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RFC 1583


OSPF Version 2

Part 2 of 9, p. 21 to 50
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3.  Splitting the AS into Areas

    OSPF allows collections of contiguous networks and hosts to be
    grouped together.  Such a group, together with the routers having
    interfaces to any one of the included networks, is called an area.
    Each area runs a separate copy of the basic link-state routing
    algorithm.  This means that each area has its own topological
    database and corresponding graph, as explained in the previous

    The topology of an area is invisible from the outside of the area.
    Conversely, routers internal to a given area know nothing of the
    detailed topology external to the area.  This isolation of knowledge
    enables the protocol to effect a marked reduction in routing traffic
    as compared to treating the entire Autonomous System as a single
    link-state domain.

    With the introduction of areas, it is no longer true that all
    routers in the AS have an identical topological database.  A router
    actually has a separate topological database for each area it is
    connected to.  (Routers connected to multiple areas are called area
    border routers).  Two routers belonging to the same area have, for
    that area, identical area topological databases.

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    Routing in the Autonomous System takes place on two levels,
    depending on whether the source and destination of a packet reside
    in the same area (intra-area routing is used) or different areas
    (inter-area routing is used).  In intra-area routing, the packet is
    routed solely on information obtained within the area; no routing
    information obtained from outside the area can be used.  This
    protects intra-area routing from the injection of bad routing
    information.  We discuss inter-area routing in Section 3.2.

    3.1.  The backbone of the Autonomous System

        The backbone consists of those networks not contained in any
        area, their attached routers, and those routers that belong to
        multiple areas.  The backbone must be contiguous.

        It is possible to define areas in such a way that the backbone
        is no longer contiguous.  In this case the system administrator
        must restore backbone connectivity by configuring virtual links.

        Virtual links can be configured between any two backbone routers
        that have an interface to a common non-backbone area.  Virtual
        links belong to the backbone.  The protocol treats two routers
        joined by a virtual link as if they were connected by an
        unnumbered point-to-point network.  On the graph of the
        backbone, two such routers are joined by arcs whose costs are
        the intra-area distances between the two routers.  The routing
        protocol traffic that flows along the virtual link uses intra-
        area routing only.

        The backbone is responsible for distributing routing information
        between areas.  The backbone itself has all of the properties of
        an area.  The topology of the backbone is invisible to each of
        the areas, while the backbone itself knows nothing of the
        topology of the areas.

    3.2.  Inter-area routing

        When routing a packet between two areas the backbone is used.
        The path that the packet will travel can be broken up into three
        contiguous pieces: an intra-area path from the source to an area
        border router, a backbone path between the source and
        destination areas, and then another intra-area path to the
        destination.  The algorithm finds the set of such paths that
        have the smallest cost.

        Looking at this another way, inter-area routing can be pictured

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        as forcing a star configuration on the Autonomous System, with
        the backbone as hub and each of the areas as spokes.

        The topology of the backbone dictates the backbone paths used
        between areas.  The topology of the backbone can be enhanced by
        adding virtual links.  This gives the system administrator some
        control over the routes taken by inter-area traffic.

        The correct area border router to use as the packet exits the
        source area is chosen in exactly the same way routers
        advertising external routes are chosen.  Each area border router
        in an area summarizes for the area its cost to all networks
        external to the area.  After the SPF tree is calculated for the
        area, routes to all other networks are calculated by examining
        the summaries of the area border routers.

    3.3.  Classification of routers

        Before the introduction of areas, the only OSPF routers having a
        specialized function were those advertising external routing
        information, such as Router RT5 in Figure 2.  When the AS is
        split into OSPF areas, the routers are further divided according
        to function into the following four overlapping categories:

        Internal routers
            A router with all directly connected networks belonging to
            the same area.  Routers with only backbone interfaces also
            belong to this category.  These routers run a single copy of
            the basic routing algorithm.

        Area border routers
            A router that attaches to multiple areas.  Area border
            routers run multiple copies of the basic algorithm, one copy
            for each attached area and an additional copy for the
            backbone.  Area border routers condense the topological
            information of their attached areas for distribution to the
            backbone.  The backbone in turn distributes the information
            to the other areas.

        Backbone routers
            A router that has an interface to the backbone.  This
            includes all routers that interface to more than one area
            (i.e., area border routers).  However, backbone routers do
            not have to be area border routers.  Routers with all
            interfaces connected to the backbone are considered to be
            internal routers.

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        AS boundary routers
            A router that exchanges routing information with routers
            belonging to other Autonomous Systems.  Such a router has AS
            external routes that are advertised throughout the
            Autonomous System.  The path to each AS boundary router is
            known by every router in the AS.  This classification is
            completely independent of the previous classifications: AS
            boundary routers may be internal or area border routers, and
            may or may not participate in the backbone.

    3.4.  A sample area configuration

        Figure 6 shows a sample area configuration.  The first area
        consists of networks N1-N4, along with their attached routers
        RT1-RT4.  The second area consists of networks N6-N8, along with
        their attached routers RT7, RT8, RT10 and RT11.  The third area
        consists of networks N9-N11 and Host H1, along with their
        attached routers RT9, RT11 and RT12.  The third area has been
        configured so that networks N9-N11 and Host H1 will all be
        grouped into a single route, when advertised external to the
        area (see Section 3.5 for more details).

        In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
        internal routers.  Routers RT3, RT4, RT7, RT10 and RT11 are area
        border routers.  Finally, as before, Routers RT5 and RT7 are AS
        boundary routers.

        Figure 7 shows the resulting topological database for the Area
        1.  The figure completely describes that area's intra-area
        routing.  It also shows the complete view of the internet for
        the two internal routers RT1 and RT2.  It is the job of the area
        border routers, RT3 and RT4, to advertise into Area 1 the
        distances to all destinations external to the area.  These are
        indicated in Figure 7 by the dashed stub routes.  Also, RT3 and
        RT4 must advertise into Area 1 the location of the AS boundary
        routers RT5 and RT7.  Finally, external advertisements from RT5
        and RT7 are flooded throughout the entire AS, and in particular
        throughout Area 1.  These advertisements are included in Area
        1's database, and yield routes to Networks N12-N15.

        Routers RT3 and RT4 must also summarize Area 1's topology for
        distribution to the backbone.  Their backbone advertisements are
        shown in Table 4.  These summaries show which networks are
        contained in Area 1 (i.e., Networks N1-N4), and the distance to
        these networks from the routers RT3 and RT4 respectively.

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             .   +                     .
             .   | 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

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                     Network   RT3 adv.   RT4 adv.
                     N1        4          4
                     N2        4          4
                     N3        1          1
                     N4        2          3

              Table 4: Networks advertised to the backbone
                        by Routers RT3 and RT4.

        The topological database for the backbone is shown in Figure 8.
        The set of routers pictured are the backbone routers.  Router
        RT11 is a backbone router because it belongs to two areas.  In
        order to make the backbone connected, a virtual link has been
        configured between Routers R10 and R11.

        Again, Routers RT3, RT4, RT7, RT10 and RT11 are area border
        routers.  As Routers RT3 and RT4 did above, they have condensed
        the routing information of their attached areas for distribution
        via the backbone; these are the dashed stubs that appear in
        Figure 8.  Remember that the third area has been configured to
        condense Networks N9-N11 and Host H1 into a single route.  This
        yields a single dashed line for networks N9-N11 and Host H1 in
        Figure 8.  Routers RT5 and RT7 are AS boundary routers; their
        externally derived information also appears on the graph in
        Figure 8 as stubs.

        The backbone enables the exchange of summary information between
        area border routers.  Every area border router hears the area
        summaries from all other area border routers.  It then forms a
        picture of the distance to all networks outside of its area by
        examining the collected advertisements, and adding in the
        backbone distance to each advertising router.

        Again using Routers RT3 and RT4 as an example, the procedure
        goes as follows: They first calculate the SPF tree for the
        backbone.  This gives the distances to all other area border
        routers.  Also noted are the distances to networks (Ia and Ib)
        and AS boundary routers (RT5 and RT7) that belong to the
        backbone.  This calculation is shown in Table 5.

        Next, by looking at the area summaries from these area border
        routers, RT3 and RT4 can determine the distance to all networks
        outside their area.  These distances are then advertised
        internally to the area by RT3 and RT4.  The advertisements that
        Router RT3 and RT4 will make into Area 1 are shown in Table 6.

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                          |1 |2 |3 |4 |5 |7 |N3|
                       ----- -------------------
                       RT1|  |  |  |  |  |  |0 |
                       RT2|  |  |  |  |  |  |0 |
                       RT3|  |  |  |  |  |  |0 |
                   *   RT4|  |  |  |  |  |  |0 |
                   *   RT5|  |  |14|8 |  |  |  |
                   T   RT7|  |  |20|14|  |  |  |
                   O    N1|3 |  |  |  |  |  |  |
                   *    N2|  |3 |  |  |  |  |  |
                   *    N3|1 |1 |1 |1 |  |  |  |
                        N4|  |  |2 |  |  |  |  |
                     Ia,Ib|  |  |15|22|  |  |  |
                        N6|  |  |16|15|  |  |  |
                        N7|  |  |20|19|  |  |  |
                        N8|  |  |18|18|  |  |  |
                 N9-N11,H1|  |  |19|16|  |  |  |
                       N12|  |  |  |  |8 |2 |  |
                       N13|  |  |  |  |8 |  |  |
                       N14|  |  |  |  |8 |  |  |
                       N15|  |  |  |  |  |9 |  |

                      Figure 7: Area 1's Database.

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

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                            |3 |4 |5 |6 |7 |10|11|
                         RT3|  |  |  |6 |  |  |  |
                         RT4|  |  |8 |  |  |  |  |
                         RT5|  |8 |  |6 |6 |  |  |
                         RT6|8 |  |7 |  |  |5 |  |
                         RT7|  |  |6 |  |  |  |  |
                     *  RT10|  |  |  |7 |  |  |2 |
                     *  RT11|  |  |  |  |  |3 |  |
                     T    N1|4 |4 |  |  |  |  |  |
                     O    N2|4 |4 |  |  |  |  |  |
                     *    N3|1 |1 |  |  |  |  |  |
                     *    N4|2 |3 |  |  |  |  |  |
                          Ia|  |  |  |  |  |5 |  |
                          Ib|  |  |  |7 |  |  |  |
                          N6|  |  |  |  |1 |1 |3 |
                          N7|  |  |  |  |5 |5 |7 |
                          N8|  |  |  |  |4 |3 |2 |
                   N9-N11,H1|  |  |  |  |  |  |1 |
                         N12|  |  |8 |  |2 |  |  |
                         N13|  |  |8 |  |  |  |  |
                         N14|  |  |8 |  |  |  |  |
                         N15|  |  |  |  |9 |  |  |

                     Figure 8: The backbone's database.

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

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                 Area  border   dist  from   dist  from
                 router         RT3          RT4
                 to  RT3        *            21
                 to  RT4        22           *
                 to  RT7        20           14
                 to  RT10       15           22
                 to  RT11       18           25
                 to  Ia         20           27
                 to  Ib         15           22
                 to  RT5        14           8
                 to  RT7        20           14

                 Table 5: Backbone distances calculated
                        by Routers RT3 and RT4.

        Note that Table 6 assumes that an area range has been configured
        for the backbone which groups Ia and Ib into a single

        The information imported into Area 1 by Routers RT3 and RT4
        enables an internal router, such as RT1, to choose an area
        border router intelligently.  Router RT1 would use RT4 for
        traffic to Network N6, RT3 for traffic to Network N10, and would
        load share between the two for traffic to Network N8.

                   Destination   RT3 adv.   RT4 adv.
                   Ia,Ib         15         22
                   N6            16         15
                   N7            20         19
                   N8            18         18
                   N9-N11,H1     19         26
                   RT5           14         8
                   RT7           20         14

              Table 6: Destinations advertised into Area 1
                        by Routers RT3 and RT4.

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        Router RT1 can also determine in this manner the shortest path
        to the AS boundary routers RT5 and RT7.  Then, by looking at RT5
        and RT7's external advertisements, Router RT1 can decide between
        RT5 or RT7 when sending to a destination in another Autonomous
        System (one of the networks N12-N15).

        Note that a failure of the line between Routers RT6 and RT10
        will cause the backbone to become disconnected.  Configuring a
        virtual link between Routers RT7 and RT10 will give the backbone
        more connectivity and more resistance to such failures. Also, a
        virtual link between RT7 and RT10 would allow a much shorter
        path between the third area (containing N9) and the router RT7,
        which is advertising a good route to external network N12.

    3.5.  IP subnetting support

        OSPF attaches an IP address mask to each advertised route.  The
        mask indicates the range of addresses being described by the
        particular route.  For example, a summary advertisement for the
        destination with a mask of 0xffff0000 actually is
        describing a single route to the collection of destinations -  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 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

                  Network address   IP address mask   Subnet size
              0xfffff000        4K
               0xffffff00        256
               0xfffffff8        8

                         Table 7: Some sample subnet sizes.

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        There are many possible ways of dividing up a class A, B, and C
        network into variable sized subnets.  The precise procedure for
        doing so is beyond the scope of this specification.  This
        specification however establishes the following guideline: When
        an IP packet is forwarded, it is always forwarded to the network
        that is the best match for the packet's destination.  Here best
        match is synonymous with the longest or most specific match.
        For example, the default route with destination of and
        mask 0x00000000 is always a match for every IP destination.  Yet
        it is always less specific than any other match.  Subnet masks
        must be assigned so that the best match for any IP destination
        is unambiguous.

        The OSPF area concept is modelled after an IP subnetted network.
        OSPF areas have been loosely defined to be a collection of
        networks.  In actuality, an OSPF area is specified to be a list
        of address ranges (see Section C.2 for more details).  Each
        address range is defined as an [address,mask] pair.  Many
        separate networks may then be contained in a single address
        range, just as a subnetted network is composed of many separate
        subnets.  Area border routers then summarize the area contents
        (for distribution to the backbone) by advertising a single route
        for each address range.  The cost of the route is the minimum
        cost to any of the networks falling in the specified range.

        For example, an IP subnetted network can be configured as a
        single OSPF area.  In that case, the area would be defined as a
        single address range: a class A, B, or C network number along
        with its natural IP mask.  Inside the area, any number of
        variable sized subnets could be defined.  External to the area,
        a single route for the entire subnetted network would be
        distributed, hiding even the fact that the network is subnetted
        at all.  The cost of this route is the minimum of the set of
        costs to the component subnets.

    3.6.  Supporting stub areas

        In some Autonomous Systems, the majority of the topological
        database may consist of AS external advertisements.  An OSPF AS
        external advertisement is usually flooded throughout the entire
        AS.  However, OSPF allows certain areas to be configured as
        "stub areas".  AS external advertisements are not flooded
        into/throughout stub areas; routing to AS external destinations
        in these areas is based on a (per-area) default only.  This
        reduces the topological database size, and therefore the memory
        requirements, for a stub area's internal routers.

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        In order to take advantage of the OSPF stub area support,
        default routing must be used in the stub area.  This is
        accomplished as follows.  One or more of the stub area's area
        border routers must advertise a default route into the stub area
        via summary link advertisements.  These summary defaults are
        flooded throughout the stub area, but no further.  (For this
        reason these defaults pertain only to the particular stub area).
        These summary default routes will match any destination that is
        not explicitly reachable by an intra-area or inter-area path
        (i.e., AS external destinations).

        An area can be configured as stub when there is a single exit
        point from the area, or when the choice of exit point need not
        be made on a per-external-destination basis.  For example, Area
        3 in Figure 6 could be configured as a stub area, because all
        external traffic must travel though its single area border
        router RT11.  If Area 3 were configured as a stub, Router RT11
        would advertise a default route for distribution inside Area 3
        (in a summary link advertisement), instead of flooding the AS
        external advertisements for Networks N12-N15 into/throughout the

        The OSPF protocol ensures that all routers belonging to an area
        agree on whether the area has been configured as a stub.  This
        guarantees that no confusion will arise in the flooding of AS
        external advertisements.

        There are a couple of restrictions on the use of stub areas.
        Virtual links cannot be configured through stub areas.  In
        addition, AS boundary routers cannot be placed internal to stub

    3.7.  Partitions of areas

        OSPF does not actively attempt to repair area partitions.  When
        an area becomes partitioned, each component simply becomes a
        separate area.  The backbone then performs routing between the
        new areas.  Some destinations reachable via intra-area routing
        before the partition will now require inter-area routing.

        In the previous section, an area was described as a list of
        address ranges.  Any particular address range must still be
        completely contained in a single component of the area
        partition.  This has to do with the way the area contents are
        summarized to the backbone.  Also, the backbone itself must not
        partition.  If it does, parts of the Autonomous System will
        become unreachable.  Backbone partitions can be repaired by

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        configuring virtual links (see Section 15).

        Another way to think about area partitions is to look at the
        Autonomous System graph that was introduced in Section 2.  Area
        IDs can be viewed as colors for the graph's edges.[1] Each edge
        of the graph connects to a network, or is itself a point-to-
        point network.  In either case, the edge is colored with the
        network's Area ID.

        A group of edges, all having the same color, and interconnected
        by vertices, represents an area.  If the topology of the
        Autonomous System is intact, the graph will have several regions
        of color, each color being a distinct Area ID.

        When the AS topology changes, one of the areas may become
        partitioned.  The graph of the AS will then have multiple
        regions of the same color (Area ID).  The routing in the
        Autonomous System will continue to function as long as these
        regions of same color are connected by the single backbone

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4.  Functional Summary

    A separate copy of OSPF's basic routing algorithm runs in each area.
    Routers having interfaces to multiple areas run multiple copies of
    the algorithm.  A brief summary of the routing algorithm follows.

    When a router starts, it first initializes the routing protocol data
    structures.  The router then waits for indications from the lower-
    level protocols that its interfaces are functional.

    A router then uses the OSPF's Hello Protocol to acquire neighbors.
    The router sends Hello packets to its neighbors, and in turn
    receives their Hello packets.  On broadcast and point-to-point
    networks, the router dynamically detects its neighboring routers by
    sending its Hello packets to the multicast address AllSPFRouters.
    On non-broadcast networks, some configuration information is
    necessary in order to discover neighbors.  On all multi-access
    networks (broadcast or non-broadcast), the Hello Protocol also
    elects a Designated router for the network.

    The router will attempt to form adjacencies with some of its newly
    acquired neighbors.  Topological databases are synchronized between
    pairs of adjacent routers.  On multi-access networks, the Designated
    Router determines which routers should become adjacent.

    Adjacencies control the distribution of routing protocol packets.
    Routing protocol packets are sent and received only on adjacencies.
    In particular, distribution of topological database updates proceeds
    along adjacencies.

    A router periodically advertises its state, which is also called
    link state.  Link state is also advertised when a router's state
    changes.  A router's adjacencies are reflected in the contents of
    its link state advertisements.  This relationship between
    adjacencies and link state allows the protocol to detect dead
    routers in a timely fashion.

    Link state advertisements are flooded throughout the area.  The
    flooding algorithm is reliable, ensuring that all routers in an area
    have exactly the same topological database.  This database consists
    of the collection of link state advertisements received from each
    router belonging to the area.  From this database each router
    calculates a shortest-path tree, with itself as root.  This
    shortest-path tree in turn yields a routing table for the protocol.

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    4.1.  Inter-area routing

        The previous section described the operation of the protocol
        within a single area.  For intra-area routing, no other routing
        information is pertinent.  In order to be able to route to
        destinations outside of the area, the area border routers inject
        additional routing information into the area.  This additional
        information is a distillation of the rest of the Autonomous
        System's topology.

        This distillation is accomplished as follows: Each area border
        router is by definition connected to the backbone.  Each area
        border router summarizes the topology of its attached areas for
        transmission on the backbone, and hence to all other area border
        routers.  An area border router then has complete topological
        information concerning the backbone, and the area summaries from
        each of the other area border routers.  From this information,
        the router calculates paths to all destinations not contained in
        its attached areas.  The router then advertises these paths into
        its attached areas.  This enables the area's internal routers to
        pick the best exit router when forwarding traffic to
        destinations in other areas.

    4.2.  AS external routes

        Routers that have information regarding other Autonomous Systems
        can flood this information throughout the AS.  This external
        routing information is distributed verbatim to every
        participating router.  There is one exception: external routing
        information is not flooded into "stub" areas (see Section 3.6).

        To utilize external routing information, the path to all routers
        advertising external information must be known throughout the AS
        (excepting the stub areas).  For that reason, the locations of
        these AS boundary routers are summarized by the (non-stub) area
        border routers.

    4.3.  Routing protocol packets

        The OSPF protocol runs directly over IP, using IP protocol 89.
        OSPF does not provide any explicit fragmentation/reassembly
        support.  When fragmentation is necessary, IP
        fragmentation/reassembly is used.  OSPF protocol packets have
        been designed so that large protocol packets can generally be
        split into several smaller protocol packets.  This practice is
        recommended; IP fragmentation should be avoided whenever

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        Routing protocol packets should always be sent with the IP TOS
        field set to 0.  If at all possible, routing protocol packets
        should be given preference over regular IP data traffic, both
        when being sent and received.  As an aid to accomplishing this,
        OSPF protocol packets should have their IP precedence field set
        to the value Internetwork Control (see [RFC 791]).

        All OSPF protocol packets share a common protocol header that is
        described in Appendix A.  The OSPF packet types are listed below
        in Table 8.  Their formats are also described in Appendix A.

             Type   Packet  name           Protocol  function
             1      Hello                  Discover/maintain  neighbors
             2      Database Description   Summarize database contents
             3      Link State Request     Database download
             4      Link State Update      Database update
             5      Link State Ack         Flooding acknowledgment

                            Table 8: OSPF packet types.

        OSPF's Hello protocol uses Hello packets to discover and
        maintain neighbor relationships.  The Database Description and
        Link State Request packets are used in the forming of
        adjacencies.  OSPF's reliable update mechanism is implemented by
        the Link State Update and Link State Acknowledgment packets.

        Each Link State Update packet carries a set of new link state
        advertisements one hop further away from their point of
        origination.  A single Link State Update packet may contain the
        link state advertisements of several routers.  Each
        advertisement is tagged with the ID of the originating router
        and a checksum of its link state contents.  The five different
        types of OSPF link state advertisements are listed below in
        Table 9.

        As mentioned above, OSPF routing packets (with the exception of
        Hellos) are sent only over adjacencies.  Note that this means
        that all OSPF protocol packets travel a single IP hop, except
        those that are sent over virtual adjacencies.  The IP source
        address of an OSPF protocol packet is one end of a router
        adjacency, and the IP destination address is either the other

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       LS     Advertisement      Advertisement description
       type   name
       1      Router links       Originated by all routers.
              advertisements     This advertisement describes
                                 the collected states of the
                                 router's interfaces to an
                                 area. Flooded throughout a
                                 single area only.
       2      Network links      Originated for multi-access
              advertisements     networks by the Designated
                                 Router. This advertisement
                                 contains the list of routers
                                 connected to the network.
                                 Flooded throughout a single
                                 area only.
       3,4    Summary link       Originated by area border
              advertisements     routers, and flooded through-
                                 out the advertisement's
                                 associated area. Each summary
                                 link advertisement describes
                                 a route to a destination out-
                                 side the area, yet still inside
                                 the AS (i.e., an inter-area
                                 route). Type 3 advertisements
                                 describe routes to networks.
                                 Type 4 advertisements describe
                                 routes to AS boundary routers.
       5      AS external link   Originated by AS boundary
              advertisements     routers, and flooded through-
                                 out the AS. Each AS external
                                 link advertisement describes
                                 a route to a destination in
                                 another Autonomous System.
                                 Default routes for the AS can
                                 also be described by AS
                                 external link advertisements.

                Table 9: OSPF link state advertisements.

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        end of the adjacency or an IP multicast address.

    4.4.  Basic implementation requirements

        An implementation of OSPF requires the following pieces of
        system support:

            Two different kind of timers are required.  The first kind,
            called single shot timers, fire once and cause a protocol
            event to be processed.  The second kind, called interval
            timers, fire at continuous intervals.  These are used for
            the sending of packets at regular intervals.  A good example
            of this is the regular broadcast of Hello packets (on
            broadcast networks).  The granularity of both kinds of
            timers is one second.

            Interval timers should be implemented to avoid drift.  In
            some router implementations, packet processing can affect
            timer execution.  When multiple routers are attached to a
            single network, all doing broadcasts, this can lead to the
            synchronization of routing packets (which should be
            avoided).  If timers cannot be implemented to avoid drift,
            small random amounts should be added to/subtracted from the
            timer interval at each firing.

        IP multicast
            Certain OSPF packets take the form of IP multicast
            datagrams.  Support for receiving and sending IP multicast
            datagrams, along with the appropriate lower-level protocol
            support, is required.  The IP multicast datagrams used by
            OSPF never travel more than one hop. For this reason, the
            ability to forward IP multicast datagrams is not required.
            For information on IP multicast, see [RFC 1112].

        Variable-length subnet support
            The router's IP protocol support must include the ability to
            divide a single IP class A, B, or C network number into many
            subnets of various sizes.  This is commonly called
            variable-length subnetting; see Section 3.5 for details.

        IP supernetting support
            The router's IP protocol support must include the ability to
            aggregate contiguous collections of IP class A, B, and C
            networks into larger quantities called supernets.
            Supernetting has been proposed as one way to improve the

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            scaling of IP routing in the worldwide Internet. For more
            information on IP supernetting, see [RFC 1519].

        Lower-level protocol support
            The lower level protocols referred to here are the network
            access protocols, such as the Ethernet data link layer.
            Indications must be passed from these protocols to OSPF as
            the network interface goes up and down.  For example, on an
            ethernet it would be valuable to know when the ethernet
            transceiver cable becomes unplugged.

        Non-broadcast lower-level protocol support
            Remember that non-broadcast networks are multi-access
            networks such as a X.25 PDN.  On these networks, the Hello
            Protocol can be aided by providing an indication to OSPF
            when an attempt is made to send a packet to a dead or non-
            existent router.  For example, on an X.25 PDN a dead
            neighboring router may be indicated by the reception of a
            X.25 clear with an appropriate cause and diagnostic, and
            this information would be passed to OSPF.

        List manipulation primitives
            Much of the OSPF functionality is described in terms of its
            operation on lists of link state advertisements.  For
            example, the collection of advertisements that will be
            retransmitted to an adjacent router until acknowledged are
            described as a list.  Any particular advertisement may be on
            many such lists.  An OSPF implementation needs to be able to
            manipulate these lists, adding and deleting constituent
            advertisements as necessary.

        Tasking support
            Certain procedures described in this specification invoke
            other procedures.  At times, these other procedures should
            be executed in-line, that is, before the current procedure
            is finished.  This is indicated in the text by instructions
            to execute a procedure.  At other times, the other
            procedures are to be executed only when the current
            procedure has finished.  This is indicated by instructions
            to schedule a task.

    4.5.  Optional OSPF capabilities

        The OSPF protocol defines several optional capabilities.  A
        router indicates the optional capabilities that it supports in
        its OSPF Hello packets, Database Description packets and in its
        link state advertisements.  This enables routers supporting a

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        mix of optional capabilities to coexist in a single Autonomous

        Some capabilities must be supported by all routers attached to a
        specific area.  In this case, a router will not accept a
        neighbor's Hello Packet unless there is a match in reported
        capabilities (i.e., a capability mismatch prevents a neighbor
        relationship from forming).  An example of this is the
        ExternalRoutingCapability (see below).

        Other capabilities can be negotiated during the Database
        Exchange process.  This is accomplished by specifying the
        optional capabilities in Database Description packets.  A
        capability mismatch with a neighbor in this case will result in
        only a subset of link state advertisements being exchanged
        between the two neighbors.

        The routing table build process can also be affected by the
        presence/absence of optional capabilities.  For example, since
        the optional capabilities are reported in link state
        advertisements, routers incapable of certain functions can be
        avoided when building the shortest path tree.  An example of
        this is the TOS routing capability (see below).

        The current OSPF optional capabilities are listed below.  See
        Section A.2 for more information.

            Entire OSPF areas can be configured as "stubs" (see Section
            3.6).  AS external advertisements will not be flooded into
            stub areas.  This capability is represented by the E-bit in
            the OSPF options field (see Section A.2).  In order to
            ensure consistent configuration of stub areas, all routers
            interfacing to such an area must have the E-bit clear in
            their Hello packets (see Sections 9.5 and 10.5).

        TOS capability
            All OSPF implementations must be able to calculate separate
            routes based on IP Type of Service.  However, to save
            routing table space and processing resources, an OSPF router
            can be configured to ignore TOS when forwarding packets.  In
            this case, the router calculates routes for TOS 0 only.
            This capability is represented by the T-bit in the OSPF
            options field (see Section A.2).  TOS-capable routers will
            attempt to avoid non-TOS-capable routers when calculating
            non-zero TOS paths.

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5.  Protocol Data Structures

    The OSPF protocol is described in this specification in terms of its
    operation on various protocol data structures.  The following list
    comprises the top-level OSPF data structures.  Any initialization
    that needs to be done is noted.  OSPF areas, interfaces and
    neighbors also have associated data structures that are described
    later in this specification.

    Router ID
        A 32-bit number that uniquely identifies this router in the AS.
        One possible implementation strategy would be to use the
        smallest IP interface address belonging to the router. If a
        router's OSPF Router ID is changed, the router's OSPF software
        should be restarted before the new Router ID takes effect.
        Before restarting in order to change its Router ID, the router
        should flush its self-originated link state advertisements from
        the routing domain (see Section 14.1), or they will persist for
        up to MaxAge minutes.

    Area structures
        Each one of the areas to which the router is connected has its
        own data structure.  This data structure describes the working
        of the basic algorithm.  Remember that each area runs a separate
        copy of the basic algorithm.

    Backbone (area) structure
        The basic algorithm operates on the backbone as if it were an
        area.  For this reason the backbone is represented as an area

    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

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        with another routing protocol (such as EGP), or through
        configuration information, or through a combination of the two
        (e.g., dynamic external information to be advertised by OSPF
        with configured metric). Any router having these external routes
        is called an AS boundary router.  These routes are advertised by
        the router into the OSPF routing domain via AS external link

    List of AS external link advertisements
        Part of the topological database.  These have originated from
        the AS boundary routers.  They comprise routes to destinations
        external to the Autonomous System.  Note that, if the router is
        itself an AS boundary router, some of these AS external link
        advertisements have been self-originated.

    The routing table
        Derived from the topological database.  Each destination that
        the router can forward to is represented by a cost and a set of
        paths.  A path is described by its type and next hop.  For more
        information, see Section 11.

    TOS capability
        This item indicates whether the router will calculate separate
        routes based on TOS.  This is a configurable parameter.  For
        more information, see Sections 4.5 and 16.9.

    Figure 9 shows the collection of data structures present in a
    typical router.  The router pictured is RT10, from the map in Figure
    6.  Note that Router RT10 has a virtual link configured to Router
    RT11, with Area 2 as the link's Transit area.  This is indicated by
    the dashed line in Figure 9.  When the virtual link becomes active,
    through the building of the shortest path tree for Area 2, it
    becomes an interface to the backbone (see the two backbone
    interfaces depicted in Figure 9).

6.  The Area Data Structure

    The area data structure contains all the information used to run the
    basic routing algorithm. Each area maintains its own topological
    database. A network belongs to a single area, and a router interface
    connects to a single area. Each router adjacency also belongs to a
    single area.

    The OSPF backbone has all the properties of an area.  For that
    reason it is also represented by an area data structure.  Note that
    some items in the structure apply differently to the backbone than
    to non-backbone areas.

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                              +----+       \+-------------+
                             /      \       |Routing Table|
                            /        \      +-------------+
                           /          \
              +------+    /            \    +--------+
              |Area 2|---+              +---|Backbone|
              +------+***********+          +--------+
             /        \           *        /          \
            /          \           *      /            \
       +---------+  +---------+    +------------+       +------------+
       |Interface|  |Interface|    |Virtual Link|       |Interface Ib|
       |  to N6  |  |  to N8  |    |   to RT11  |       +------------+
       +---------+  +---------+    +------------+             |
           /  \           |               |                   |
          /    \          |               |                   |
   +--------+ +--------+  |        +-------------+      +------------+
   |Neighbor| |Neighbor|  |        |Neighbor RT11|      |Neighbor RT6|
   |  RT8   | |  RT7   |  |        +-------------+      +------------+
   +--------+ +--------+  |
                     |Neighbor RT11|

                Figure 9: Router RT10's Data structures

    The area topological (or link state) database consists of the
    collection of router links, network links and summary link
    advertisements that have originated from the area's routers.  This
    information is flooded throughout a single area only.  The list of
    AS external link advertisements (see Section 5) is also considered
    to be part of each area's topological database.

    Area ID
        A 32-bit number identifying the area. is reserved for
        the Area ID of the backbone.  If assigning subnetted networks as
        separate areas, the IP network number could be used as the Area

    List of component address ranges
        The address ranges that define the area.  Each address range is

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        specified by an [address,mask] pair and a status indication of
        either Advertise or DoNotAdvertise (see Section 12.4.3). Each
        network is then assigned to an area depending on the address
        range that it falls into (specified address ranges are not
        allowed to overlap).  As an example, if an IP subnetted network
        is to be its own separate OSPF area, the area is defined to
        consist of a single address range - an IP network number with
        its natural (class A, B or C) mask.

    Associated router interfaces
        This router's interfaces connecting to the area.  A router
        interface belongs to one and only one area (or the backbone).
        For the backbone structure this list includes all the virtual
        links.  A virtual link is identified by the Router ID of its
        other endpoint; its cost is the cost of the shortest intra-area
        path through the Transit area that exists between the two

    List of router links advertisements
        A router links advertisement is generated by each router in the
        area.  It describes the state of the router's interfaces to the

    List of network links advertisements
        One network links advertisement is generated for each transit
        multi-access network in the area.  A network links advertisement
        describes the set of routers currently connected to the network.

    List of summary link advertisements
        Summary link advertisements originate from the area's area
        border routers.  They describe routes to destinations internal
        to the Autonomous System, yet external to the area.

    Shortest-path tree
        The shortest-path tree for the area, with this router itself as
        root.  Derived from the collected router links and network links
        advertisements by the Dijkstra algorithm (see Section 16.1).

        The type of authentication used for this area.  Authentication
        types are defined in Appendix D.  All OSPF packet exchanges are
        authenticated.  Different authentication schemes may be used in
        different areas.

        Set to TRUE if and only if there are one or more active virtual
        links using the area as a Transit area. Equivalently, this
        parameter indicates whether the area can carry data traffic that

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        neither originates nor terminates in the area itself. This
        parameter is calculated when the area's shortest-path tree is
        built (see Section 16.1, and is used as an input to a subsequent
        step of the routing table build process (see Section 16.3).

        Whether AS external advertisements will be flooded
        into/throughout the area.  This is a configurable parameter.  If
        AS external advertisements are excluded from the area, the area
        is called a "stub".  Internal to stub areas, routing to AS
        external destinations will be based solely on a default summary
        route.  The backbone cannot be configured as a stub area.  Also,
        virtual links cannot be configured through stub areas.  For more
        information, see Section 3.6.

        If the area has been configured as a stub area, and the router
        itself is an area border router, then the StubDefaultCost
        indicates the cost of the default summary link that the router
        should advertise into the area.  There can be a separate cost
        configured for each IP TOS.  See Section 12.4.3 for more

    Unless otherwise specified, the remaining sections of this document
    refer to the operation of the protocol in a single area.

7.  Bringing Up Adjacencies

    OSPF creates adjacencies between neighboring routers for the purpose
    of exchanging routing information.  Not every two neighboring
    routers will become adjacent.  This section covers the generalities
    involved in creating adjacencies.  For further details consult
    Section 10.

    7.1.  The Hello Protocol

        The Hello Protocol is responsible for establishing and
        maintaining neighbor relationships.  It also ensures that
        communication between neighbors is bidirectional.  Hello packets
        are sent periodically out all router interfaces.  Bidirectional
        communication is indicated when the router sees itself listed in
        the neighbor's Hello Packet.

        On multi-access networks, the Hello Protocol elects a Designated
        Router for the network.  Among other things, the Designated

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        Router controls what adjacencies will be formed over the network
        (see below).

        The Hello Protocol works differently on broadcast networks, as
        compared to non-broadcast networks.  On broadcast networks, each
        router advertises itself by periodically multicasting Hello
        Packets.  This allows neighbors to be discovered dynamically.
        These Hello Packets contain the router's view of the Designated
        Router's identity, and the list of routers whose Hello Packets
        have been seen recently.

        On non-broadcast networks some configuration information is
        necessary for the operation of the Hello Protocol.  Each router
        that may potentially become Designated Router has a list of all
        other routers attached to the network.  A router, having
        Designated Router potential, sends Hello Packets to all other
        potential Designated Routers when its interface to the non-
        broadcast network first becomes operational.  This is an attempt
        to find the Designated Router for the network.  If the router
        itself is elected Designated Router, it begins sending Hello
        Packets to all other routers attached to the network.

        After a neighbor has been discovered, bidirectional
        communication ensured, and (if on a multi-access network) a
        Designated Router elected, a decision is made regarding whether
        or not an adjacency should be formed with the neighbor (see
        Section 10.4).  An attempt is always made to establish
        adjacencies over point-to-point networks and virtual links.  The
        first step in bringing up an adjacency is to synchronize the
        neighbors' topological databases.  This is covered in the next

    7.2.  The Synchronization of Databases

        In a link-state routing algorithm, it is very important for all
        routers' topological databases to stay synchronized.  OSPF
        simplifies this by requiring only adjacent routers to remain
        synchronized.  The synchronization process begins as soon as the
        routers attempt to bring up the adjacency.  Each router
        describes its database by sending a sequence of Database
        Description packets to its neighbor.  Each Database Description
        Packet describes a set of link state advertisements belonging to
        the router's database.  When the neighbor sees a link state
        advertisement that is more recent than its own database copy, it
        makes a note that this newer advertisement should be requested.

        This sending and receiving of Database Description packets is

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        called the "Database Exchange Process".  During this process,
        the two routers form a master/slave relationship.  Each Database
        Description Packet has a sequence number.  Database Description
        Packets sent by the master (polls) are acknowledged by the slave
        through echoing of the sequence number.  Both polls and their
        responses contain summaries of link state data.  The master is
        the only one allowed to retransmit Database Description Packets.
        It does so only at fixed intervals, the length of which is the
        configured constant RxmtInterval.

        Each Database Description contains an indication that there are
        more packets to follow --- the M-bit.  The Database Exchange
        Process is over when a router has received and sent Database
        Description Packets with the M-bit off.

        During and after the Database Exchange Process, each router has
        a list of those link state advertisements for which the neighbor
        has more up-to-date instances.  These advertisements are
        requested in Link State Request Packets.  Link State Request
        packets that are not satisfied are retransmitted at fixed
        intervals of time RxmtInterval.  When the Database Description
        Process has completed and all Link State Requests have been
        satisfied, the databases are deemed synchronized and the routers
        are marked fully adjacent.  At this time the adjacency is fully
        functional and is advertised in the two routers' link state

        The adjacency is used by the flooding procedure as soon as the
        Database Exchange Process begins.  This simplifies database
        synchronization, and guarantees that it finishes in a
        predictable period of time.

    7.3.  The Designated Router

        Every multi-access network has a Designated Router.  The
        Designated Router performs two main functions for the routing

        o   The Designated Router originates a network links
            advertisement on behalf of the network.  This advertisement
            lists the set of routers (including the Designated Router
            itself) currently attached to the network.  The Link State
            ID for this advertisement (see Section 12.1.4) is the IP
            interface address of the Designated Router.  The IP network
            number can then be obtained by using the subnet/network

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        o   The Designated Router becomes adjacent to all other routers
            on the network.  Since the link state databases are
            synchronized across adjacencies (through adjacency bring-up
            and then the flooding procedure), the Designated Router
            plays a central part in the synchronization process.

        The Designated Router is elected by the Hello Protocol.  A
        router's Hello Packet contains its Router Priority, which is
        configurable on a per-interface basis.  In general, when a
        router's interface to a network first becomes functional, it
        checks to see whether there is currently a Designated Router for
        the network.  If there is, it accepts that Designated Router,
        regardless of its Router Priority.  (This makes it harder to
        predict the identity of the Designated Router, but ensures that
        the Designated Router changes less often.  See below.)
        Otherwise, the router itself becomes Designated Router if it has
        the highest Router Priority on the network.  A more detailed
        (and more accurate) description of Designated Router election is
        presented in Section 9.4.

        The Designated Router is the endpoint of many adjacencies.  In
        order to optimize the flooding procedure on broadcast networks,
        the Designated Router multicasts its Link State Update Packets
        to the address AllSPFRouters, rather than sending separate
        packets over each adjacency.

        Section 2 of this document discusses the directed graph
        representation of an area.  Router nodes are labelled with their
        Router ID.  Multi-access network nodes are actually labelled
        with the IP address of their Designated Router.  It follows that
        when the Designated Router changes, it appears as if the network
        node on the graph is replaced by an entirely new node.  This
        will cause the network and all its attached routers to originate
        new link state advertisements.  Until the topological databases
        again converge, some temporary loss of connectivity may result.
        This may result in ICMP unreachable messages being sent in
        response to data traffic.  For that reason, the Designated
        Router should change only infrequently.  Router Priorities
        should be configured so that the most dependable router on a
        network eventually becomes Designated Router.

    7.4.  The Backup Designated Router

        In order to make the transition to a new Designated Router
        smoother, there is a Backup Designated Router for each multi-
        access network.  The Backup Designated Router is also adjacent

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        to all routers on the network, and becomes Designated Router
        when the previous Designated Router fails.  If there were no
        Backup Designated Router, when a new Designated Router became
        necessary, new adjacencies would have to be formed between the
        new Designated Router and all other routers attached to the
        network.  Part of the adjacency forming process is the
        synchronizing of topological databases, which can potentially
        take quite a long time.  During this time, the network would not
        be available for transit data traffic.  The Backup Designated
        obviates the need to form these adjacencies, since they already
        exist.  This means the period of disruption in transit traffic
        lasts only as long as it takes to flood the new link state
        advertisements (which announce the new Designated Router).

        The Backup Designated Router does not generate a network links
        advertisement for the network.  (If it did, the transition to a
        new Designated Router would be even faster.  However, this is a
        tradeoff between database size and speed of convergence when the
        Designated Router disappears.)

        The Backup Designated Router is also elected by the Hello
        Protocol.  Each Hello Packet has a field that specifies the
        Backup Designated Router for the network.

        In some steps of the flooding procedure, the Backup Designated
        Router plays a passive role, letting the Designated Router do
        more of the work.  This cuts down on the amount of local routing
        traffic.  See Section 13.3 for more information.

    7.5.  The graph of adjacencies

        An adjacency is bound to the network that the two routers have
        in common.  If two routers have multiple networks in common,
        they may have multiple adjacencies between them.

        One can picture the collection of adjacencies on a network as
        forming an undirected graph.  The vertices consist of routers,
        with an edge joining two routers if they are adjacent.  The
        graph of adjacencies describes the flow of routing protocol
        packets, and in particular Link State Update Packets, through
        the Autonomous System.

        Two graphs are possible, depending on whether the common network
        is multi-access.  On physical point-to-point networks (and
        virtual links), the two routers joined by the network will be
        adjacent after their databases have been synchronized.  On
        multi-access networks, both the Designated Router and the Backup

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        Designated Router are adjacent to all other routers attached to
        the network, and these account for all adjacencies.

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

(page 50 continued on part 3)

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