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RFC 2328

 Errata 
STD 54
Pages: 244
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OSPF Version 2

Part 1 of 8, p. 1 to 26
None       Next RFC Part

Obsoletes:    2178
Updated by:    5709    6549    6845    6860    7474    8042


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


                             OSPF Version 2


Status of this Memo

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

Copyright Notice

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

Abstract

    This memo documents version 2 of the OSPF protocol.  OSPF is a
    link-state routing protocol.  It is designed to be run internal to a
    single Autonomous System.  Each OSPF router maintains an identical
    database describing the Autonomous System's topology.  From this
    database, a routing table is calculated by constructing a shortest-
    path tree.

    OSPF recalculates routes quickly in the face of topological changes,
    utilizing a minimum of routing protocol traffic.  OSPF provides
    support for equal-cost multipath.  An area routing capability is
    provided, enabling an additional level of routing protection and a
    reduction in routing protocol traffic.  In addition, all OSPF
    routing protocol exchanges are authenticated.

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

Page 2 
    Implementations of this memo and of RFCs 2178, 1583, and 1247 will
    interoperate.

    Please send comments to ospf@gated.cornell.edu.

Table of Contents

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

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

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

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

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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.

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        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.

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    1.2.  Definitions of commonly used terms

        This section provides definitions for terms that have a specific
        meaning to the OSPF protocol and that are used throughout the
        text.  The reader unfamiliar with the Internet Protocol Suite is
        referred to [Ref13] for an introduction to IP.


        Router
            A level three Internet Protocol packet switch.  Formerly
            called a gateway in much of the IP literature.

        Autonomous System
            A group of routers exchanging routing information via a
            common routing protocol.  Abbreviated as AS.

        Interior Gateway Protocol
            The routing protocol spoken by the routers belonging to an
            Autonomous system.  Abbreviated as IGP.  Each Autonomous
            System has a single IGP.  Separate Autonomous Systems may be
            running different IGPs.

        Router ID
            A 32-bit number assigned to each router running the OSPF
            protocol.  This number uniquely identifies the router within
            an Autonomous System.

        Network
            In this memo, an IP network/subnet/supernet.  It is possible
            for one physical network to be assigned multiple IP
            network/subnet numbers.  We consider these to be separate
            networks.  Point-to-point physical networks are an exception
            - they are considered a single network no matter how many
            (if any at all) IP network/subnet numbers are assigned to
            them.

        Network mask
            A 32-bit number indicating the range of IP addresses
            residing on a single IP network/subnet/supernet.  This
            specification displays network masks as hexadecimal numbers.

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            For example, the network mask for a class C IP network is
            displayed as 0xffffff00.  Such a mask is often displayed
            elsewhere in the literature as 255.255.255.0.

        Point-to-point networks
            A network that joins a single pair of routers.  A 56Kb
            serial line is an example of a point-to-point network.

        Broadcast networks
            Networks supporting many (more than two) attached routers,
            together with the capability to address a single physical
            message to all of the attached routers (broadcast).
            Neighboring routers are discovered dynamically on these nets
            using OSPF's Hello Protocol.  The Hello Protocol itself
            takes advantage of the broadcast capability.  The OSPF
            protocol makes further use of multicast capabilities, if
            they exist.  Each pair of routers on a broadcast network is
            assumed to be able to communicate directly. An ethernet is
            an example of a broadcast network.

        Non-broadcast networks
            Networks supporting many (more than two) routers, but having
            no broadcast capability.  Neighboring routers are maintained
            on these nets using OSPF's Hello Protocol.  However, due to
            the lack of broadcast capability, some configuration
            information may be necessary to aid in the discovery of
            neighbors.  On non-broadcast networks, OSPF protocol packets
            that are normally multicast need to be sent to each
            neighboring router, in turn. An X.25 Public Data Network
            (PDN) is an example of a non-broadcast network.

            OSPF runs in one of two modes over non-broadcast networks.
            The first mode, called non-broadcast multi-access or NBMA,
            simulates the operation of OSPF on a broadcast network. The
            second mode, called Point-to-MultiPoint, treats the non-
            broadcast network as a collection of point-to-point links.
            Non-broadcast networks are referred to as NBMA networks or
            Point-to-MultiPoint networks, depending on OSPF's mode of
            operation over the network.

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        Interface
            The connection between a router and one of its attached
            networks.  An interface has state information associated
            with it, which is obtained from the underlying lower level
            protocols and the routing protocol itself.  An interface to
            a network has associated with it a single IP address and
            mask (unless the network is an unnumbered point-to-point
            network).  An interface is sometimes also referred to as a
            link.

        Neighboring routers
            Two routers that have interfaces to a common network.
            Neighbor relationships are maintained by, and usually
            dynamically discovered by, OSPF's Hello Protocol.

        Adjacency
            A relationship formed between selected neighboring routers
            for the purpose of exchanging routing information.  Not
            every pair of neighboring routers become adjacent.

        Link state advertisement
            Unit of data describing the local state of a router or
            network. For a router, this includes the state of the
            router's interfaces and adjacencies.  Each link state
            advertisement is flooded throughout the routing domain. The
            collected link state advertisements of all routers and
            networks forms the protocol's link state database.
            Throughout this memo, link state advertisement is
            abbreviated as LSA.

        Hello Protocol
            The part of the OSPF protocol used to establish and maintain
            neighbor relationships.  On broadcast networks the Hello
            Protocol can also dynamically discover neighboring routers.

        Flooding
            The part of the OSPF protocol that distributes and
            synchronizes the link-state database between OSPF routers.

        Designated Router
            Each broadcast and NBMA network that has at least two
            attached routers has a Designated Router.  The Designated

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            Router generates an LSA for the network and has other
            special responsibilities in the running of the protocol.
            The Designated Router is elected by the Hello Protocol.

            The Designated Router concept enables a reduction in the
            number of adjacencies required on a broadcast or NBMA
            network.  This in turn reduces the amount of routing
            protocol traffic and the size of the link-state database.

        Lower-level protocols
            The underlying network access protocols that provide
            services to the Internet Protocol and in turn the OSPF
            protocol.  Examples of these are the X.25 packet and frame
            levels for X.25 PDNs, and the ethernet data link layer for
            ethernets.


    1.3.  Brief history of link-state routing technology

        OSPF is a link state routing protocol.  Such protocols are also
        referred to in the literature as SPF-based or distributed-
        database protocols.  This section gives a brief description of
        the developments in link-state technology that have influenced
        the OSPF protocol.

        The first link-state routing protocol was developed for use in
        the ARPANET packet switching network.  This protocol is
        described in [Ref3].  It has formed the starting point for all
        other link-state protocols.  The homogeneous ARPANET
        environment, i.e., single-vendor packet switches connected by
        synchronous serial lines, simplified the design and
        implementation of the original protocol.

        Modifications to this protocol were proposed in [Ref4].  These
        modifications dealt with increasing the fault tolerance of the
        routing protocol through, among other things, adding a checksum
        to the LSAs (thereby detecting database corruption).  The paper
        also included means for reducing the routing traffic overhead in
        a link-state protocol.  This was accomplished by introducing
        mechanisms which enabled the interval between LSA originations
        to be increased by an order of magnitude.

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        A link-state algorithm has also been proposed for use as an ISO
        IS-IS routing protocol.  This protocol is described in [Ref2].
        The protocol includes methods for data and routing traffic
        reduction when operating over broadcast networks.  This is
        accomplished by election of a Designated Router for each
        broadcast network, which then originates an LSA for the network.

        The OSPF Working Group of the IETF has extended this work in
        developing the OSPF protocol.  The Designated Router concept has
        been greatly enhanced to further reduce the amount of routing
        traffic required.  Multicast capabilities are utilized for
        additional routing bandwidth reduction.  An area routing scheme
        has been developed enabling information
        hiding/protection/reduction.  Finally, the algorithms have been
        tailored for efficient operation in TCP/IP internets.


    1.4.  Organization of this document

        The first three sections of this specification give a general
        overview of the protocol's capabilities and functions.  Sections
        4-16 explain the protocol's mechanisms in detail.  Packet
        formats, protocol constants and configuration items are
        specified in the appendices.

        Labels such as HelloInterval encountered in the text refer to
        protocol constants.  They may or may not be configurable.
        Architectural constants are summarized in Appendix B.
        Configurable constants are summarized in Appendix C.

        The detailed specification of the protocol is presented in terms
        of data structures.  This is done in order to make the
        explanation more precise.  Implementations of the protocol are
        required to support the functionality described, but need not
        use the precise data structures that appear in this memo.


    1.5.  Acknowledgments

        The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
        Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
        Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui

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        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.

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                                                  **FROM**

                                           *      |RT1|RT2|
                +---+Ia    +---+           *   ------------
                |RT1|------|RT2|           T   RT1|   | X |
                +---+    Ib+---+           O   RT2| X |   |
                                           *    Ia|   | X |
                                           *    Ib| X |   |

                     Physical point-to-point networks


                                                  **FROM**
                      +---+                *
                      |RT7|                *      |RT7| N3|
                      +---+                T   ------------
                        |                  O   RT7|   |   |
            +----------------------+       *    N3| X |   |
                       N3                  *

                              Stub networks

                                                  **FROM**
                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|N2 |
                +---+      +---+        *  ------------------------
                  |    N2    |          *  RT3|   |   |   |   | X |
            +----------------------+    T  RT4|   |   |   |   | X |
                  |          |          O  RT5|   |   |   |   | X |
                +---+      +---+        *  RT6|   |   |   |   | X |
                |RT5|      |RT6|        *   N2| X | X | X | X |   |
                +---+      +---+

                          Broadcast or NBMA networks



                    Figure 1a: Network map components

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             Networks and routers are represented by vertices.
             An edge connects Vertex A to Vertex B iff the
             intersection of Column A and Row B is marked with
                                  an X.



        The top of Figure 1a shows two routers connected by a point-to-
        point link. In the resulting link-state database graph, the two
        router vertices are directly connected by a pair of edges, one
        in each direction. Interfaces to point-to-point networks need
        not be assigned IP addresses.  When interface addresses are
        assigned, they are modelled as stub links, with each router
        advertising a stub connection to the other router's interface
        address. Optionally, an IP subnet can be assigned to the point-
        to-point network. In this case, both routers advertise a stub
        link to the IP subnet, instead of advertising each others' IP
        interface addresses.

        The middle of Figure 1a shows a network with only one attached
        router (i.e., a stub network). In this case, the network appears
        on the end of a stub connection in the link-state database's
        graph.

        When multiple routers are attached to a broadcast network, the
        link-state database graph shows all routers bidirectionally
        connected to the network vertex. This is pictured at the bottom
        of Figure 1a.

        Each network (stub or transit) in the graph has an IP address
        and associated network mask.  The mask indicates the number of
        nodes on the network.  Hosts attached directly to routers
        (referred to as host routes) appear on the graph as stub
        networks.  The network mask for a host route is always
        0xffffffff, which indicates the presence of a single node.


        2.1.1.  Representation of non-broadcast networks

            As mentioned previously, OSPF can run over non-broadcast
            networks in one of two modes: NBMA or Point-to-MultiPoint.
            The choice of mode determines the way that the Hello

Top      ToC       Page 16 
            protocol and flooding work over the non-broadcast network,
            and the way that the network is represented in the link-
            state database.

            In NBMA mode, OSPF emulates operation over a broadcast
            network: a Designated Router is elected for the NBMA
            network, and the Designated Router originates an LSA for the
            network. The graph representation for broadcast networks and
            NBMA networks is identical. This representation is pictured
            in the middle of Figure 1a.

            NBMA mode is the most efficient way to run OSPF over non-
            broadcast networks, both in terms of link-state database
            size and in terms of the amount of routing protocol traffic.
            However, it has one significant restriction: it requires all
            routers attached to the NBMA network to be able to
            communicate directly. This restriction may be met on some
            non-broadcast networks, such as an ATM subnet utilizing
            SVCs. But it is often not met on other non-broadcast
            networks, such as PVC-only Frame Relay networks. On non-
            broadcast networks where not all routers can communicate
            directly you can break the non-broadcast network into
            logical subnets, with the routers on each subnet being able
            to communicate directly, and then run each separate subnet
            as an NBMA network (see [Ref15]). This however requires
            quite a bit of administrative overhead, and is prone to
            misconfiguration. It is probably better to run such a non-
            broadcast network in Point-to-Multipoint mode.

            In Point-to-MultiPoint mode, OSPF treats all router-to-
            router connections over the non-broadcast network as if they
            were point-to-point links. No Designated Router is elected
            for the network, nor is there an LSA generated for the
            network. In fact, a vertex for the Point-to-MultiPoint
            network does not appear in the graph of the link-state
            database.

            Figure 1b illustrates the link-state database representation
            of a Point-to-MultiPoint network. On the left side of the
            figure, a Point-to-MultiPoint network is pictured. It is
            assumed that all routers can communicate directly, except
            for routers RT4 and RT5. I3 though I6 indicate the routers'

Top      ToC       Page 17 
            IP interface addresses on the Point-to-MultiPoint network.
            In the graphical representation of the link-state database,
            routers that can communicate directly over the Point-to-
            MultiPoint network are joined by bidirectional edges, and
            each router also has a stub connection to its own IP
            interface address (which is in contrast to the
            representation of real point-to-point links; see Figure 1a).

            On some non-broadcast networks, use of Point-to-MultiPoint
            mode and data-link protocols such as Inverse ARP (see
            [Ref14]) will allow autodiscovery of OSPF neighbors even
            though broadcast support is not available.






                                                  **FROM**
                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|
                +---+      +---+        *  --------------------
                I3|    N2    |I4        *  RT3|   | X | X | X |
            +----------------------+    T  RT4| X |   |   | X |
                I5|          |I6        O  RT5| X |   |   | X |
                +---+      +---+        *  RT6| X | X | X |   |
                |RT5|      |RT6|        *   I3| X |   |   |   |
                +---+      +---+            I4|   | X |   |   |
                                            I5|   |   | X |   |
                                            I6|   |   |   | X |



                    Figure 1b: Network map components
                       Point-to-MultiPoint networks

             All routers can communicate directly over N2, except
                routers RT4 and RT5. I3 through I6 indicate IP
                           interface addresses

Top      ToC       Page 18 
        2.1.2.  An example link-state database

            Figure 2 shows a sample map of an Autonomous System.  The
            rectangle labelled H1 indicates a host, which has a SLIP
            connection to Router RT12.  Router RT12 is therefore
            advertising a host route.  Lines between routers indicate
            physical point-to-point networks.  The only point-to-point
            network that has been assigned interface addresses is the
            one joining Routers RT6 and RT10.  Routers RT5 and RT7 have
            BGP connections to other Autonomous Systems.  A set of BGP-
            learned routes have been displayed for both of these
            routers.

            A cost is associated with the output side of each router
            interface.  This cost is configurable by the system
            administrator.  The lower the cost, the more likely the
            interface is to be used to forward data traffic.  Costs are
            also associated with the externally derived routing data
            (e.g., the BGP-learned routes).

            The directed graph resulting from the map in Figure 2 is
            depicted in Figure 3.  Arcs are labelled with the cost of
            the corresponding router output interface.  Arcs having no
            labelled cost have a cost of 0.  Note that arcs leading from
            networks to routers always have cost 0; they are significant
            nonetheless.  Note also that the externally derived routing
            data appears on the graph as stubs.

            The link-state database is pieced together from LSAs
            generated by the routers.  In the associated graphical
            representation, the neighborhood of each router or transit
            network is represented in a single, separate LSA.  Figure 4
            shows these LSAs graphically. Router RT12 has an interface
            to two broadcast networks and a SLIP line to a host.
            Network N6 is a broadcast network with three attached
            routers.  The cost of all links from Network N6 to its
            attached routers is 0.  Note that the LSA for Network N6 is
            actually generated by one of the network's attached routers:
            the router that has been elected Designated Router for the
            network.

Top      ToC       Page 19 
                 +
                 | 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

Top      ToC       Page 20 
                    Figure 2: A sample Autonomous System

                                **FROM**

                 |RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
                 |1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
              ----- ---------------------------------------------
              RT1|  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |  |
              RT2|  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |  |
              RT3|  |  |  |  |  |6 |  |  |  |  |  |  |0 |  |  |  |
              RT4|  |  |  |  |8 |  |  |  |  |  |  |  |0 |  |  |  |
              RT5|  |  |  |8 |  |6 |6 |  |  |  |  |  |  |  |  |  |
              RT6|  |  |8 |  |7 |  |  |  |  |5 |  |  |  |  |  |  |
              RT7|  |  |  |  |6 |  |  |  |  |  |  |  |  |0 |  |  |
          *   RT8|  |  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |
          *   RT9|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |
          T  RT10|  |  |  |  |  |7 |  |  |  |  |  |  |  |0 |0 |  |
          O  RT11|  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |0 |
          *  RT12|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |
          *    N1|3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N2|  |3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N3|1 |1 |1 |1 |  |  |  |  |  |  |  |  |  |  |  |  |
               N4|  |  |2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N6|  |  |  |  |  |  |1 |1 |  |1 |  |  |  |  |  |  |
               N7|  |  |  |  |  |  |  |4 |  |  |  |  |  |  |  |  |
               N8|  |  |  |  |  |  |  |  |  |3 |2 |  |  |  |  |  |
               N9|  |  |  |  |  |  |  |  |1 |  |1 |1 |  |  |  |  |
              N10|  |  |  |  |  |  |  |  |  |  |  |2 |  |  |  |  |
              N11|  |  |  |  |  |  |  |  |3 |  |  |  |  |  |  |  |
              N12|  |  |  |  |8 |  |2 |  |  |  |  |  |  |  |  |  |
              N13|  |  |  |  |8 |  |  |  |  |  |  |  |  |  |  |  |
              N14|  |  |  |  |8 |  |  |  |  |  |  |  |  |  |  |  |
              N15|  |  |  |  |  |  |9 |  |  |  |  |  |  |  |  |  |
               H1|  |  |  |  |  |  |  |  |  |  |  |10|  |  |  |  |


                     Figure 3: The resulting directed graph

                 Networks and routers are represented by vertices.
                 An edge of cost X connects Vertex A to Vertex B iff
                 the intersection of Column A and Row B is marked
                                     with an X.

Top      ToC       Page 21 
                     **FROM**                       **FROM**

                  |RT12|N9|N10|H1|                 |RT9|RT11|RT12|N9|
           *  --------------------          *  ----------------------
           *  RT12|    |  |   |  |          *   RT9|   |    |    |0 |
           T    N9|1   |  |   |  |          T  RT11|   |    |    |0 |
           O   N10|2   |  |   |  |          O  RT12|   |    |    |0 |
           *    H1|10  |  |   |  |          *    N9|   |    |    |  |
           *                                *
                RT12's router-LSA              N9's network-LSA

                  Figure 4: Individual link state components

              Networks and routers are represented by vertices.
              An edge of cost X connects Vertex A to Vertex B iff
              the intersection of Column A and Row B is marked
                                  with an X.

    2.2.  The shortest-path tree

        When no OSPF areas are configured, each router in the Autonomous
        System has an identical link-state database, leading to an
        identical graphical representation.  A router generates its
        routing table from this graph by calculating a tree of shortest
        paths with the router itself as root.  Obviously, the shortest-
        path tree depends on the router doing the calculation.  The
        shortest-path tree for Router RT6 in our example is depicted in
        Figure 5.

        The tree gives the entire path to any destination network or
        host.  However, only the next hop to the destination is used in
        the forwarding process.  Note also that the best route to any
        router has also been calculated.  For the processing of external
        data, we note the next hop and distance to any router
        advertising external routes.  The resulting routing table for
        Router RT6 is pictured in Table 2.  Note that there is a
        separate route for each end of a numbered point-to-point network
        (in this case, the serial line between Routers RT6 and RT10).


        Routes to networks belonging to other AS'es (such as N12) appear
        as dashed lines on the shortest path tree in Figure 5.  Use of

Top      ToC       Page 22 
                                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

Top      ToC       Page 23 
                   Destination   Next  Hop   Distance
                   __________________________________
                   N1            RT3         10
                   N2            RT3         10
                   N3            RT3         7
                   N4            RT3         8
                   Ib            *           7
                   Ia            RT10        12
                   N6            RT10        8
                   N7            RT10        12
                   N8            RT10        10
                   N9            RT10        11
                   N10           RT10        13
                   N11           RT10        14
                   H1            RT10        21
                   __________________________________
                   RT5           RT5         6
                   RT7           RT10        8


    Table 2: The portion of Router RT6's routing table listing local
                             destinations.

        this externally derived routing information is considered in the
        next section.


    2.3.  Use of external routing information

        After the tree is created the external routing information is
        examined.  This external routing information may originate from
        another routing protocol such as BGP, or be statically
        configured (static routes).  Default routes can also be included
        as part of the Autonomous System's external routing information.

        External routing information is flooded unaltered throughout the
        AS.  In our example, all the routers in the Autonomous System
        know that Router RT7 has two external routes, with metrics 2 and
        9.

        OSPF supports two types of external metrics.  Type 1 external
        metrics are expressed in the same units as OSPF interface cost

Top      ToC       Page 24 
        (i.e., in terms of the link state metric).  Type 2 external
        metrics are an order of magnitude larger; any Type 2 metric is
        considered greater than the cost of any path internal to the AS.
        Use of Type 2 external metrics assumes that routing between
        AS'es is the major cost of routing a packet, and eliminates the
        need for conversion of external costs to internal link state
        metrics.

        As an example of Type 1 external metric processing, suppose that
        the Routers RT7 and RT5 in Figure 2 are advertising Type 1
        external metrics.  For each advertised external route, the total
        cost from Router RT6 is calculated as the sum of the external
        route's advertised cost and the distance from Router RT6 to the
        advertising router.  When two routers are advertising the same
        external destination, RT6 picks the advertising router providing
        the minimum total cost. RT6 then sets the next hop to the
        external destination equal to the next hop that would be used
        when routing packets to the chosen advertising router.

        In Figure 2, both Router RT5 and RT7 are advertising an external
        route to destination Network N12.  Router RT7 is preferred since
        it is advertising N12 at a distance of 10 (8+2) to Router RT6,
        which is better than Router RT5's 14 (6+8).  Table 3 shows the
        entries that are added to the routing table when external routes
        are examined:



                         Destination   Next  Hop   Distance
                         __________________________________
                         N12           RT10        10
                         N13           RT5         14
                         N14           RT5         14
                         N15           RT10        17


                 Table 3: The portion of Router RT6's routing table
                           listing external destinations.


        Processing of Type 2 external metrics is simpler.  The AS
        boundary router advertising the smallest external metric is

Top      ToC       Page 25 
        chosen, regardless of the internal distance to the AS boundary
        router.  Suppose in our example both Router RT5 and Router RT7
        were advertising Type 2 external routes.  Then all traffic
        destined for Network N12 would be forwarded to Router RT7, since
        2 < 8.  When several equal-cost Type 2 routes exist, the
        internal distance to the advertising routers is used to break
        the tie.

        Both Type 1 and Type 2 external metrics can be present in the AS
        at the same time.  In that event, Type 1 external metrics always
        take precedence.

        This section has assumed that packets destined for external
        destinations are always routed through the advertising AS
        boundary router.  This is not always desirable.  For example,
        suppose in Figure 2 there is an additional router attached to
        Network N6, called Router RTX.  Suppose further that RTX does
        not participate in OSPF routing, but does exchange BGP
        information with the AS boundary router RT7.  Then, Router RT7
        would end up advertising OSPF external routes for all
        destinations that should be routed to RTX.  An extra hop will
        sometimes be introduced if packets for these destinations need
        always be routed first to Router RT7 (the advertising router).

        To deal with this situation, the OSPF protocol allows an AS
        boundary router to specify a "forwarding address" in its AS-
        external-LSAs.  In the above example, Router RT7 would specify
        RTX's IP address as the "forwarding address" for all those
        destinations whose packets should be routed directly to RTX.

        The "forwarding address" has one other application.  It enables
        routers in the Autonomous System's interior to function as
        "route servers".  For example, in Figure 2 the router RT6 could
        become a route server, gaining external routing information
        through a combination of static configuration and external
        routing protocols.  RT6 would then start advertising itself as
        an AS boundary router, and would originate a collection of OSPF
        AS-external-LSAs.  In each AS-external-LSA, Router RT6 would
        specify the correct Autonomous System exit point to use for the
        destination through appropriate setting of the LSA's "forwarding
        address" field.

Top      ToC       Page 26 
    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.




(page 26 continued on part 2)

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