas ................................... 33
  4        Functional Summary .................................... 34
  4.1      Inter-area routing .................................... 35
  4.2      AS external routes .................................... 35
  4.3      Routing protocol packets .............................. 35
  4.4      Basic implementation requirements ..................... 38
  4.5      Optional OSPF capabilities ............................ 39
  5        Protocol data structures .............................. 40
  6        The Area Data Structure ............................... 42
  7        Bringing Up Adjacencies ............................... 44
  7.1      The Hello Protocol .................................... 44
  7.2      The Synchronization of Databases ...................... 45
  7.3      The Designated Router ................................. 46
  7.4      The Backup Designated Router .......................... 47
  7.5      The graph of adjacencies .............................. 48
  8        Protocol Packet Processing ............................ 49
  8.1      Sending protocol packets .............................. 49
  8.2      Receiving protocol packets ............................ 51
  9        The Interface Data Structure .......................... 54
  9.1      Interface states ...................................... 57
  9.2      Events causing interface state changes ................ 59
  9.3      The Interface state machine ........................... 61
  9.4      Electing the Designated Router ........................ 64
  9.5      Sending Hello packets ................................. 66
  9.5.1    Sending Hello packets on NBMA networks ................ 67
  10       The Neighbor Data Structure ........................... 68
  10.1     Neighbor states ....................................... 70
  10.2     Events causing neighbor state changes ................. 75
  10.3     The Neighbor state machine ............................ 76
  10.4     Whether tocome adjacent    ............................ 82
  10.5     Receiving Hello Packets ............................... 83
  10.6     Receiving Database Description Packets ................ 85
  10.7     Receiving Link State Request Packets .................. 88
  10.8     Sending Database Description Packets .................. 89
  10.9     Sending Link State Request Packets .................... 90
  10.10    An Example ............................................ 91

Moy Standards Track [Page 2] RFC 2178 OSPF Version 2 July 1997

  11       The Routing Table Structure ........................... 93
  11.1     Routing table lookup .................................. 96
  11.2     Sample routing table, without areas ................... 97
  11.3     Sample routing table, with areas ...................... 97
  12       Link State Advertisements (LSAs) ......................100
  12.1     The LSA Header ........................................100
  12.1.1   LS age ............................................... 101
  12.1.2   Options .............................................. 101
  12.1.3   LS type .............................................. 102
  12.1.4   Link State ID ........................................ 102
  12.1.5   Advertising Router ................................... 104
  12.1.6   LS sequence number ................................... 104
  12.1.7   LS checksum .......................................... 105
  12.2     The link state database .............................. 105
  12.3     Representation of TOS ................................ 106
  12.4     Originating LSAs ..................................... 107
  12.4.1   Router-LSAs .......................................... 110
  12.4.1.1 Describing point-to-point interfaces ................. 112
  12.4.1.2 Describing broadcast and NBMA interfaces ............. 113
  12.4.1.3 Describing virtual links ............................. 113
  12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 114
  12.4.1.5 Examples of router-LSAs .............................. 114
  12.4.2   Network-LSAs ......................................... 116
  12.4.2.1 Examples of network-LSAs ............................. 116
  12.4.3   Summary-LSAs ......................................... 117
  12.4.3.1 Originating summary-LSAs into stub areas ............. 119
  12.4.3.2 Examples of summary-LSAs ............................. 119
  12.4.4   AS-external-LSAs ..................................... 120
  12.4.4.1 Examples of AS-external-LSAs ......................... 121
  13       The Flooding Procedure ............................... 122
  13.1     Determining which LSA is newer ....................... 126
  13.2     Installing LSAs in the database ...................... 127
  13.3     Next step in the flooding procedure .................. 128
  13.4     Receiving self-originated LSAs ....................... 130
  13.5     Sending Link State Acknowledgment packets ............ 131
  13.6     Retransmitting LSAs .................................. 133
  13.7     Receiving link state acknowledgments ................. 134
  14       Aging The Link State Database ........................ 134
  14.1     Premature aging of LSAs .............................. 135
  15       Virtual Links ........................................ 135
  16       Calculation of the routing table ..................... 137
  16.1     Calculating the shortest-path tree for an area ....... 138
  16.1.1   The next hop calculation ............................. 144
  16.2     Calculating the inter-area routes .................... 145
  16.3     Examining transit areas' summary-LSAs ................ 146
  16.4     Calculating AS external routes ....................... 149
  16.4.1   External path preferences ............................ 151
  16.5     Incremental updates -- summary-LSAs .................. 151

Moy Standards Track [Page 3] RFC 2178 OSPF Version 2 July 1997

  16.6     Incremental updates -- AS-external-LSAs .............. 152
  16.7     Events generated as a result of routing table changes  153
  16.8     Equal-cost multipath ................................. 154
           Footnotes ............................................ 155
           References ........................................... 158
  A        OSPF data formats .................................... 160
  A.1      Encapsulation of OSPF packets ........................ 160
  A.2      The Options field .................................... 162
  A.3      OSPF Packet Formats .................................. 163
  A.3.1    The OSPF packet header ............................... 164
  A.3.2    The Hello packet ..................................... 166
  A.3.3    The Database Description packet ...................... 168
  A.3.4    The Link State Request packet ........................ 170
  A.3.5    The Link State Update packet ......................... 171
  A.3.6    The Link State Acknowledgment packet ................. 172
  A.4      LSA formats .......................................... 173
  A.4.1    The LSA header ....................................... 174
  A.4.2    Router-LSAs .......................................... 176
  A.4.3    Network-LSAs ......................................... 179
  A.4.4    Summary-LSAs ......................................... 180
  A.4.5    AS-external-LSAs ..................................... 182
  B        Architectural Constants .............................. 184
  C        Configurable Constants ............................... 186
  C.1      Global parameters .................................... 186
  C.2      Area parameters ...................................... 187
  C.3      Router interface parameters .......................... 188
  C.4      Virtual link parameters .............................. 190
  C.5      NBMA network parameters .............................. 191
  C.6      Point-to-MultiPoint network parameters ............... 191
  C.7      Host route parameters ................................ 192
  D        Authentication ....................................... 193
  D.1      Null authentication .................................. 193
  D.2      Simple password authentication ....................... 193
  D.3      Cryptographic authentication ......................... 194
  D.4      Message generation ................................... 196
  D.4.1    Generating Null authentication ....................... 196
  D.4.2    Generating Simple password authentication ............ 197
  D.4.3    Generating Cryptographic authentication .............. 197
  D.5      Message verification ................................. 198
  D.5.1    Verifying Null authentication ........................ 199
  D.5.2    Verifying Simple password authentication ............. 199
  D.5.3    Verifying Cryptographic authentication ............... 199
  E        An algorithm for assigning Link State IDs ............ 201
  F        Multiple interfaces to the same network/subnet ....... 203
  G        Differences from RFC 1583 ............................ 204
  G.1      Enhancements to OSPF authentication .................. 204
  G.2      Addition of Point-to-MultiPoint interface ............ 204
  G.3      Support for overlapping area ranges .................. 205

Moy Standards Track [Page 4] RFC 2178 OSPF Version 2 July 1997

  G.4      A modification to the flooding algorithm ............. 206
  G.5      Introduction of the MinLSArrival constant ............ 206
  G.6      Optionally advertising point-to-point links as subnets 207
  G.7      Advertising same external route from multiple areas .. 207
  G.8      Retransmission of initial Database Description packets 209
  G.9      Detecting interface MTU mismatches ................... 209
  G.10     Deleting the TOS routing option ...................... 209
           Security Considerations .............................. 210
           Author's Address ..................................... 211

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 5] RFC 2178 OSPF Version 2 July 1997

 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.

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.

Moy Standards Track [Page 6] RFC 2178 OSPF Version 2 July 1997

 Router
    A level three Internet Protocol packet switch.  Formerly called a
    gateway in much of the IP literature.
 Autonomous System
    A group of routers exchanging routing information via a common
    routing protocol.  Abbreviated as AS.
 Interior Gateway Protocol
    The routing protocol spoken by the routers belonging to an
    Autonomous system. Abbreviated as IGP.  Each Autonomous System has
    a single IGP.  Separate Autonomous Systems may be running
    different IGPs.
 Router ID
    A 32-bit number assigned to each router running the OSPF protocol.
    This number uniquely identifies the router within an Autonomous
    System.
 Network
    In this memo, an IP network/subnet/supernet.  It is possible for
    one physical network to be assigned multiple IP network/subnet
    numbers.  We consider these to be separate networks.  Point-to-
    point physical networks are an exception - they are considered a
    single network no matter how many (if any at all) IP
    network/subnet numbers are assigned to them.
 Network mask
    A 32-bit number indicating the range of IP addresses residing on a
    single IP network/subnet/supernet.  This specification displays
    network masks as hexadecimal numbers.  For example, the network
    mask for a class C IP network is displayed as 0xffffff00.  Such a
    mask is often displayed elsewhere in the literature as
    255.255.255.0.
 Point-to-point networks
    A network that joins a single pair of routers.  A 56Kb serial line
    is an example of a point-to-point network.

Moy Standards Track [Page 7] RFC 2178 OSPF Version 2 July 1997

 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.
 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.

Moy Standards Track [Page 8] RFC 2178 OSPF Version 2 July 1997

 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 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

Moy Standards Track [Page 9] RFC 2178 OSPF Version 2 July 1997

 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.
 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.

Moy Standards Track [Page 10] RFC 2178 OSPF Version 2 July 1997

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 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 11] RFC 2178 OSPF Version 2 July 1997

  • *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 +—+ +—+ |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 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 Moy Standards Track [Page 12] RFC 2178 OSPF Version 2 July 1997 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 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. Moy Standards Track [Page 13] RFC 2178 OSPF Version 2 July 1997 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' 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. 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. Moy Standards Track [Page 14] RFC 2178 OSPF Version 2 July 1997 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 15] RFC 2178 OSPF Version 2 July 1997 + | 3+—+ N12 N14 N1|–|RT1|\ 1 \ N13 / | +—+ \ 8\ |8/8 + \ \|/ / \ 1+—+8 8+—+6 * N3 *—|RT4|——|RT5|——–+ \/ +—+ +—+ | + / | |7 | | 3+—+ / | | | N2|–|RT2|/1 |1 |6 | | +—+ +—+8 6+—+ | + |RT3|————–|RT6| | +—+ +—+ | |2 Ia|7 | | | | +———+ | | N4 | | | | | | N11 | | +———+ | | | | | N12 |3 | |6 2/ +—+ | +—+/ |RT9| | |RT7|—N15 +—+ | +—+ 9 |1 + | |1 _| | Ib|5 |_ / \ 1+—-+2 | 3+—-+1 / \ * N9 *——|RT11|—-|—|RT10|—* N6 * \/ +—-+ | +—-+ \/ | | | |1 + |1 +–+ 10+—-+ N8 +—+ |H1|—–|RT12| |RT8| +–+SLIP +—-+ +—+ |2 |4 | | +———+ +——–+ N10 N7 Figure 2: A sample Autonomous System Moy Standards Track [Page 16] RFC 2178 OSPF Version 2 July 1997 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. 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. Moy Standards Track [Page 17] RFC 2178 OSPF Version 2 July 1997 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. 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). 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. Moy Standards Track [Page 18] RFC 2178 OSPF Version 2 July 1997 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 19] RFC 2178 OSPF Version 2 July 1997 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. Routes to networks belonging to other AS'es (such as N12) appear as dashed lines on the shortest path tree in Figure 5. Use of this externally derived routing information is considered in the next section. 2.3. 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 (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. Moy Standards Track [Page 20] RFC 2178 OSPF Version 2 July 1997 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 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 Moy Standards Track [Page 21] RFC 2178 OSPF Version 2 July 1997 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. 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 Moy Standards Track [Page 22] RFC 2178 OSPF Version 2 July 1997 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 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. Moy Standards Track [Page 23] RFC 2178 OSPF Version 2 July 1997 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. 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 24] RFC 2178 OSPF Version 2 July 1997 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. Moy Standards Track [Page 25] RFC 2178 OSPF Version 2 July 1997 ……………………… . + . . | 3+—+ . N12 N14 . N1|–|RT1|\ 1 . \ N13 / . | +—+ \ . 8\ |8/8 . + \ . \|/ . / \ 1+—+8 8+—+6 . * N3 *—|RT4|——|RT5|——–+ . \/ +—+ +—+ | . + / \ . |7 | . | 3+—+ / \ . | | . N2|–|RT2|/1 1\ . |6 | . | +—+ +—+8 6+—+ | . + |RT3|——|RT6| | . +—+ +—+ | . 2/ . Ia|7 | . / . | | . +———+ . | | .Area 1 N4 . | | ……………………… | | …………………….. | | . N11 . | | . +———+ . | | . | . | | N12 . |3 . Ib|5 |6 2/ . +—+ . +—-+ +—+/ . |RT9| . ………|RT10|…..|RT7|—N15. . +—+ . . +—-+ +—+ 9 . . |1 . . + /3 1\ |1 . . _| . . | / \ |_ . . / \ 1+—-+2 |/ \ / \ . . * N9 *——|RT11|—-| * N6 * . . \/ +—-+ | \/ . . | . . | | . . |1 . . + |1 . . +–+ 10+—-+ . . N8 +—+ . . |H1|—–|RT12| . . |RT8| . . +–+SLIP +—-+ . . +—+ . . |2 . . |4 . . | . . | . . +———+ . . +——–+ . . N10 . . N7 . . . .Area 2 . .Area 3 . ………………………….. …………………….. Figure 6: A sample OSPF area configuration Moy Standards Track [Page 26] RFC 2178 OSPF Version 2 July 1997 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 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 27] RFC 2178 OSPF Version 2 July 1997 |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 28] RFC 2178 OSPF Version 2 July 1997 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. 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. Moy Standards Track [Page 29] RFC 2178 OSPF Version 2 July 1997 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 load share between the two for traffic to Network N8. 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. 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. Moy Standards Track [Page 30] RFC 2178 OSPF Version 2 July 1997 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. 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 Moy Standards Track [Page 31] RFC 2178 OSPF Version 2 July 1997 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]). 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 Moy Standards Track [Page 32] RFC 2178 OSPF Version 2 July 1997 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 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 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. Moy Standards Track [Page 33] RFC 2178 OSPF Version 2 July 1997 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. 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 34] RFC 2178 OSPF Version 2 July 1997 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. 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 Moy Standards Track [Page 35] RFC 2178 OSPF Version 2 July 1997 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. 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. Moy Standards Track [Page 36] RFC 2178 OSPF Version 2 July 1997 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. Table 9: OSPF link state advertisements (LSAs). Moy Standards Track [Page 37] RFC 2178 OSPF Version 2 July 1997 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. 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]. Moy Standards Track [Page 38] RFC 2178 OSPF Version 2 July 1997 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 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). Moy Standards Track [Page 39] RFC 2178 OSPF Version 2 July 1997 Other capabilities can be negotiated during the Database Exchange process. This is accomplished by specifying the optional capabilities in Database Description packets. A capability mismatch with a neighbor in this case will result in only a subset of 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. 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. Moy Standards Track [Page 40] RFC 2178 OSPF Version 2 July 1997 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. 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. Moy Standards Track [Page 41] RFC 2178 OSPF Version 2 July 1997 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). +—-+ |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

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.

Moy Standards Track [Page 42] RFC 2178 OSPF Version 2 July 1997

 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).
 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.
 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).

Moy Standards Track [Page 43] RFC 2178 OSPF Version 2 July 1997

 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.
 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.

Moy Standards Track [Page 44] RFC 2178 OSPF Version 2 July 1997

 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 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

Moy Standards Track [Page 45] RFC 2178 OSPF Version 2 July 1997

 Packet has a sequence number.  Database Description Packets sent by
 the master (polls) are acknowledged by the slave through echoing of
 the sequence number.  Both polls and their responses contain
 summaries of link state data.  The master is the only one allowed to
 retransmit Database Description Packets.  It does so only at fixed
 intervals, the length of which is the configured 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.
 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.

Moy Standards Track [Page 46] RFC 2178 OSPF Version 2 July 1997

 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.

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

Moy Standards Track [Page 47] RFC 2178 OSPF Version 2 July 1997

 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.
 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.
 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.

Moy Standards Track [Page 48] RFC 2178 OSPF Version 2 July 1997

        +---+            +---+
        |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

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:

Moy Standards Track [Page 49] RFC 2178 OSPF Version 2 July 1997

 Version #
    Set to 2, the version number of the protocol as documented in this
    specification.
 Packet type
    The type of OSPF packet, such as Link state Update or Hello
    Packet.
 Packet length
    The length of the entire OSPF packet in bytes, including the
    standard OSPF packet header.
 Router ID
    The identity of the router itself (who is originating the packet).
 Area ID
    The OSPF area that the packet is being sent into.
 Checksum
    The standard IP 16-bit one's complement checksum of the entire
    OSPF packet, excluding the 64-bit authentication field.  This
    checksum 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.

Moy Standards Track [Page 50] RFC 2178 OSPF Version 2 July 1997

 The IP destination address for the packet is selected as follows.  On
 physical point-to-point networks, the IP destination is always set to
 the address AllSPFRouters.  On all other network types (including
 virtual links), the majority of OSPF packets are sent as unicasts,
 i.e., sent directly to the other end of the adjacency.  In this case,
 the IP destination is just the Neighbor IP address associated with
 the other end of the adjacency (see Section 10).  The only packets
 not sent as unicasts are on broadcast networks; on these networks
 Hello packets are sent to the multicast destination AllSPFRouters,
 the Designated Router and its Backup send both Link State Update
 Packets and Link State Acknowledgment Packets to the multicast
 address AllSPFRouters, while all other routers send both their Link
 State Update and Link State Acknowledgment Packets to the multicast
 address AllDRouters.
 Retransmissions of Link State Update packets are ALWAYS sent as
 unicasts.
 The IP source address should be set to the IP address of the sending
 interface.  Interfaces to unnumbered point-to-point networks have no
 associated IP address.  On these interfaces, the IP source should be
 set to any of the other IP addresses belonging to the router.  For
 this reason, there must be at least one IP address assigned to the
 router.[2] Note that, for most purposes, virtual links act precisely
 the same as unnumbered point-to-point networks.  However, each
 virtual link does have an IP interface address (discovered during the
 routing table build process) which is used as the IP source when
 sending packets over the virtual link.
 For more information on the format of specific OSPF packet types,
 consult the sections listed in Table 10.
           Type   Packet name            detailed section (transmit)
           _________________________________________________________
           1      Hello                  Section  9.5
           2      Database description   Section 10.8
           3      Link state request     Section 10.9
           4      Link state update      Section 13.3
           5      Link state ack         Section 13.5
  Table 10: Sections describing OSPF protocol packet transmission.

8.2. Receiving protocol packets

 Whenever a protocol packet is received by the router it is marked
 with the interface it was received on.  For routers that have virtual
 links configured, it may not be immediately obvious which interface

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 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:
     (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.

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     (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.
 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.

Moy Standards Track [Page 53] RFC 2178 OSPF Version 2 July 1997

    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.
 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.

Moy Standards Track [Page 54] RFC 2178 OSPF Version 2 July 1997

    Masking the IP interface address with the IP interface mask yields
    the IP network number of the attached network.  On point-to-point
    networks and virtual links, the IP interface mask is not defined.
    On these networks, the link itself is not assigned an IP network
    number, and so the addresses of each side of the link are assigned
    independently, if they are assigned at all.
 Area ID
    The Area ID of the area to which the attached network belongs.
    All routing protocol packets originating from the interface are
    labelled with this Area ID.
 HelloInterval
    The length of time, in seconds, between the Hello packets that the
    router sends on the interface.  Advertised in Hello packets sent
    out this interface.
 RouterDeadInterval
    The number of seconds before the router's neighbors will declare
    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.

Moy Standards Track [Page 55] RFC 2178 OSPF Version 2 July 1997

 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
    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.

Moy Standards Track [Page 56] RFC 2178 OSPF Version 2 July 1997

 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.

Moy Standards Track [Page 57] RFC 2178 OSPF Version 2 July 1997

                                +----+   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
    All interface timers should be disabled, and there should be no
    adjacencies associated with the interface.
 Loopback
    In this state, the router's interface to the network is looped
    back.  The interface may be looped back in hardware or software.
    The interface will be unavailable for regular data traffic.
    However, it may still be desirable to gain information on the
    quality of this interface, either through sending ICMP pings to
    the interface or through something like a bit error test.  For
    this reason, IP packets may still be addressed to an interface in
    Loopback state.  To facilitate this, such interfaces are
    advertised in router-LSAs as single host routes, whose destination
    is the IP interface address.[4]

Moy Standards Track [Page 58] RFC 2178 OSPF Version 2 July 1997

 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 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.

Moy Standards Track [Page 59] RFC 2178 OSPF Version 2 July 1997

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

Moy Standards Track [Page 60] RFC 2178 OSPF Version 2 July 1997

     o   The advertised Router Priority for a bidirectional
         neighbor has changed.  This is again detected through
         examination of that neighbor's Hello Packets.
 LoopInd
    An indication has been received that the interface is now looped
    back to itself.  This indication can be received either from
    network management or from the lower level protocols.
 UnloopInd
    An indication has been received that the interface is no longer
    looped back.  As with the LoopInd event, this indication can be
    received either from network management or from the lower level
    protocols.
 InterfaceDown
    Lower-level protocols indicate that this interface is no longer
    functional. No matter what the current interface state is, the new
    interface state will be Down.

9.3. The Interface state machine

 A detailed description of the interface state changes follows.  Each
 state change is invoked by an event (Section 9.2).  This event may
 produce different effects, depending on the current state of the
 interface.  For this reason, the state machine below is organized by
 current interface state and received event. Each entry in the state
 machine describes the resulting new interface state and the required
 set of additional actions.
 When an interface's state changes, it may be necessary to originate a
 new router-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.

Moy Standards Track [Page 61] RFC 2178 OSPF Version 2 July 1997

  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.
             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.

Moy Standards Track [Page 62] RFC 2178 OSPF Version 2 July 1997

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

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 New state:  Down
    Action:  No actions are necessary.  For example, the
             interface variables have already been reset upon
             entering the Loopback state.  Note that reception of
             an InterfaceUp event is necessary before the
             interface again becomes fully functional.

9.4. Electing the Designated Router

 This section describes the algorithm used for calculating a network's
 Designated Router and Backup Designated Router.  This algorithm is
 invoked by the Interface state machine.  The initial time a router
 runs the election algorithm for a network, the network's Designated
 Router and Backup Designated Router are initialized to 0.0.0.0.  This
 indicates the lack of both a Designated Router and a Backup
 Designated Router.
 The Designated Router election algorithm proceeds as follows: Call
 the router doing the calculation Router X.  The list of neighbors
 attached to the network and having established bidirectional
 communication with Router X is examined.  This list is precisely the
 collection of Router X's neighbors (on this network) whose state is
 greater than or equal to 2-Way (see Section 10.1).  Router X itself
 is also considered to be on the list.  Discard all routers from the
 list that are ineligible to become Designated Router.  (Routers
 having Router Priority of 0 are ineligible to become Designated
 Router.)  The following steps are then executed, considering only
 those routers that remain on the list:
 (1) Note the current values for the network's Designated Router
     and Backup Designated Router.  This is used later for
     comparison purposes.
 (2) Calculate the new Backup Designated Router for the network
     as follows.  Only those routers on the list that have not
     declared themselves to be Designated Router are eligible to
     become Backup Designated Router.  If one or more of these
     routers have declared themselves Backup Designated Router
     (i.e., they are currently listing themselves as Backup
     Designated Router, but not as Designated Router, in their
     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.
 (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

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     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.
 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-

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 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.

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.

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 If the router is not eligible to become Designated Router, it must
 periodically send Hello Packets to both the Designated Router and the
 Backup Designated Router (if they exist).  It must also send an Hello
 Packet in reply to an Hello Packet received from any eligible
 neighbor (other than the current Designated Router and Backup
 Designated Router).  This is needed to establish an initial
 bidirectional relationship with any potential Designated Router.
 When sending Hello packets periodically to any neighbor, the interval
 between Hello Packets is determined by the neighbor's state.  If the
 neighbor is in state Down, Hello Packets are sent every PollInterval
 seconds.  Otherwise, Hello Packets are sent every HelloInterval
 seconds.

10. The Neighbor Data Structure

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

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 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.

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.

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 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 Inactivity Timer 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

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 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 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.

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 Full
    In this state, the neighboring routers are fully adjacent.  These
    adjacencies will now appear in router-LSAs and network-LSAs.

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 portions
    of the database.  This is indicated by the Link state request list
    becoming empty after the Database Exchange process has completed.

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 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 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.

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 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
     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.

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  State(s):  Init or greater
     Event:  HelloReceived
 New state:  No state change.
    Action:  Restart the Inactivity Timer for the neighbor, since
             the neighbor has again been heard from.
  State(s):  Init
     Event:  2-WayReceived
 New state:  Depends upon action routine.
    Action:  Determine whether an adjacency should be established
             with the neighbor (see Section 10.4).  If not, the
             new neighbor state is 2-Way.
             Otherwise (an adjacency should be established) the
             neighbor state transitions to ExStart.  Upon
             entering this state, the router increments the DD
             sequence number 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-

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             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.
  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.

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  State(s):  2-Way
     Event:  AdjOK?
 New state:  Depends upon action routine.
    Action:  Determine whether an adjacency should be formed with
             the neighboring router (see Section 10.4).  If not,
             the neighbor state remains at 2-Way.  Otherwise,
             transition the neighbor state to ExStart and perform
             the actions associated with the above state machine
             entry for state Init and event 2-WayReceived.
  State(s):  ExStart or greater
     Event:  AdjOK?
 New state:  Depends upon action routine.
    Action:  Determine whether the neighboring router should
             still be adjacent.  If yes, there is no state change
             and no further action is necessary.
             Otherwise, the (possibly partially formed) adjacency
             must be destroyed.  The neighbor state transitions
             to 2-Way.  The Link state retransmission list,
             Database summary list and Link state request list
             are cleared of 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.

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             This Database Description Packet should be otherwise
             empty (see Section 10.8).
  State(s):  Exchange or greater
     Event:  BadLSReq
 New state:  ExStart
    Action:  The action for event BadLSReq is exactly the same as
             for the neighbor event SeqNumberMismatch.  The
             (possibly partially formed) adjacency is torn down,
             and then an attempt is made at reestablishment.  For
             more information, see the neighbor state machine
             entry that is invoked when event SeqNumberMismatch
             is generated in state Exchange or greater.
  State(s):  Any state
     Event:  KillNbr
 New state:  Down
    Action:  The Link state retransmission list, Database summary
             list and Link state request list are cleared of
             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.

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  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.
  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.

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 The adjacency-forming decision occurs in two places in the neighbor
 state machine.  First, when bidirectional communication is initially
 established with the neighbor, and secondly, when the identity of the
 attached network's (Backup) Designated Router changes.  If the
 decision is made to not attempt an adjacency, the state of the
 neighbor communication stops at 2-Way.
 An adjacency should be established with a bidirectional neighbor when
 at least one of the following conditions holds:
 o   The underlying network type is point-to-point
 o   The underlying network type is 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 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.

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 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.
 When receiving an Hello on a point-to-point network (but not on a
 virtual link) set the neighbor structure's Neighbor IP address to the
 packet's IP source address.
 Now the rest of the Hello Packet is examined, generating events to be
 given to the neighbor and interface state machines.  These state
 machines are specified either to be executed or scheduled (see
 Section 4.4).  For example, by specifying below that the neighbor
 state machine be executed in line, several neighbor state transitions
 may be effected by a single received Hello:
 o   Each Hello Packet causes the neighbor state machine to be
     executed with the event HelloReceived.
 o   Then the list of neighbors contained in the Hello Packet is
     examined.  If the router itself appears in this list, the
     neighbor state machine should be executed with the event 2-
     WayReceived.  Otherwise, the neighbor state machine should
     be executed with the event 1-WayReceived, and the processing
     of the packet stops.
 o   Next, the Hello Packet's Router Priority field is examined.
     If this field is different than the one previously received
     from the neighbor, the receiving interface's state machine
     is scheduled with the event NeighborChange.  In any case,
     the Router Priority field in the neighbor data structure
     should be updated accordingly.
 o   Next the Designated Router field in the Hello Packet is
     examined.  If the neighbor is both declaring itself to be
     Designated Router (Designated Router field = Neighbor IP
     address) and the Backup Designated Router field in the

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     packet is equal to 0.0.0.0 and the receiving interface is in
     state Waiting, the receiving interface's state machine is
     scheduled with the event BackupSeen.  Otherwise, if the
     neighbor is declaring itself to be Designated Router and it
     had not previously, or the neighbor is not declaring itself
     Designated Router where it had previously, the receiving
     interface's state machine is scheduled with the event
     NeighborChange.  In any case, the Neighbors' Designated
     Router item in the neighbor structure is updated
     accordingly.
 o   Finally, the Backup Designated Router field in the Hello
     Packet is examined.  If the neighbor is declaring itself to
     be Backup Designated Router (Backup Designated Router field
     = Neighbor IP address) and the receiving interface is in
     state Waiting, the receiving interface's state machine is
     scheduled with the event BackupSeen.  Otherwise, if the
     neighbor is declaring itself to be Backup Designated Router
     and it had not previously, or the neighbor is not declaring
     itself Backup Designated Router where it had previously, the
     receiving interface's state machine is scheduled with the
     event NeighborChange.  In any case, the Neighbor's Backup
     Designated Router item in the neighbor structure is updated
     accordingly.
 On 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.

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.

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 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.
 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.

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 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.
     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.

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 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 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 88] RFC 2178 OSPF Version 2 July 1997

 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 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:

Moy Standards Track [Page 89] RFC 2178 OSPF Version 2 July 1997

 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.
 When the neighbor responds to these requests with the proper Link
 State Update packet(s), the Link state request list is truncated and
 a new Link State Request packet is sent.  This process continues
 until the Link state request list becomes empty.  Unsatisfied Link
 State Request packets are retransmitted 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.

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10.10. An Example

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

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

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11. The Routing Table Structure

 The routing table data structure contains all the information
 necessary to forward an IP data packet toward its destination.  Each
 routing table entry describes the collection of best paths to a
 particular destination.  When forwarding an IP data packet, the
 routing table entry providing the best match for the packet's IP
 destination is located.  The matching routing table entry then
 provides the next hop towards the packet's destination.  OSPF also
 provides for the existence of a default route (Destination ID =
 DefaultDestination, Address Mask = 0x00000000).  When the default
 route exists, it matches all IP destinations (although any other
 matching entry is a better match). Finding the routing table entry
 that best matches an IP destination is further described in Section
 11.1.
 There is a single routing table in each router.  Two sample routing
 tables are described in Sections 11.2 and 11.3.  The building of the
 routing table is discussed in Section 16.
 The rest of this section defines the fields found in a routing table
 entry.  The first set of fields describes the routing table entry's
 destination.
 Destination Type
    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]

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 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.
 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 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.

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 Cost
    The link state cost of the path to the destination.  For all paths
    except type 2 external paths this describes the entire path's
    cost.  For Type 2 external paths, this field describes the cost of
    the portion of the path internal to the AS.  This cost is
    calculated as the sum of the costs of the path's constituent
    links.
 Type 2 cost
    Only valid for type 2 external paths.  For these paths, this field
    indicates the cost of the path's external portion.  This cost has
    been advertised by an AS boundary router, and is the most
    significant part of the total path cost.  For example, a type 2
    external path with type 2 cost of 5 is always preferred over a
    path with type 2 cost of 10, regardless of the cost of the two
    paths' internal components.
 Link State Origin
    Valid only for intra-area paths, this field indicates the 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.

Moy Standards Track [Page 95] RFC 2178 OSPF Version 2 July 1997

11.1. Routing table lookup

 When an IP data packet is received, an OSPF router finds the routing
 table entry that best matches the packet's destination.  This routing
 table entry then provides the outgoing interface and next hop router
 to use in forwarding the packet. This section describes the process
 of finding the best matching routing table entry. The process
 consists of a number of steps, wherein the collection of routing
 table entries is progressively pruned.  In the end, the single
 routing table entry remaining is called the "best match".
 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]
 Note that the steps described below may fail to produce a best match
 routing table entry (i.e., all existing routing table entries are
 pruned for some reason or another), or the best match routing table
 entry may be one of the above "discard" routing table entries. In
 these cases, the packet's IP destination is considered unreachable.
 Instead of being forwarded, the packet should be discarded and an
 ICMP destination unreachable message should be returned to the
 packet's source.
 (1) Select the complete set of "matching" routing table entries
     from the routing table.  Each routing table entry describes
     a (set of) path(s) to a range of IP addresses. If the data
     packet's IP destination falls into an entry's range of IP
     addresses, the routing table entry is called a match. (It is
     quite likely that multiple entries will match the data
     packet.  For example, a default route will match all
     packets.)
 (2) Reduce the set of matching entries to those having the most
     preferential path-type (see Section 11). OSPF has a four
     level hierarchy of paths. Intra-area paths are the most
     preferred, followed in order by inter-area, type 1 external
     and type 2 external paths.
 (3) Select the remaining routing table entry that provides the
     most specific (longest) match. Another way of saying this is
     to choose the remaining 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

Moy Standards Track [Page 96] RFC 2178 OSPF Version 2 July 1997

     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.

11.2. Sample routing table, without areas

 Consider the Autonomous System pictured in Figure 2.  No OSPF areas
 have been configured.  A single metric is shown per outbound
 interface.  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.

Moy Standards Track [Page 97] RFC 2178 OSPF Version 2 July 1997

    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).
 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 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.

Moy Standards Track [Page 98] RFC 2178 OSPF Version 2 July 1997

 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).
 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.
 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

Moy Standards Track [Page 99] RFC 2178 OSPF Version 2 July 1997

 link are shown 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.

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

Moy Standards Track [Page 100] RFC 2178 OSPF Version 2 July 1997

 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 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.

Moy Standards Track [Page 101] RFC 2178 OSPF Version 2 July 1997

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 16.
 Actually, for Type 3 summary-LSAs (LS type = 3) and AS-external-LSAs
 (LS type = 5), the Link State ID may additionally have one or more of
 the destination network's "host" bits set. For example, when
 originating an AS-external-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.

Moy Standards Track [Page 102] RFC 2178 OSPF Version 2 July 1997

          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).
          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.

Moy Standards Track [Page 103] RFC 2178 OSPF Version 2 July 1997

 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 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 104] RFC 2178 OSPF Version 2 July 1997

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 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).

Moy Standards Track [Page 105] RFC 2178 OSPF Version 2 July 1997

 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 integer, and the IP
 packet header's TOS field is expressed in the binary TOS values used
 in [Ref12].

Moy Standards Track [Page 106] RFC 2178 OSPF Version 2 July 1997

                  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.
 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

Moy Standards Track [Page 107] RFC 2178 OSPF Version 2 July 1997

 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).
 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).

Moy Standards Track [Page 108] RFC 2178 OSPF Version 2 July 1997

 (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).
 (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.

Moy Standards Track [Page 109] RFC 2178 OSPF Version 2 July 1997

 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.  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
                ....................................
                . 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
 (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

Moy Standards Track [Page 110] RFC 2178 OSPF Version 2 July 1997

 area.  Bit E should never be set in a router-LSA for a stub area
 (stub areas cannot contain AS boundary routers).
 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.
 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:

Moy Standards Track [Page 111] RFC 2178 OSPF Version 2 July 1997

 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).

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:

Moy Standards Track [Page 112] RFC 2178 OSPF Version 2 July 1997

 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:
 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).

Moy Standards Track [Page 113] RFC 2178 OSPF Version 2 July 1997

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.
 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.

Moy Standards Track [Page 114] RFC 2178 OSPF Version 2 July 1997

   ; 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
   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

Moy Standards Track [Page 115] RFC 2178 OSPF Version 2 July 1997

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.
 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):

Moy Standards Track [Page 116] RFC 2178 OSPF Version 2 July 1997

   ; Network-LSA for Network N3
   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.
 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]

Moy Standards Track [Page 117] RFC 2178 OSPF Version 2 July 1997

 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 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.

Moy Standards Track [Page 118] RFC 2178 OSPF Version 2 July 1997

 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 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

Moy Standards Track [Page 119] RFC 2178 OSPF Version 2 July 1997

 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.
 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.

Moy Standards Track [Page 120] RFC 2178 OSPF Version 2 July 1997

 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
 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

Moy Standards Track [Page 121] RFC 2178 OSPF Version 2 July 1997

 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, 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.

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

Moy Standards Track [Page 122] RFC 2178 OSPF Version 2 July 1997

 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.
                              +
                              |
                    +---+.....|.BGP
                    |RTA|-----|.....+---+
                    +---+     |-----|RTX|
                              |     +---+
                    +---+     |
                    |RTB|-----|
                    +---+     |
                              |
                    +---+     |
                    |RTC|-----|
                    +---+     |
                              |
                              +
               Figure 16: Forwarding address example
 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

Moy Standards Track [Page 123] RFC 2178 OSPF Version 2 July 1997

      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, 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).
      (b) Purge all outstanding requests for equal or previous
          instances of the LSA from the sending neighbor's Link State
          Request list (see Section 10).
      (c) If the sending neighbor is in state Exchange or in state
          Loading, then install the MaxAge LSA in the link state
          database.  Otherwise, simply discard the LSA.  In either
          case, 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 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

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          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.
      (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, send the database copy back to the sending neighbor,

Moy Standards Track [Page 125] RFC 2178 OSPF Version 2 July 1997

      encapsulated within a Link State Update Packet. The Link State
      Update Packet should be unicast to the neighbor. In so doing, do
      not put the 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.

Moy Standards Track [Page 126] RFC 2178 OSPF Version 2 July 1997

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.
 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).

Moy Standards Track [Page 127] RFC 2178 OSPF Version 2 July 1997

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
    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:

Moy Standards Track [Page 128] RFC 2178 OSPF Version 2 July 1997

 (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.
     (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.

Moy Standards Track [Page 129] RFC 2178 OSPF Version 2 July 1997

 (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
     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)

Moy Standards Track [Page 130] RFC 2178 OSPF Version 2 July 1997

 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).

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 (as a
 unicast) 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 to a particular neighbor in response
 to the receipt of duplicate LSAs.  These acknowledgments are sent as
 unicasts, and are sent immediately when the duplicate is received.
 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

Moy Standards Track [Page 131] RFC 2178 OSPF Version 2 July 1997

 as multicasts.  The Destination IP address used depends 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).
                                  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 (see
  Section 13, step 4).
           Table 19: Sending link state acknowledgments.

Moy Standards Track [Page 132] RFC 2178 OSPF Version 2 July 1997

 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 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 as
 unicasts (directly to the physical address of the neighbor).  They
 are never sent as multicasts.  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.

Moy Standards Track [Page 133] RFC 2178 OSPF Version 2 July 1997

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:
 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.

Moy Standards Track [Page 134] RFC 2178 OSPF Version 2 July 1997

 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.

14.1. Premature aging of LSAs

 An LSA can be flushed from the routing domain by setting its LS age
 to MaxAge and 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.

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

Moy Standards Track [Page 135] RFC 2178 OSPF Version 2 July 1997

 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-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.

Moy Standards Track [Page 136] RFC 2178 OSPF Version 2 July 1997

 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 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.

Moy Standards Track [Page 137] RFC 2178 OSPF Version 2 July 1997

 (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.
 (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.

Moy Standards Track [Page 138] RFC 2178 OSPF Version 2 July 1997

 The procedure will be explained using the graph terminology that was
 introduced in Section 2.  The area's link state database is
 represented as a directed graph.  The graph's vertices are routers,
 transit networks and stub networks.  The first stage of the procedure
 concerns only the transit vertices (routers and transit networks) and
 their connecting links.  Throughout the shortest path calculation,
 the following data is also associated with each transit vertex:
 Vertex (node) ID
     A 32-bit number uniquely identifying the vertex.  For router
     vertices this is the router's OSPF Router ID.  For network
     vertices, this is the IP address of the network's Designated
     Router.
 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

Moy Standards Track [Page 139] RFC 2178 OSPF Version 2 July 1997

 vertices are examined for possible addition to/modification of the
 candidate list.  The algorithm then iterates again.  It terminates
 when the candidate list becomes empty.
 The following steps describe the algorithm in detail.  Remember that
 we are computing the shortest path tree for Area A.  All references
 to link state database lookup below are from Area A's database.
 (1) Initialize the algorithm's data structures.  Clear the list
     of candidate vertices.  Initialize the shortest-path tree to
     only the root (which is the router doing the calculation).
     Set Area A's TransitCapability to FALSE.
 (2) Call the vertex just added to the tree vertex V.  Examine
     the 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:
         o   Greater than the value that already appears for
             vertex W on the candidate list, then examine the
             next link.

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         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.
     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

Moy Standards Track [Page 141] RFC 2178 OSPF Version 2 July 1997

     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.
 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.

Moy Standards Track [Page 142] RFC 2178 OSPF Version 2 July 1997

 (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 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].

Moy Standards Track [Page 143] RFC 2178 OSPF Version 2 July 1997

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).
 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.

Moy Standards Track [Page 144] RFC 2178 OSPF Version 2 July 1997

 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).
 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

Moy Standards Track [Page 145] RFC 2178 OSPF Version 2 July 1997

     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.
 (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 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.

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 (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.

Moy Standards Track [Page 147] RFC 2178 OSPF Version 2 July 1997

                    . 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
 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 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

Moy Standards Track [Page 148] RFC 2178 OSPF Version 2 July 1997

 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.

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.

Moy Standards Track [Page 149] RFC 2178 OSPF Version 2 July 1997

     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.

Moy Standards Track [Page 150] RFC 2178 OSPF Version 2 July 1997

     (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   Otherwise, intra-area backbone paths are preferred.
  o   Inter-area paths are the least preferred.

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 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:

Moy Standards Track [Page 151] RFC 2178 OSPF Version 2 July 1997

 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 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).

Moy Standards Track [Page 152] RFC 2178 OSPF Version 2 July 1997

 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.
    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.

Moy Standards Track [Page 153] RFC 2178 OSPF Version 2 July 1997

16.8. Equal-cost multipath

 The OSPF protocol maintains multiple equal-cost routes to all
 destinations.  This can be seen in the steps used above to calculate
 the routing table, and in the definition of the routing table
 structure.
 Each one of the multiple routes will be of the same type (intra-area,
 inter-area, type 1 external or type 2 external), cost, and will have
 the same associated area.  However, each route specifies a separate
 next hop and Advertising router.
 There is no requirement that a router running OSPF keep track of all
 possible equal-cost routes to a destination.  An implementation may
 choose to keep only a fixed number of routes to any given
 destination.  This does not affect any of the algorithms presented in
 this specification.

Moy Standards Track [Page 154] RFC 2178 OSPF Version 2 July 1997

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
 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.

Moy Standards Track [Page 155] RFC 2178 OSPF Version 2 July 1997

 [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.
 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.

Moy Standards Track [Page 156] RFC 2178 OSPF Version 2 July 1997

 [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 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 157] RFC 2178 OSPF Version 2 July 1997

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,
         USC/Information Sciences Institute, September 1981.
 [Ref6]  McKenzie, A., "ISO Transport Protocol specification ISO DP
         8073", RFC 905, ISO, April 1984.
 [Ref7]  Deering, S., "Host extensions for IP multicasting", STD 5,
         RFC 1112, Stanford University, 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, Hughes LAN Systems, Performance Systems
         International, March 1991.
 [Ref9]  Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March
         1994.
 [Ref10] Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
         Inter-Domain Routing (CIDR): an Address Assignment and
         Aggregation Strategy", RFC1519, BARRNet, cisco, MERIT,
         OARnet, September 1993.
 [Ref11] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
         1700, USC/Information Sciences Institute, October 1994.
 [Ref12] Almquist, P., "Type of Service in the Internet Protocol
         Suite", RFC 1349, July 1992.
 [Ref13] Leiner, B., et.al., "The DARPA Internet Protocol Suite", DDN
         Protocol Handbook, April 1985.

Moy Standards Track [Page 158] RFC 2178 OSPF Version 2 July 1997

 [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, Proteon,
         Inc., March 1994.
 [Ref19] Coltun, R. and V. Fuller, "The OSPF NSSA Option", RFC 1587,
         RainbowBridge Communications, Stanford University, March
         1994.
 [Ref20] Ferguson, D., "The OSPF External Attributes LSA", work in
         progress.
 [Ref21] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
         1793, Cascade, April 1995.
 [Ref22] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
         DECWRL, Stanford University, November 1990.
 [Ref23] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-
         4)", RFC 1771, T.J. Watson Research Center, IBM Corp., cisco
         Systems, March 1995.
 [Ref24] Hinden, R., "Internet Routing Protocol Standardization
         Criteria", BBN, October 1991.

Moy Standards Track [Page 159] RFC 2178 OSPF Version 2 July 1997

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 160] RFC 2178 OSPF Version 2 July 1997

 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 161] RFC 2178 OSPF Version 2 July 1997

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].
 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].

Moy Standards Track [Page 162] RFC 2178 OSPF Version 2 July 1997

 DC-bit
    This bit describes the router's handling of demand circuits, as
    specified in [Ref21].

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 163] RFC 2178 OSPF Version 2 July 1997

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.
                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.

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

Moy Standards Track [Page 165] RFC 2178 OSPF Version 2 July 1997

A.3.2 The Hello packet

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

Moy Standards Track [Page 166] RFC 2178 OSPF Version 2 July 1997

 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 167] RFC 2178 OSPF Version 2 July 1997

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                          -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
 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

Moy Standards Track [Page 168] RFC 2178 OSPF Version 2 July 1997

 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 169] RFC 2178 OSPF Version 2 July 1997

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                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
 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 170] RFC 2178 OSPF Version 2 July 1997

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 carried by unicast Link
 State Update packets.  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                              |
     +-                                                            +-+
     |                              ...                              |
 # 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 171] RFC 2178 OSPF Version 2 July 1997

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 Acknowledgment packets is documented in Section 13.5.  The
 reception of Link State Acknowledgment 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 172] RFC 2178 OSPF Version 2 July 1997

 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.

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 173] RFC 2178 OSPF Version 2 July 1997

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):
      LS Type   Description
      ___________________________________
      1         Router-LSAs
      2         Network-LSAs
      3         Summary-LSAs (IP network)
      4         Summary-LSAs (ASBR)
      5         AS-external-LSAs

Moy Standards Track [Page 174] RFC 2178 OSPF Version 2 July 1997

 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 175] RFC 2178 OSPF Version 2 July 1997

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                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
 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).

Moy Standards Track [Page 176] RFC 2178 OSPF Version 2 July 1997

 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.
       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.

Moy Standards Track [Page 177] RFC 2178 OSPF Version 2 July 1997

     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.
 # 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 178] RFC 2178 OSPF Version 2 July 1997

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.
 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 179] RFC 2178 OSPF Version 2 July 1997

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                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
 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.

Moy Standards Track [Page 180] RFC 2178 OSPF Version 2 July 1997

 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 181] RFC 2178 OSPF Version 2 July 1997

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                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      External Route Tag                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

Moy Standards Track [Page 182] RFC 2178 OSPF Version 2 July 1997

 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.
 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 183] RFC 2178 OSPF Version 2 July 1997

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.
 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.

Moy Standards Track [Page 184] RFC 2178 OSPF Version 2 July 1997

 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 185] RFC 2178 OSPF Version 2 July 1997

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 Section 16.4.1, which
     prevent routing loops when AS- external-LSAs for the same
     destination have been originated from different areas (see
     Section G.7). Set to "enabled" by default.

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     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.
     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.

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         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:
 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.

Moy Standards Track [Page 188] RFC 2178 OSPF Version 2 July 1997

 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 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.

Moy Standards Track [Page 189] RFC 2178 OSPF Version 2 July 1997

 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
     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.

Moy Standards Track [Page 190] RFC 2178 OSPF Version 2 July 1997

 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.
 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.

Moy Standards Track [Page 191] RFC 2178 OSPF Version 2 July 1997

 Alternatively, neighbors on Point-to-MultiPoint networks may be
 dynamically discovered by lower-level protocols such as Inverse ARP
 ([Ref14]).

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.

Moy Standards Track [Page 192] RFC 2178 OSPF Version 2 July 1997

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.

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.

Moy Standards Track [Page 193] RFC 2178 OSPF Version 2 July 1997

 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
      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
 contains a new field called the "cryptographic sequence number".
 This field is initialized to zero, and is also set to zero 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.

Moy Standards Track [Page 194] RFC 2178 OSPF Version 2 July 1997

 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:
 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.

Moy Standards Track [Page 195] RFC 2178 OSPF Version 2 July 1997

 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.
 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.

Moy Standards Track [Page 196] RFC 2178 OSPF Version 2 July 1997

 (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.

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.
 (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.

Moy Standards Track [Page 197] RFC 2178 OSPF Version 2 July 1997

 (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 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.

Moy Standards Track [Page 198] RFC 2178 OSPF Version 2 July 1997

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.

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.

Moy Standards Track [Page 199] RFC 2178 OSPF Version 2 July 1997

    (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.

Moy Standards Track [Page 200] RFC 2178 OSPF Version 2 July 1997

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,
          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).

Moy Standards Track [Page 201] RFC 2178 OSPF Version 2 July 1997

 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]:
      (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.

Moy Standards Track [Page 202] RFC 2178 OSPF Version 2 July 1997

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.

Moy Standards Track [Page 203] RFC 2178 OSPF Version 2 July 1997

G. Differences from RFC 1583

 This section documents the differences between this memo and RFC
 1583.  All differences are backward-compatible. Implementations of
 this memo and of RFC 1583 will interoperate.

G.1 Enhancements to OSPF authentication

 An additional OSPF authentication type has been added: the
 Cryptographic authentication type. This has been defined so that any
 arbitrary "Keyed Message Digest" algorithm can be used for packet
 authentication. Operation using the MD5 algorithm is completely
 specified (see Appendix D).
 A number of other changes were also made to OSPF packet
 authentication, affecting the following Sections:
 o   The authentication type is now specified per-interface,
     rather than per-area (Sections 6, 9, C.2 and C.3).
 o   The OSPF packet header checksum is now considered part of
     the authentication procedure, and so has been moved out of the
     packet send and receive logic (Sections 8.1 and 8.2) and into the
     description of authentication types (Appendix D).
 o   In Appendix D, sections detailing message generation and
     message verification have been added.
 o   For the OSPF Cryptographic authentication type, a discussion
     of key management, including the requirement for simultaneous
     support of multiple keys, key lifetimes and smooth key
     transition, has been added to Appendix D.

G.2 Addition of Point-to-MultiPoint interface

 This memo adds an additional method for running OSPF over non-
 broadcast networks: the Point-to-Multipoint network. To implement
 this addition, the language of RFC 1583 has been altered slightly.
 References to "multi-access" networks have been deleted. The term
 "non-broadcast networks" is now used to describe networks which can
 connect many routers, but which do not natively support
 broadcast/multicast (such as a public Frame relay network).  Over
 non-broadcast networks, there are two options for running OSPF:
 modelling them as "NBMA networks" or as "Point-to-MultiPoint
 networks".  NBMA networks require full mesh connectivity between
 routers; when employing NBMA networks in the presence of partial mesh
 connectivity, multiple NBMA networks must be configured, as described
 in [Ref15].  In contrast, Point-to-Multipoint networks have been

Moy Standards Track [Page 204] RFC 2178 OSPF Version 2 July 1997

 designed to work simply and naturally when faced with partial mesh
 connectivity.
 The addition of Point-to-MultiPoint networks has impacted the text in
 many places, which are briefly summarized below:
 o   Section 2 describing the OSPF link-state database has been
     split into additional subsections, with one of the subsections
     (Section 2.1.1) describing the differing map representations of
     the two non-broadcast network options.  This subsection also
     contrasts the NBMA network and Point- to-MultiPoint network
     options, and describes the situations when one is preferable to
     the other.
 o   In contrast to NBMA networks, Point-to-MultiPoint networks
     have the following properties. Adjacencies are established
     between all neighboring routers (Sections 4, 7.1, 7.5, 9.5 and
     10.4). There is no Designated Router or Backup Designated Router
     for a Point-to-MultiPoint network (Sections 7.3 and 7.4). No
     network-LSA is originated for Point-to-MultiPoint networks
     (Sections 12.4.2 and A.4.3).  Router Priority is not configured
     for Point-to-MultiPoint interfaces, nor for neighbors on Point-
     to-MultiPoint networks (Sections C.3 and C.6).
 o   The Interface FSM for a Point-to-MultiPoint interface is
     identical to that used for point-to-point interfaces. Two states
     are possible: "Down" and "Point-to-Point" (Section 9.3).
 o   When originating a router-LSA, and Point-to-MultiPoint
     interface is reported as a collection of "point-to-point links"
     to all of the interface's adjacent neighbors, together with a
     single stub link advertising the interface's IP address with a
     cost of 0 (Section 12.4.1.4).
 o   When flooding out a non-broadcast interface (when either in
     NBMA or Point-to-MultiPoint mode) the Link State Update or Link
     State Acknowledgment packet must be replicated in order to be
     sent to each of the interface's neighbors (see Sections 13.3 and
     13.5).

G.3 Support for overlapping area ranges

 RFC 1583 requires that all networks falling into a given area range
 actually belong to a single area. This memo relaxes that restriction.
 This is useful in the following example. Suppose that [10.0.0.0,
 255.0.0.0] is carved up into subnets. Most of these subnets are
 assigned to a single OSPF area (call it Area X), while a few subnets
 are assigned to other areas. In order to get this configuration to

Moy Standards Track [Page 205] RFC 2178 OSPF Version 2 July 1997

 work with RFC 1583, you must not summarize the subnets of Area X with
 the single range [10.0.0.0, 255.0.0.0], because then the subnets of
 10.0.0.0 belonging to other areas would become unreachable. However,
 with this memo you can summarize the subnets in Area X, provided that
 the subnets belonging to other areas are not summarized.
 Implementation details for this change can be found in Sections 11.1
 and 16.2.

G.4 A modification to the flooding algorithm

 The OSPF flooding algorithm has been modified as follows. When a Link
 State Update Packet is received that contains an LSA instance which
 is actually less recent than the the router's current database copy,
 the router will now in most cases respond by flooding back its
 database copy. This is in contrast to the RFC 1583 behavior, which
 was to simply throw the received LSA away.
 Detailed description of the change can be found in Step 8 of Section
 13.
 This change improves MaxAge processing. There are times when MaxAge
 LSAs stay in a router's database for extended intervals: 1) when they
 are stuck in a retransmission queue on a slow link or 2) when a
 router is not properly flushing them from its database, due to
 software bugs. The prolonged existence of these MaxAge LSAs can
 inhibit the flooding of new instances of the LSA. New instances
 typically start with LS sequence number equal to
 InitialSequenceNumber, and are treated as less recent (and hence were
 discarded according to RFC 1583) by routers still holding MaxAge
 instances. However, with the above change to flooding, a router
 holding a MaxAge instance will flood back the MaxAge instance. When
 this flood reaches the LSA's originator, it will then pick the next
 highest LS sequence number and reflood, overwriting the MaxAge
 instance.

G.5 Introduction of the MinLSArrival constant

 OSPF limits the frequency that new instances of any particular LSA
 can be accepted during flooding. This is extra protection, just in
 case a neighboring router is violating the mandated limit on LSA
 (re)originations (namely, one per LSA in any MinLSInterval).

Moy Standards Track [Page 206] RFC 2178 OSPF Version 2 July 1997

 In RFC 1583, the frequency at which new LSA instances were accepted
 was also set equal to once every MinLSInterval seconds.  However, in
 some circumstances this led to unwanted link state retransmissions,
 even when the LSA originator was obeying the MinLSInterval limit on
 originations. This was due to either 1) choice of clock granularity
 in some OSPF implementations or 2) differing clock speed in
 neighboring routers.
 To alleviate this problem, the frequency at which new LSA instances
 are accepted during flooding has now been increased to once every
 MinLSArrival seconds, whose value is set to 1.  This change is
 reflected in Steps 5a and 5d of Section 13, and in Appendix B.

G.6 Optionally advertising point-to-point links as subnets

 When describing a point-to-point interface in its router-LSA, a
 router may now advertise a stub link to the point-to-point network's
 subnet. This is specified as an alternative to the RFC 1583 behavior,
 which is to advertise a stub link to the neighbor's IP address. See
 Sections 12.4.1 and 12.4.1.1 for details.

G.7 Advertising same external route from multiple areas

 This document fixes routing loops which can occur in RFC 1583 when
 the same external destination is advertised by AS boundary routers in
 separate areas. There are two manifestations of this problem. The
 first, discovered by Dennis Ferguson, occurs when an aggregated
 forwarding address is in use. In this case, the desirability of the
 forwarding address can change for the worse as a packet crosses an
 area aggregation boundary on the way to the forwarding address, which
 in turn can cause the preference of AS-external-LSAs to change,
 resulting in a routing loop.
 The second manifestation was discovered by Richard Woundy. It is
 caused by an incomplete application of OSPF's preference of intra-
 area routes over inter-area routes: paths to any given
 ASBR/forwarding address are selected first based on intra-area
 preference, while the comparison between separate ASBRs/forwarding
 addresses is driven only by cost, ignoring intra-area preference. His
 example is replicated in Figure 19.  Both router A3 and router B3 are
 originating an AS-external-LSA for 10.0.0.0/8, with the same type 2
 metric. Router A1 selects B1 as its next hop towards 10.0.0.0/8,
 based on shorter cost to ASBR B3 (via B1->B2->B3). However, the
 shorter route to B3 is not available to B1, due to B1's preference
 for the (higher cost) intra-area route to B3 through Area A. This
 leads B1 to select A1 as its next hop to 10.0.0.0/8, resulting in a
 routing loop.

Moy Standards Track [Page 207] RFC 2178 OSPF Version 2 July 1997

 The following two changes have been made to prevent these routing
 loops:
 o   When originating a type 3 summary-LSA for a configured area
     address range, the cost of the summary-LSA is now set to the
     maximum cost of the range's component networks (instead of the
     previous algorithm which set the cost to the minimum component
     cost).  This change affects Sections 3.5 and 12.4.3, Figures 7
     and 8, and Tables 6 and 13.
 o   The preference rules for choosing among multiple AS-
     external-LSAs have been changed. Where previously cost was the
     only determining factor, now the preference is driven first by
     type of path (intra-area or inter-area, through non-backbone area
     or through backbone) to the ASBR/forwarding address, using cost
     only to break ties. This change affects Sections 16.4 and 16.4.1.
 After implementing this change, the example in Figure 19 is modified
 as follows. Router A1 now chooses A3 as the next
                            10.0.0.0/8
                            ----------
                                 |
                              +----+
                              | XX |
                              +----+
                 RIP          /    \        RIP
         ---------------------      --------------------
         !                                             !
         !                                             !
       +----+      +----+       1       +----+......+----+....
       | A3 |------| A1 |---------------| B1 |------| B3 |   .
       +----+   6  +----+               +----+  8   +----+   .
                                         1|  .         /     .
                     OSPF backbone        |  .        /      .
                                        +----+  2    /       .
                                        | B2 |-------  Area A.
                                        +----+................
              Figure 19: Example routing loop when the
          same external route is advertised from multiple
                               areas
 hop to 10.0.0.0/8, while B1 chooses B3 as next hop. The reason for
 both choices is that ASBRs/forwarding addresses are now chosen based
 first on intra-area preference, and then by cost.

Moy Standards Track [Page 208] RFC 2178 OSPF Version 2 July 1997

 Unfortunately, this change is not backward compatible. While the
 change prevents routing loops when all routers run the new preference
 rules, it can actually create routing loops when some routers are
 running the new preference rules and other routers implement RFC
 1583.  For this reason, a new configuration parameter has been added:
 RFC1583Compatibility. Only when RFC1583Compatibility is set to
 "disabled" will the new preference rules take effect. See Appendix C
 for more details.

G.8 Retransmission of initial Database Description packets

 This memo allows retransmission of initial Database Description
 packets, without resetting the state of the adjacency. In some
 environments, retransmission of the initial Database Description
 packet may be unavoidable. For example, the link delay incurred by a
 satellite link may exceed the value configured for an interface's
 RxmtInterval. In RFC 1583 such an environment prevents a full
 adjacency from ever forming.
 In this memo, changes have been made in the reception of Database
 Description packets so that retransmitted initial Database
 Description packets are treated identically to any other
 retransmitted Database Description packets. See Section 10.6 for
 details.

G.9 Detecting interface MTU mismatches

 When two neighboring routers have a different interface MTU for their
 common network segment, serious problems can ensue: large packets are
 prevented from being successfully transferred from one router to the
 other, impairing OSPF's flooding algorithm and possibly creating
 "black holes" for user data traffic.
 This memo provides a fix for the interface MTU mismatch problem by
 advertising the interface MTU in Database Description packets. When a
 router receives a Database description packet advertising an MTU
 larger than the router can receive, the router drops the Database
 Description packet. This prevents an adjacency from forming, telling
 OSPF flooding and user data traffic to avoid the connection between
 the two routers. For more information, see Sections 10.6, 10.8, and
 A.3.3.

G.10 Deleting the TOS routing option

 The TOS routing option has been deleted from OSPF. This action was
 required by the Internet standards process ([Ref24]), due to lack of
 implementation experience with OSPF's TOS routing.  However, for
 backward compatibility the formats of OSPF's various LSAs remain

Moy Standards Track [Page 209] RFC 2178 OSPF Version 2 July 1997

 unchanged, maintaining the ability to specify TOS metrics in router-
 LSAs, summary-LSAs, ASBR-summary-LSAs, and AS-external-LSAs (see
 Sections 12.3, A.4.2, A.4.4, and A.4.5).
 To see OSPF's original TOS routing design, consult [Ref9].

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.

Moy Standards Track [Page 210] RFC 2178 OSPF Version 2 July 1997

Author's Address

 John Moy
 Cascade Communications Corp.
 5 Carlisle Road
 Westford, MA 01886
 Phone: 508-952-1367
 Fax:   508-692-9214
 Email: jmoy@casc.com

Moy Standards Track [Page 211]

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