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RFC 2328

 
 
 

OSPF Version 2

Part 7 of 8, p. 168 to 202
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    16.2.  Calculating the inter-area routes

        The inter-area routes are calculated by examining summary-LSAs.
        If the router has active attachments to multiple areas, only
        backbone summary-LSAs are examined.  Routers attached to a
        single area examine that area's summary-LSAs.  In either case,
        the summary-LSAs examined below are all part of a single area's
        link state database (call it Area A).

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        Summary-LSAs are originated by the area border routers.  Each
        summary-LSA in Area A is considered in turn.  Remember that the
        destination described by a summary-LSA is either a network (Type
        3 summary-LSAs) or an AS boundary router (Type 4 summary-LSAs).
        For each summary-LSA:


        (1) If the cost specified by the LSA is LSInfinity, or if the
            LSA's LS age is equal to MaxAge, then examine the the next
            LSA.

        (2) If the LSA was originated by the calculating router itself,
            examine the next LSA.

        (3) If it is a Type 3 summary-LSA, and the collection of
            destinations described by the summary-LSA equals one of the
            router's configured area address ranges (see Section 3.5),
            and the particular area address range is active, then the
            summary-LSA should be ignored.  "Active" means that there
            are one or more reachable (by intra-area paths) networks
            contained in the area range.

        (4) Else, call the destination described by the LSA N (for Type
            3 summary-LSAs, N's address is obtained by masking the LSA's
            Link State ID with the network/subnet mask contained in the
            body of the LSA), and the area border originating the LSA
            BR.  Look up the routing table entry for BR having Area A as
            its associated area.  If no such entry exists for router BR
            (i.e., BR is unreachable in Area A), do nothing with this
            LSA and consider the next in the list.  Else, this LSA
            describes an inter-area path to destination N, whose cost is
            the distance to BR plus the cost specified in the LSA. Call
            the cost of this inter-area path IAC.

        (5) Next, look up the routing table entry for the destination N.
            (If N is an AS boundary router, look up the "router" routing
            table entry associated with Area A).  If no entry exists for
            N or if the entry's path type is "type 1 external" or "type
            2 external", then install the inter-area path to N, with
            associated area Area A, cost IAC, next hop equal to the list
            of next hops to router BR, and Advertising router equal to
            BR.

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        (6) Else, if the paths present in the table are intra-area
            paths, do nothing with the LSA (intra-area paths are always
            preferred).

        (7) Else, the paths present in the routing table are also
            inter-area paths.  Install the new path through BR if it is
            cheaper, overriding the paths in the routing table.
            Otherwise, if the new path is the same cost, add it to the
            list of paths that appear in the routing table entry.

    16.3.  Examining transit areas' summary-LSAs

        This step is only performed by area border routers attached to
        one or more non-backbone areas that are capable of carrying
        transit traffic (i.e., "transit areas", or those areas whose
        TransitCapability parameter has been set to TRUE in Step 2 of
        the Dijkstra algorithm (see Section 16.1).

        The purpose of the calculation below is to examine the transit
        areas to see whether they provide any better (shorter) paths
        than the paths previously calculated in Sections 16.1 and 16.2.
        Any paths found that are better than or equal to previously
        discovered paths are installed in the routing table.

        The calculation also determines the actual next hop(s) for those
        destinations whose next hop was calculated as a virtual link in
        Sections 16.1 and 16.2.  After completion of the calculation
        below, any paths calculated in Sections 16.1 and 16.2 that still
        have unresolved virtual next hops should be discarded.

        The calculation proceeds as follows. All the transit areas'
        summary-LSAs are examined in turn.  Each such summary-LSA
        describes a route through a transit area Area A to a Network N
        (N's address is obtained by masking the LSA's Link State ID with
        the network/subnet mask contained in the body of the LSA) or in
        the case of a Type 4 summary-LSA, to an AS boundary router N.
        Suppose also that the summary-LSA was originated by an area
        border router BR.

        (1) If the cost advertised by the summary-LSA is LSInfinity, or
            if the LSA's LS age is equal to MaxAge, then examine the
            next LSA.

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        (2) If the summary-LSA was originated by the calculating router
            itself, examine the next LSA.

        (3) Look up the routing table entry for N. (If N is an AS
            boundary router, look up the "router" routing table entry
            associated with the backbone area). If it does not exist, or
            if the route type is other than intra-area or inter-area, or
            if the area associated with the routing table entry is not
            the backbone area, then examine the next LSA. In other
            words, this calculation only updates backbone intra-area
            routes found in Section 16.1 and inter-area routes found in
            Section 16.2.

        (4) Look up the routing table entry for the advertising router
            BR associated with the Area A. If it is unreachable, examine
            the next LSA. Otherwise, the cost to destination N is the
            sum of the cost in BR's Area A routing table entry and the
            cost advertised in the LSA. Call this cost IAC.

        (5) If this cost is less than the cost occurring in N's routing
            table entry, overwrite N's list of next hops with those used
            for BR, and set N's routing table cost to IAC. Else, if IAC
            is the same as N's current cost, add BR's list of next hops
            to N's list of next hops. In any case, the area associated
            with N's routing table entry must remain the backbone area,
            and the path type (either intra-area or inter-area) must
            also remain the same.

        It is important to note that the above calculation never makes
        unreachable destinations reachable, but instead just potentially
        finds better paths to already reachable destinations.  The
        calculation installs any better cost found into the routing
        table entry, from which it may be readvertised in summary-LSAs
        to other areas.

        As an example of the calculation, consider the Autonomous System
        pictured in Figure 17.  There is a single non-backbone area
        (Area 1) that physically divides the backbone into two separate
        pieces. To maintain connectivity of the backbone, a virtual link
        has been configured between routers RT1 and RT4. On the right
        side of the figure, Network N1 belongs to the backbone. The
        dotted lines indicate that there is a much shorter intra-area

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                      ........................
                      . Area 1 (transit)     .            +
                      .                      .            |
                      .      +---+1        1+---+100      |
                      .      |RT2|----------|RT4|=========|
                      .    1/+---+********* +---+         |
                      .    /*******          .            |
                      .  1/*Virtual          .            |
                   1+---+/*  Link            .         Net|work
             =======|RT1|*                   .            | N1
                    +---+\                   .            |
                      .   \                  .            |
                      .    \                 .            |
                      .    1\+---+1        1+---+20       |
                      .      |RT3|----------|RT5|=========|
                      .      +---+          +---+         |
                      .                      .            |
                      ........................            +

                    Figure 17: Routing through transit areas

        backbone path between router RT5 and Network N1 (cost 20) than
        there is between Router RT4 and Network N1 (cost 100). Both
        Router RT4 and Router RT5 will inject summary-LSAs for Network
        N1 into Area 1.

        After the shortest-path tree has been calculated for the
        backbone in Section 16.1, Router RT1 (left end of the virtual
        link) will have calculated a path through Router RT4 for all
        data traffic destined for Network N1. However, since Router RT5
        is so much closer to Network N1, all routers internal to Area 1
        (e.g., Routers RT2 and RT3) will forward their Network N1
        traffic towards Router RT5, instead of RT4. And indeed, after
        examining Area 1's summary-LSAs by the above calculation, Router
        RT1 will also forward Network N1 traffic towards RT5. Note that
        in this example the virtual link enables transit data traffic to
        be forwarded through Area 1, but the actual path the transit
        data traffic takes does not follow the virtual link.  In other
        words, virtual links allow transit traffic to be forwarded
        through an area, but do not dictate the precise path that the
        traffic will take.

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    16.4.  Calculating AS external routes

        AS external routes are calculated by examining AS-external-LSAs.
        Each of the AS-external-LSAs is considered in turn.  Most AS-
        external-LSAs describe routes to specific IP destinations.  An
        AS-external-LSA can also describe a default route for the
        Autonomous System (Destination ID = DefaultDestination,
        network/subnet mask = 0x00000000).  For each AS-external-LSA:


        (1) If the cost specified by the LSA is LSInfinity, or if the
            LSA's LS age is equal to MaxAge, then examine the next LSA.

        (2) If the LSA was originated by the calculating router itself,
            examine the next LSA.

        (3) Call the destination described by the LSA N.  N's address is
            obtained by masking the LSA's Link State ID with the
            network/subnet mask contained in the body of the LSA.  Look
            up the routing table entries (potentially one per attached
            area) for the AS boundary router (ASBR) that originated the
            LSA. If no entries exist for router ASBR (i.e., ASBR is
            unreachable), do nothing with this LSA and consider the next
            in the list.

            Else, this LSA describes an AS external path to destination
            N.  Examine the forwarding address specified in the AS-
            external-LSA.  This indicates the IP address to which
            packets for the destination should be forwarded.

            If the forwarding address is set to 0.0.0.0, packets should
            be sent to the ASBR itself. Among the multiple routing table
            entries for the ASBR, select the preferred entry as follows.
            If RFC1583Compatibility is set to "disabled", prune the set
            of routing table entries for the ASBR as described in
            Section 16.4.1. In any case, among the remaining routing
            table entries, select the routing table entry with the least
            cost; when there are multiple least cost routing table
            entries the entry whose associated area has the largest OSPF
            Area ID (when considered as an unsigned 32-bit integer) is
            chosen.

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            If the forwarding address is non-zero, look up the
            forwarding address in the routing table.[24] The matching
            routing table entry must specify an intra-area or inter-area
            path; if no such path exists, do nothing with the LSA and
            consider the next in the list.

        (4) Let X be the cost specified by the preferred routing table
            entry for the ASBR/forwarding address, and Y the cost
            specified in the LSA.  X is in terms of the link state
            metric, and Y is a type 1 or 2 external metric.

        (5) Look up the routing table entry for the destination N.  If
            no entry exists for N, install the AS external path to N,
            with next hop equal to the list of next hops to the
            forwarding address, and advertising router equal to ASBR.
            If the external metric type is 1, then the path-type is set
            to type 1 external and the cost is equal to X+Y.  If the
            external metric type is 2, the path-type is set to type 2
            external, the link state component of the route's cost is X,
            and the type 2 cost is Y.

        (6) Compare the AS external path described by the LSA with the
            existing paths in N's routing table entry, as follows. If
            the new path is preferred, it replaces the present paths in
            N's routing table entry.  If the new path is of equal
            preference, it is added to N's routing table entry's list of
            paths.

            (a) Intra-area and inter-area paths are always preferred
                over AS external paths.

            (b) Type 1 external paths are always preferred over type 2
                external paths. When all paths are type 2 external
                paths, the paths with the smallest advertised type 2
                metric are always preferred.

            (c) If the new AS external path is still indistinguishable
                from the current paths in the N's routing table entry,
                and RFC1583Compatibility is set to "disabled", select
                the preferred paths based on the intra-AS paths to the
                ASBR/forwarding addresses, as specified in Section
                16.4.1.

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            (d) If the new AS external path is still indistinguishable
                from the current paths in the N's routing table entry,
                select the preferred path based on a least cost
                comparison.  Type 1 external paths are compared by
                looking at the sum of the distance to the forwarding
                address and the advertised type 1 metric (X+Y).  Type 2
                external paths advertising equal type 2 metrics are
                compared by looking at the distance to the forwarding
                addresses.

        16.4.1.  External path preferences

            When multiple intra-AS paths are available to
            ASBRs/forwarding addresses, the following rules indicate
            which paths are preferred. These rules apply when the same
            ASBR is reachable through multiple areas, or when trying to
            decide which of several AS-external-LSAs should be
            preferred. In the former case the paths all terminate at the
            same ASBR, while in the latter the paths terminate at
            separate ASBRs/forwarding addresses. In either case, each
            path is represented by a separate routing table entry as
            defined in Section 11.

            This section only applies when RFC1583Compatibility is set
            to "disabled".

            The path preference rules, stated from highest to lowest
            preference, are as follows. Note that as a result of these
            rules, there may still be multiple paths of the highest
            preference. In this case, the path to use must be determined
            based on cost, as described in Section 16.4.

            o   Intra-area paths using non-backbone areas are always the
                most preferred.

            o   The other paths, intra-area backbone paths and inter-
                area paths, are of equal preference.

    16.5.  Incremental updates -- summary-LSAs

        When a new summary-LSA is received, it is not necessary to
        recalculate the entire routing table.  Call the destination

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        described by the summary-LSA N (N's address is obtained by
        masking the LSA's Link State ID with the network/subnet mask
        contained in the body of the LSA), and let Area A be the area to
        which the LSA belongs. There are then two separate cases:

        Case 1: Area A is the backbone and/or the router is not an area
            border router.
            In this case, the following calculations must be performed.
            First, if there is presently an inter-area route to the
            destination N, N's routing table entry is invalidated,
            saving the entry's values for later comparisons. Then the
            calculation in Section 16.2 is run again for the single
            destination N. In this calculation, all of Area A's
            summary-LSAs that describe a route to N are examined.  In
            addition, if the router is an area border router attached to
            one or more transit areas, the calculation in Section 16.3
            must be run again for the single destination.  If the
            results of these calculations have changed the cost/path to
            an AS boundary router (as would be the case for a Type 4
            summary-LSA) or to any forwarding addresses, all AS-
            external-LSAs will have to be reexamined by rerunning the
            calculation in Section 16.4.  Otherwise, if N is now newly
            unreachable, the calculation in Section 16.4 must be rerun
            for the single destination N, in case an alternate external
            route to N exists.

        Case 2: Area A is a transit area and the router is an area
            border router.
            In this case, the following calculations must be performed.
            First, if N's routing table entry presently contains one or
            more inter-area paths that utilize the transit area Area A,
            these paths should be removed. If this removes all paths
            from the routing table entry, the entry should be
            invalidated.  The entry's old values should be saved for
            later comparisons. Next the calculation in Section 16.3 must
            be run again for the single destination N. If the results of
            this calculation have caused the cost to N to increase, the
            complete routing table calculation must be rerun starting
            with the Dijkstra algorithm specified in Section 16.1.
            Otherwise, if the cost/path to an AS boundary router (as
            would be the case for a Type 4 summary-LSA) or to any
            forwarding addresses has changed, all AS-external-LSAs will

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            have to be reexamined by rerunning the calculation in
            Section 16.4.  Otherwise, if N is now newly unreachable, the
            calculation in Section 16.4 must be rerun for the single
            destination N, in case an alternate external route to N
            exists.

    16.6.  Incremental updates -- AS-external-LSAs

        When a new AS-external-LSA is received, it is not necessary to
        recalculate the entire routing table.  Call the destination
        described by the AS-external-LSA N.  N's address is obtained by
        masking the LSA's Link State ID with the network/subnet mask
        contained in the body of the LSA. If there is already an intra-
        area or inter-area route to the destination, no recalculation is
        necessary (internal routes take precedence).

        Otherwise, the procedure in Section 16.4 will have to be
        performed, but only for those AS-external-LSAs whose destination
        is N.  Before this procedure is performed, the present routing
        table entry for N should be invalidated.

    16.7.  Events generated as a result of routing table changes

        Changes to routing table entries sometimes cause the OSPF area
        border routers to take additional actions.  These routers need
        to act on the following routing table changes:

        o   The cost or path type of a routing table entry has changed.
            If the destination described by this entry is a Network or
            AS boundary router, and this is not simply a change of AS
            external routes, new summary-LSAs may have to be generated
            (potentially one for each attached area, including the
            backbone).  See Section 12.4.3 for more information.  If a
            previously advertised entry has been deleted, or is no
            longer advertisable to a particular area, the LSA must be
            flushed from the routing domain by setting its LS age to
            MaxAge and reflooding (see Section 14.1).

        o   A routing table entry associated with a configured virtual
            link has changed.  The destination of such a routing table
            entry is an area border router.  The change indicates a
            modification to the virtual link's cost or viability.

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            If the entry indicates that the area border router is newly
            reachable, the corresponding virtual link is now
            operational.  An InterfaceUp event should be generated for
            the virtual link, which will cause a virtual adjacency to
            begin to form (see Section 10.3).  At this time the virtual
            link's IP interface address and the virtual neighbor's
            Neighbor IP address are also calculated.

            If the entry indicates that the area border router is no
            longer reachable, the virtual link and its associated
            adjacency should be destroyed.  This means an InterfaceDown
            event should be generated for the associated virtual link.

            If the cost of the entry has changed, and there is a fully
            established virtual adjacency, a new router-LSA for the
            backbone must be originated.  This in turn may cause further
            routing table changes.

    16.8.  Equal-cost multipath

        The OSPF protocol maintains multiple equal-cost routes to all
        destinations.  This can be seen in the steps used above to
        calculate the routing table, and in the definition of the
        routing table structure.

        Each one of the multiple routes will be of the same type
        (intra-area, inter-area, type 1 external or type 2 external),
        cost, and will have the same associated area.  However, each
        route may specify a separate next hop and Advertising router.

        There is no requirement that a router running OSPF keep track of
        all possible equal-cost routes to a destination.  An
        implementation may choose to keep only a fixed number of routes
        to any given destination.  This does not affect any of the
        algorithms presented in this specification.

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Footnotes


    [1]The graph's vertices represent either routers, transit networks,
    or stub networks.  Since routers may belong to multiple areas, it is
    not possible to color the graph's vertices.

    [2]It is possible for all of a router's interfaces to be unnumbered
    point-to-point links.  In this case, an IP address must be assigned
    to the router.  This address will then be advertised in the router's
    router-LSA as a host route.

    [3]Note that in these cases both interfaces, the non-virtual and the
    virtual, would have the same IP address.

    [4]Note that no host route is generated for, and no IP packets can
    be addressed to, interfaces to unnumbered point-to-point networks.
    This is regardless of such an interface's state.

    [5]It is instructive to see what happens when the Designated Router
    for the network crashes.  Call the Designated Router for the network
    RT1, and the Backup Designated Router RT2.  If Router RT1 crashes
    (or maybe its interface to the network dies), the other routers on
    the network will detect RT1's absence within RouterDeadInterval
    seconds.  All routers may not detect this at precisely the same
    time; the routers that detect RT1's absence before RT2 does will,
    for a time, select RT2 to be both Designated Router and Backup
    Designated Router.  When RT2 detects that RT1 is gone it will move
    itself to Designated Router.  At this time, the remaining router
    having highest Router Priority will be selected as Backup Designated
    Router.

    [6]On point-to-point networks, the lower level protocols indicate
    whether the neighbor is up and running.  Likewise, existence of the
    neighbor on virtual links is indicated by the routing table
    calculation.  However, in both these cases, the Hello Protocol is
    still used.  This ensures that communication between the neighbors
    is bidirectional, and that each of the neighbors has a functioning
    routing protocol layer.

    [7]When the identity of the Designated Router is changing, it may be
    quite common for a neighbor in this state to send the router a

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    Database Description packet; this means that there is some momentary
    disagreement on the Designated Router's identity.

    [8]Note that it is possible for a router to resynchronize any of its
    fully established adjacencies by setting the adjacency's state back
    to ExStart.  This will cause the other end of the adjacency to
    process a SeqNumberMismatch event, and therefore to also go back to
    ExStart state.

    [9]The address space of IP networks and the address space of OSPF
    Router IDs may overlap.  That is, a network may have an IP address
    which is identical (when considered as a 32-bit number) to some
    router's Router ID.

    [10]"Discard" entries are necessary to ensure that route
    summarization at area boundaries will not cause packet looping.

    [11]It is assumed that, for two different address ranges matching
    the destination, one range is more specific than the other. Non-
    contiguous subnet masks can be configured to violate this
    assumption. Such subnet mask configurations cannot be handled by the
    OSPF protocol.

    [12]MaxAgeDiff is an architectural constant.  It indicates the
    maximum dispersion of ages, in seconds, that can occur for a single
    LSA instance as it is flooded throughout the routing domain.  If two
    LSAs differ by more than this, they are assumed to be different
    instances of the same LSA.  This can occur when a router restarts
    and loses track of the LSA's previous LS sequence number.  See
    Section 13.4 for more details.

    [13]When two LSAs have different LS checksums, they are assumed to
    be separate instances.  This can occur when a router restarts, and
    loses track of the LSA's previous LS sequence number.  In the case
    where the two LSAs have the same LS sequence number, it is not
    possible to determine which LSA is actually newer.  However, if the
    wrong LSA is accepted as newer, the originating router will simply
    originate another instance.  See Section 13.4 for further details.

    [14]There is one instance where a lookup must be done based on
    partial information.  This is during the routing table calculation,
    when a network-LSA must be found based solely on its Link State ID.

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    The lookup in this case is still well defined, since no two
    network-LSAs can have the same Link State ID.

    [15]This is the way RFC 1583 specified point-to-point
    representation.  It has three advantages: a) it does not require
    allocating a subnet to the point-to-point link, b) it tends to bias
    the routing so that packets destined for the point-to-point
    interface will actually be received over the interface (which is
    useful for diagnostic purposes) and c) it allows network
    bootstrapping of a neighbor, without requiring that the bootstrap
    program contain an OSPF implementation.

    [16]This is the more traditional point-to-point representation used
    by protocols such as RIP.

    [17]This clause covers the case: Inter-area routes are not
    summarized to the backbone.  This is because inter-area routes are
    always associated with the backbone area.

    [18]This clause is only invoked when a non-backbone Area A supports
    transit data traffic (i.e., has TransitCapability set to TRUE).  For
    example, in the area configuration of Figure 6, Area 2 can support
    transit traffic due to the configured virtual link between Routers
    RT10 and RT11. As a result, Router RT11 need only originate a single
    summary-LSA into Area 2 (having the collapsed destination N9-
    N11,H1), since all of Router RT11's other eligible routes have next
    hops belonging to Area 2 itself (and as such only need be advertised
    by other area border routers; in this case, Routers RT10 and RT7).

    [19]By keeping more information in the routing table, it is possible
    for an implementation to recalculate the shortest path tree for only
    a single area.  In fact, there are incremental algorithms that allow
    an implementation to recalculate only a portion of a single area's
    shortest path tree [Ref1].  However, these algorithms are beyond the
    scope of this specification.

    [20]This is how the Link state request list is emptied, which
    eventually causes the neighbor state to transition to Full.  See
    Section 10.9 for more details.

    [21]It should be a relatively rare occurrence for an LSA's LS age to
    reach MaxAge in this fashion.  Usually, the LSA will be replaced by

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    a more recent instance before it ages out.

    [22]Strictly speaking, because of equal-cost multipath, the
    algorithm does not create a tree.  We continue to use the "tree"
    terminology because that is what occurs most often in the existing
    literature.

    [23]Note that the presence of any link back to V is sufficient; it
    need not be the matching half of the link under consideration from V
    to W. This is enough to ensure that, before data traffic flows
    between a pair of neighboring routers, their link state databases
    will be synchronized.

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

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References

    [Ref1]  McQuillan, J., I. Richer and E. Rosen, "ARPANET Routing
            Algorithm Improvements", BBN Technical Report 3803, April
            1978.

    [Ref2]  Digital Equipment Corporation, "Information processing
            systems -- Data communications -- Intermediate System to
            Intermediate System Intra-Domain Routing Protocol", October
            1987.

    [Ref3]  McQuillan, J., et.al., "The New Routing Algorithm for the
            ARPANET", IEEE Transactions on Communications, May 1980.

    [Ref4]  Perlman, R., "Fault-Tolerant Broadcast of Routing
            Information", Computer Networks, December 1983.

    [Ref5]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.

    [Ref6]  McKenzie, A., "ISO Transport Protocol specification ISO DP
            8073", RFC 905, April 1984.

    [Ref7]  Deering, S., "Host extensions for IP multicasting", STD 5,
            RFC 1112, May 1988.

    [Ref8]  McCloghrie, K., and M. Rose, "Management Information Base
            for network management of TCP/IP-based internets: MIB-II",
            STD 17, RFC 1213, March 1991.

    [Ref9]  Moy, J., "OSPF Version 2", RFC 1583, March 1994.

    [Ref10] Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
            Inter-Domain Routing (CIDR): an Address Assignment and
            Aggregation Strategy", RFC1519, September 1993.

    [Ref11] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
            1700, October 1994.

    [Ref12] Almquist, P., "Type of Service in the Internet Protocol
            Suite", RFC 1349, July 1992.

Top      Up      ToC       Page 184 
    [Ref13] Leiner, B., et.al., "The DARPA Internet Protocol Suite", DDN
            Protocol Handbook, April 1985.

    [Ref14] Bradley, T., and C. Brown, "Inverse Address Resolution
            Protocol", RFC 1293, January 1992.

    [Ref15] deSouza, O., and M. Rodrigues, "Guidelines for Running OSPF
            Over Frame Relay Networks", RFC 1586, March 1994.

    [Ref16] Bellovin, S., "Security Problems in the TCP/IP Protocol
            Suite", ACM Computer Communications Review, Volume 19,
            Number 2, pp. 32-38, April 1989.

    [Ref17] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
            April 1992.

    [Ref18] Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
            1994.

    [Ref19] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC 1587,
            March 1994.

    [Ref20] Ferguson, D., "The OSPF External Attributes LSA", work in
            progress.

    [Ref21] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
            1793, April 1995.

    [Ref22] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
            November 1990.

    [Ref23] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-
            4)", RFC 1771, March 1995.

    [Ref24] Hinden, R., "Internet Routing Protocol Standardization
            Criteria", BBN, October 1991.

    [Ref25] Moy, J., "OSPF Version 2", RFC 2178, July 1997.

    [Ref26] Rosen, E., "Vulnerabilities of Network Control Protocols: An
            Example", Computer Communication Review, July 1981.

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A. OSPF data formats

    This appendix describes the format of OSPF protocol packets and OSPF
    LSAs.  The OSPF protocol runs directly over the IP network layer.
    Before any data formats are described, the details of the OSPF
    encapsulation are explained.

    Next the OSPF Options field is described.  This field describes
    various capabilities that may or may not be supported by pieces of
    the OSPF routing domain. The OSPF Options field is contained in OSPF
    Hello packets, Database Description packets and in OSPF LSAs.

    OSPF packet formats are detailed in Section A.3.  A description of
    OSPF LSAs appears in Section A.4.

A.1 Encapsulation of OSPF packets

    OSPF runs directly over the Internet Protocol's network layer.  OSPF
    packets are therefore encapsulated solely by IP and local data-link
    headers.

    OSPF does not define a way to fragment its protocol packets, and
    depends on IP fragmentation when transmitting packets larger than
    the network MTU. If necessary, the length of OSPF packets can be up
    to 65,535 bytes (including the IP header).  The OSPF packet types
    that are likely to be large (Database Description Packets, Link
    State Request, Link State Update, and Link State Acknowledgment
    packets) can usually be split into several separate protocol
    packets, without loss of functionality.  This is recommended; IP
    fragmentation should be avoided whenever possible.  Using this
    reasoning, an attempt should be made to limit the sizes of OSPF
    packets sent over virtual links to 576 bytes unless Path MTU
    Discovery is being performed (see [Ref22]).

    The other important features of OSPF's IP encapsulation are:

    o   Use of IP multicast.  Some OSPF messages are multicast, when
        sent over broadcast networks.  Two distinct IP multicast
        addresses are used.  Packets sent to these multicast addresses
        should never be forwarded; they are meant to travel a single hop
        only.  To ensure that these packets will not travel multiple
        hops, their IP TTL must be set to 1.

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        AllSPFRouters
            This multicast address has been assigned the value
            224.0.0.5.  All routers running OSPF should be prepared to
            receive packets sent to this address.  Hello packets are
            always sent to this destination.  Also, certain OSPF
            protocol packets are sent to this address during the
            flooding procedure.

        AllDRouters
            This multicast address has been assigned the value
            224.0.0.6.  Both the Designated Router and Backup Designated
            Router must be prepared to receive packets destined to this
            address.  Certain OSPF protocol packets are sent to this
            address during the flooding procedure.

    o   OSPF is IP protocol number 89.  This number has been registered
        with the Network Information Center.  IP protocol number
        assignments are documented in [Ref11].

    o   All OSPF routing protocol packets are sent using the normal
        service TOS value of binary 0000 defined in [Ref12].

    o   Routing protocol packets are sent with IP precedence set to
        Internetwork Control.  OSPF protocol packets should be given
        precedence over regular IP data traffic, in both sending and
        receiving.  Setting the IP precedence field in the IP header to
        Internetwork Control [Ref5] may help implement this objective.

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A.2 The Options field

    The OSPF Options field is present in OSPF Hello packets, Database
    Description packets and all LSAs.  The Options field enables OSPF
    routers to support (or not support) optional capabilities, and to
    communicate their capability level to other OSPF routers.  Through
    this mechanism routers of differing capabilities can be mixed within
    an OSPF routing domain.

    When used in Hello packets, the Options field allows a router to
    reject a neighbor because of a capability mismatch.  Alternatively,
    when capabilities are exchanged in Database Description packets a
    router can choose not to forward certain LSAs to a neighbor because
    of its reduced functionality.  Lastly, listing capabilities in LSAs
    allows routers to forward traffic around reduced functionality
    routers, by excluding them from parts of the routing table
    calculation.

    Five bits of the OSPF Options field have been assigned, although
    only one (the E-bit) is described completely by this memo. Each bit
    is described briefly below. Routers should reset (i.e.  clear)
    unrecognized bits in the Options field when sending Hello packets or
    Database Description packets and when originating LSAs. Conversely,
    routers encountering unrecognized Option bits in received Hello
    Packets, Database Description packets or LSAs should ignore the
    capability and process the packet/LSA normally.

                       +------------------------------------+
                       | * | * | DC | EA | N/P | MC | E | * |
                       +------------------------------------+

                             The Options field


    E-bit
        This bit describes the way AS-external-LSAs are flooded, as
        described in Sections 3.6, 9.5, 10.8 and 12.1.2 of this memo.

    MC-bit
        This bit describes whether IP multicast datagrams are forwarded
        according to the specifications in [Ref18].

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    N/P-bit
        This bit describes the handling of Type-7 LSAs, as specified in
        [Ref19].

    EA-bit
        This bit describes the router's willingness to receive and
        forward External-Attributes-LSAs, as specified in [Ref20].

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

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A.3 OSPF Packet Formats

    There are five distinct OSPF packet types.  All OSPF packet types
    begin with a standard 24 byte header.  This header is described
    first.  Each packet type is then described in a succeeding section.
    In these sections each packet's division into fields is displayed,
    and then the field definitions are enumerated.

    All OSPF packet types (other than the OSPF Hello packets) deal with
    lists of LSAs.  For example, Link State Update packets implement the
    flooding of LSAs throughout the OSPF routing domain.  Because of
    this, OSPF protocol packets cannot be parsed unless the format of
    LSAs is also understood.  The format of LSAs is described in Section
    A.4.

    The receive processing of OSPF packets is detailed in Section 8.2.
    The sending of OSPF packets is explained in Section 8.1.

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A.3.1 The OSPF packet header

    Every OSPF packet starts with a standard 24 byte header.  This
    header contains all the information necessary to determine whether
    the packet should be accepted for further processing.  This
    determination is described in Section 8.2 of the specification.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |     Type      |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



    Version #
        The OSPF version number.  This specification documents version 2
        of the protocol.

    Type
        The OSPF packet types are as follows. See Sections A.3.2 through
        A.3.6 for details.

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                          Type   Description
                          ________________________________
                          1      Hello
                          2      Database Description
                          3      Link State Request
                          4      Link State Update
                          5      Link State Acknowledgment




    Packet length
        The length of the OSPF protocol packet in bytes.  This length
        includes the standard OSPF header.

    Router ID
        The Router ID of the packet's source.

    Area ID
        A 32 bit number identifying the area that this packet belongs
        to.  All OSPF packets are associated with a single area.  Most
        travel a single hop only.  Packets travelling over a virtual
        link are labelled with the backbone Area ID of 0.0.0.0.

    Checksum
        The standard IP checksum of the entire contents of the packet,
        starting with the OSPF packet header but excluding the 64-bit
        authentication field.  This checksum is calculated as the 16-bit
        one's complement of the one's complement sum of all the 16-bit
        words in the packet, excepting the authentication field.  If the
        packet's length is not an integral number of 16-bit words, the
        packet is padded with a byte of zero before checksumming.  The
        checksum is considered to be part of the packet authentication
        procedure; for some authentication types the checksum
        calculation is omitted.

    AuType
        Identifies the authentication procedure to be used for the
        packet.  Authentication is discussed in Appendix D of the
        specification.  Consult Appendix D for a list of the currently
        defined authentication types.

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

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A.3.2 The Hello packet

    Hello packets are OSPF packet type 1.  These packets are sent
    periodically on all interfaces (including virtual links) in order to
    establish and maintain neighbor relationships.  In addition, Hello
    Packets are multicast on those physical networks having a multicast
    or broadcast capability, enabling dynamic discovery of neighboring
    routers.

    All routers connected to a common network must agree on certain
    parameters (Network mask, HelloInterval and RouterDeadInterval).
    These parameters are included in Hello packets, so that differences
    can inhibit the forming of neighbor relationships.  A detailed
    explanation of the receive processing for Hello packets is presented
    in Section 10.5.  The sending of Hello packets is covered in Section
    9.5.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       1       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Network Mask                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         HelloInterval         |    Options    |    Rtr Pri    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     RouterDeadInterval                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Designated Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Backup Designated Router                    |

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


    Network mask
        The network mask associated with this interface.  For example,
        if the interface is to a class B network whose third byte is
        used for subnetting, the network mask is 0xffffff00.

    Options
        The optional capabilities supported by the router, as documented
        in Section A.2.

    HelloInterval
        The number of seconds between this router's Hello packets.

    Rtr Pri
        This router's Router Priority.  Used in (Backup) Designated
        Router election.  If set to 0, the router will be ineligible to
        become (Backup) Designated Router.

    RouterDeadInterval
        The number of seconds before declaring a silent router down.

    Designated Router
        The identity of the Designated Router for this network, in the
        view of the sending router.  The Designated Router is identified
        here by its IP interface address on the network.  Set to 0.0.0.0
        if there is no Designated Router.

    Backup Designated Router
        The identity of the Backup Designated Router for this network,
        in the view of the sending router.  The Backup Designated Router
        is identified here by its IP interface address on the network.
        Set to 0.0.0.0 if there is no Backup Designated Router.

    Neighbor
        The Router IDs of each router from whom valid Hello packets have
        been seen recently on the network.  Recently means in the last
        RouterDeadInterval seconds.

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A.3.3 The Database Description packet

    Database Description packets are OSPF packet type 2.  These packets
    are exchanged when an adjacency is being initialized.  They describe
    the contents of the link-state database.  Multiple packets may be
    used to describe the database.  For this purpose a poll-response
    procedure is used.  One of the routers is designated to be the
    master, the other the slave.  The master sends Database Description
    packets (polls) which are acknowledged by Database Description
    packets sent by the slave (responses).  The responses are linked to
    the polls via the packets' DD sequence numbers.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       2       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Interface MTU         |    Options    |0|0|0|0|0|I|M|MS
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     DD sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-                      An LSA Header                          -+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |

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    The format of the Database Description packet is very similar to
    both the Link State Request and Link State Acknowledgment packets.
    The main part of all three is a list of items, each item describing
    a piece of the link-state database.  The sending of Database
    Description Packets is documented in Section 10.8.  The reception of
    Database Description packets is documented in Section 10.6.

    Interface MTU
        The size in bytes of the largest IP datagram that can be sent
        out the associated interface, without fragmentation.  The MTUs
        of common Internet link types can be found in Table 7-1 of
        [Ref22]. Interface MTU should be set to 0 in Database
        Description packets sent over virtual links.

    Options
        The optional capabilities supported by the router, as documented
        in Section A.2.

    I-bit
        The Init bit.  When set to 1, this packet is the first in the
        sequence of Database Description Packets.

    M-bit
        The More bit.  When set to 1, it indicates that more Database
        Description Packets are to follow.

    MS-bit
        The Master/Slave bit.  When set to 1, it indicates that the
        router is the master during the Database Exchange process.
        Otherwise, the router is the slave.

    DD sequence number
        Used to sequence the collection of Database Description Packets.
        The initial value (indicated by the Init bit being set) should
        be unique.  The DD sequence number then increments until the
        complete database description has been sent.

    The rest of the packet consists of a (possibly partial) list of the
    link-state database's pieces.  Each LSA in the database is described
    by its LSA header.  The LSA header is documented in Section A.4.1.
    It contains all the information required to uniquely identify both
    the LSA and the LSA's current instance.

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A.3.4 The Link State Request packet

    Link State Request packets are OSPF packet type 3.  After exchanging
    Database Description packets with a neighboring router, a router may
    find that parts of its link-state database are out-of-date.  The
    Link State Request packet is used to request the pieces of the
    neighbor's database that are more up-to-date.  Multiple Link State
    Request packets may need to be used.

    A router that sends a Link State Request packet has in mind the
    precise instance of the database pieces it is requesting. Each
    instance is defined by its LS sequence number, LS checksum, and LS
    age, although these fields are not specified in the Link State
    Request Packet itself.  The router may receive even more recent
    instances in response.

    The sending of Link State Request packets is documented in Section
    10.9.  The reception of Link State Request packets is documented in
    Section 10.7.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       3       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          LS type                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Link State ID                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |

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

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A.3.5 The Link State Update packet

    Link State Update packets are OSPF packet type 4.  These packets
    implement the flooding of LSAs.  Each Link State Update packet
    carries a collection of LSAs one hop further from their origin.
    Several LSAs may be included in a single packet.

    Link State Update packets are multicast on those physical networks
    that support multicast/broadcast.  In order to make the flooding
    procedure reliable, flooded LSAs are acknowledged in Link State
    Acknowledgment packets.  If retransmission of certain LSAs is
    necessary, the retransmitted LSAs are always sent directly to the
    neighbor.  For more information on the reliable flooding of LSAs,
    consult Section 13.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       4       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            # LSAs                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                                                            +-+
       |                             LSAs                              |
       +-                                                            +-+
       |                              ...                              |

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    # LSAs
        The number of LSAs included in this update.


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

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A.3.6 The Link State Acknowledgment packet

    Link State Acknowledgment Packets are OSPF packet type 5.  To make
    the flooding of LSAs reliable, flooded LSAs are explicitly
    acknowledged.  This acknowledgment is accomplished through the
    sending and receiving of Link State Acknowledgment packets.
    Multiple LSAs can be acknowledged in a single Link State
    Acknowledgment packet.

    Depending on the state of the sending interface and the sender of
    the corresponding Link State Update packet, a Link State
    Acknowledgment packet is sent either to the multicast address
    AllSPFRouters, to the multicast address AllDRouters, or as a
    unicast.  The sending of Link State Acknowledgement packets is
    documented in Section 13.5.  The reception of Link State
    Acknowledgement packets is documented in Section 13.7.

    The format of this packet is similar to that of the Data Description
    packet.  The body of both packets is simply a list of LSA headers.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       5       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-                         An LSA Header                       -+
       |                                                               |
       +-                                                             -+

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


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


Next RFC Part