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RFC 1583

 
 
 

OSPF Version 2

Part 5 of 9, p. 100 to 126
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12.  Link State Advertisements

    Each router in the Autonomous System originates one or more link
    state advertisements.  There are five distinct types of link state
    advertisements, which are described in Section 4.3.  The collection
    of link state advertisements forms the link state or topological
    database.  Each separate type of advertisement has a separate

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    Type   Dest        Area   Path  Type   Cost   Next     Adv.
                                                  Hop(s)   Router(s)
    ________________________________________________________________
    N      Ib          0      intra-area   16     RT3      *
    N      Ia          0      intra-area   21     RT3      *
    BR     RT3         0      intra-area   1      *        *
    BR     RT10        0      intra-area   16     RT3      *
    BR     RT11        0      intra-area   19     RT3      *
    ________________________________________________________________
    N      N9-N11,H1   0      inter-area   20     RT3      RT11


                  Table 14: Changes resulting from an
                        additional virtual link.

    function.  Router links and network links advertisements describe
    how an area's routers and networks are interconnected.  Summary link
    advertisements provide a way of condensing an area's routing
    information.  AS external advertisements provide a way of
    transparently advertising externally-derived routing information
    throughout the Autonomous System.

    Each link state advertisement begins with a standard 20-byte header.
    This link state advertisement header is discussed below.


    12.1.  The Link State Advertisement Header

        The link state advertisement header contains the LS type, Link
        State ID and Advertising Router fields.  The combination of
        these three fields uniquely identifies the link state
        advertisement.

        There may be several instances of an advertisement present in
        the Autonomous System, all at the same time.  It must then be
        determined which instance is more recent.  This determination is
        made by examining the LS sequence, LS checksum and LS age
        fields.  These fields are also contained in the 20-byte link
        state advertisement header.

        Several of the OSPF packet types list link state advertisements.
        When the instance is not important, an advertisement is referred
        to by its LS type, Link State ID and Advertising Router (see
        Link State Request Packets).  Otherwise, the LS sequence number,
        LS age and LS checksum fields must also be referenced.

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        A detailed explanation of the fields contained in the link state
        advertisement header follows.


        12.1.1.  LS age

            This field is the age of the link state advertisement in
            seconds.  It should be processed as an unsigned 16-bit
            integer.  It is set to 0 when the link state advertisement
            is originated.  It must be incremented by InfTransDelay on
            every hop of the flooding procedure.  Link state
            advertisements are also aged as they are held in each
            router's database.

            The age of a link state advertisement is never incremented
            past MaxAge.  Advertisements having age MaxAge are not used
            in the routing table calculation.  When an advertisement's
            age first reaches MaxAge, it is reflooded.  A link state
            advertisement of age MaxAge is finally flushed from the
            database when it is no longer needed to ensure database
            synchronization.  For more information on the aging of link
            state advertisements, consult Section 14.

            The LS age field is examined when a router receives two
            instances of a link state advertisement, both having
            identical LS sequence numbers and LS checksums.  An instance
            of age MaxAge is then always accepted as most recent; this
            allows old advertisements to be flushed quickly from the
            routing domain.  Otherwise, if the ages differ by more than
            MaxAgeDiff, the instance having the smaller age is accepted
            as most recent.[11] See Section 13.1 for more details.


        12.1.2.  Options

            The Options field in the link state advertisement header
            indicates which optional capabilities are associated with
            the advertisement.  OSPF's optional capabilities are
            described in Section 4.5.  There are currently two optional
            capabilities defined; they are represented by the T-bit and
            E-bit found in the Options field.  The rest of the Options
            field should be set to zero.

            The E-bit represents OSPF's ExternalRoutingCapability.  This
            bit should be set in all advertisements associated with the
            backbone, and all advertisements associated with non-stub
            areas (see Section 3.6).  It should also be set in all AS
            external link advertisements.  It should be reset in all

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            router links, network links and summary link advertisements
            associated with a stub area.  For all link state
            advertisements, the setting of the E-bit is for
            informational purposes only; it does not affect the routing
            table calculation.

            The T-bit represents OSPF's TOS routing capability.  This
            bit should be set in a router links advertisement if and
            only if the router is capable of calculating separate routes
            for each IP TOS (see Section 2.4).  The T-bit should always
            be set in network links advertisements.  It should be set in
            summary link and AS external link advertisements if and only
            if the advertisement describes paths for all TOS values,
            instead of just the TOS 0 path.  Note that, with the T-bit
            set, there may still be only a single metric in the
            advertisement (the TOS 0 metric).  This would mean that
            paths for non-zero TOS exist, but are equivalent to the TOS
            0 path.  A link state advertisement's T-bit is examined when
            calculating the routing table's non-zero TOS paths (see
            Section 16.9).


        12.1.3.  LS type

            The LS type field dictates the format and function of the
            link state advertisement.  Advertisements of different types
            have different names (e.g., router links or network links).
            All advertisement types, except the AS external link
            advertisements (LS type = 5), are flooded throughout a
            single area only.  AS external link advertisements are
            flooded throughout the entire Autonomous System, excepting
            stub areas (see Section 3.6).  Each separate advertisement
            type is briefly described below in Table 15.

        12.1.4.  Link State ID

            This field identifies the piece of the routing domain that
            is being described by the advertisement.  Depending on the
            advertisement's LS type, the Link State ID takes on the
            values listed in Table 16.


            Actually, for Type 3 summary link (LS type = 3)
            advertisements and AS external link (LS type = 5)
            advertisements, the Link State ID may additionally have one
            or more of the destination network's "host" bits set. For
            example, when originating an AS external link for the
            network 10.0.0.0 with mask of 255.0.0.0, the Link State ID

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           LS Type   Advertisement description
           __________________________________________________
           1         These are the router links
                     advertisements. They describe the
                     collected states of the router's
                     interfaces. For more information,
                     consult Section 12.4.1.
           __________________________________________________
           2         These are the network links
                     advertisements. They describe the set
                     of routers attached to the network. For
                     more information, consult
                     Section 12.4.2.
           __________________________________________________
           3 or 4    These are the summary link
                     advertisements. They describe
                     inter-area routes, and enable the
                     condensation of routing information at
                     area borders. Originated by area border
                     routers, the Type 3 advertisements
                     describe routes to networks while the
                     Type 4 advertisements describe routes to
                     AS boundary routers.
           __________________________________________________
           5         These are the AS external link
                     advertisements. Originated by AS
                     boundary routers, they describe routes
                     to destinations external to the
                     Autonomous System. A default route for
                     the Autonomous System can also be
                     described by an AS external link
                     advertisement.


               Table 15: OSPF link state advertisements.

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            LS Type   Link State ID
            _______________________________________________
            1         The originating router's Router ID.
            2         The IP interface address of the
                      network's Designated Router.
            3         The destination network's IP address.
            4         The Router ID of the described AS
                      boundary router.
            5         The destination network's IP address.


              Table 16: The advertisement's Link State ID.

            can be set to anything in the range 10.0.0.0 through
            10.255.255.255 inclusive (although 10.0.0.0 should be used
            whenever possible). The freedom to set certain host bits
            allows a router to originate separate advertisements for two
            networks having the same address but different masks. See
            Appendix F for details.

            When the link state advertisement is describing a network
            (LS type = 2, 3 or 5), the network's IP address is easily
            derived by masking the Link State ID with the network/subnet
            mask contained in the body of the link state advertisement.
            When the link state advertisement is describing a router (LS
            type = 1 or 4), the Link State ID is always the described
            router's OSPF Router ID.

            When an AS external advertisement (LS Type = 5) is
            describing a default route, its Link State ID is set to
            DefaultDestination (0.0.0.0).


        12.1.5.  Advertising Router

            This field specifies the OSPF Router ID of the
            advertisement's originator.  For router links
            advertisements, this field is identical to the Link State ID
            field.  Network link advertisements are originated by the
            network's Designated Router.  Summary link advertisements
            are originated by area border routers.  AS external link
            advertisements are originated by AS boundary routers.


        12.1.6.  LS sequence number

            The sequence number field is a signed 32-bit integer.  It is
            used to detect old and duplicate link state advertisements.

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            The space of sequence numbers is linearly ordered.  The
            larger the sequence number (when compared as signed 32-bit
            integers) the more recent the advertisement.  To describe to
            sequence number space more precisely, let N refer in the
            discussion below to the constant 2**31.

            The sequence number -N (0x80000000) is reserved (and
            unused).  This leaves -N + 1 (0x80000001) as the smallest
            (and therefore oldest) sequence number.  A router uses this
            sequence number the first time it originates any link state
            advertisement.  Afterwards, the advertisement's sequence
            number is incremented each time the router originates a new
            instance of the advertisement.  When an attempt is made to
            increment the sequence number past the maximum value of N -
            1 (0x7fffffff), the current instance of the advertisement
            must first be flushed from the routing domain.  This is done
            by prematurely aging the advertisement (see Section 14.1)
            and reflooding it.  As soon as this flood has been
            acknowledged by all adjacent neighbors, a new instance can
            be originated with sequence number of -N + 1 (0x80000001).

            The router may be forced to promote the sequence number of
            one of its advertisements when a more recent instance of the
            advertisement is unexpectedly received during the flooding
            process.  This should be a rare event.  This may indicate
            that an out-of-date advertisement, originated by the router
            itself before its last restart/reload, still exists in the
            Autonomous System.  For more information see Section 13.4.


        12.1.7.  LS checksum

            This field is the checksum of the complete contents of the
            advertisement, excepting the LS age field.  The LS age field
            is excepted so that an advertisement's age can be
            incremented without updating the checksum.  The checksum
            used is the same that is used for ISO connectionless
            datagrams; it is commonly referred to as the Fletcher
            checksum.  It is documented in Annex B of [RFC 905].  The
            link state advertisement header also contains the length of
            the advertisement in bytes; subtracting the size of the LS
            age field (two bytes) yields the amount of data to checksum.

            The checksum is used to detect data corruption of an
            advertisement.  This corruption can occur while an
            advertisement is being flooded, or while it is being held in
            a router's memory.  The LS checksum field cannot take on the
            value of zero; the occurrence of such a value should be

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            considered a checksum failure.  In other words, calculation
            of the checksum is not optional.

            The checksum of a link state advertisement is verified in
            two cases: a) when it is received in a Link State Update
            Packet and b) at times during the aging of the link state
            database.  The detection of a checksum failure leads to
            separate actions in each case.  See Sections 13 and 14 for
            more details.

            Whenever the LS sequence number field indicates that two
            instances of an advertisement are the same, the LS checksum
            field is examined.  If there is a difference, the instance
            with the larger LS checksum is considered to be most
            recent.[12] See Section 13.1 for more details.


    12.2.  The link state database

        A router has a separate link state database for every area to
        which it belongs.  The link state database has been referred to
        elsewhere in the text as the topological database.  All routers
        belonging to the same area have identical topological databases
        for the area.

        The databases for each individual area are always dealt with
        separately.  The shortest path calculation is performed
        separately for each area (see Section 16).  Components of the
        area topological database are flooded throughout the area only.
        Finally, when an adjacency (belonging to Area A) is being
        brought up, only the database for Area A is synchronized between
        the two routers.

        The area database is composed of router links advertisements,
        network links advertisements, and summary link advertisements
        (all listed in the area data structure).  In addition, external
        routes (AS external advertisements) are included in all non-stub
        area databases (see Section 3.6).

        An implementation of OSPF must be able to access individual
        pieces of an area database.  This lookup function is based on an
        advertisement's LS type, Link State ID and Advertising
        Router.[13] There will be a single instance (the most up-to-
        date) of each link state advertisement in the database.  The
        database lookup function is invoked during the link state
        flooding procedure (Section 13) and the routing table
        calculation (Section 16).  In addition, using this lookup
        function the router can determine whether it has itself ever

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        originated a particular link state advertisement, and if so,
        with what LS sequence number.

        A link state advertisement is added to a router's database when
        either a) it is received during the flooding process (Section
        13) or b) it is originated by the router itself (Section 12.4).
        A link state advertisement is deleted from a router's database
        when either a) it has been overwritten by a newer instance
        during the flooding process (Section 13) or b) the router
        originates a newer instance of one of its self-originated
        advertisements (Section 12.4) or c) the advertisement ages out
        and is flushed from the routing domain (Section 14).  Whenever a
        link state advertisement is deleted from the database it must
        also be removed from all neighbors' Link state retransmission
        lists (see Section 10).


    12.3.  Representation of TOS

        All OSPF link state advertisements (with the exception of
        network links advertisements) specify metrics.  In router links
        advertisements, the metrics indicate the costs of the described
        interfaces.  In summary link and AS external link
        advertisements, the metric indicates the cost of the described
        path.  In all of these advertisements, a separate metric can be
        specified for each IP TOS.  The encoding of TOS in OSPF link
        state advertisements is specified in Table 17. That table
        relates the OSPF encoding to the IP packet header's TOS field
        (defined in [RFC 1349]).  The OSPF encoding is expressed as a
        decimal integer, and the IP packet header's TOS field is
        expressed in the binary TOS values used in [RFC 1349].

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                    OSPF encoding   RFC 1349 TOS values
                    ___________________________________________
                    0               0000 normal service
                    2               0001 minimize monetary cost
                    4               0010 maximize reliability
                    6               0011
                    8               0100 maximize throughput
                    10              0101
                    12              0110
                    14              0111
                    16              1000 minimize delay
                    18              1001
                    20              1010
                    22              1011
                    24              1100
                    26              1101
                    28              1110
                    30              1111


                        Table 17: Representing TOS in OSPF.


        Each OSPF link state advertisement must specify the TOS 0
        metric.  Other TOS metrics, if they appear, must appear in order
        of increasing TOS encoding.  For example, the TOS 8 (maximize
        throughput) metric must always appear before the TOS 16
        (minimize delay) metric when both are specified.  If a metric
        for some non-zero TOS is not specified, its cost defaults to the
        cost for TOS 0, unless the T-bit is reset in the advertisement's
        Options field (see Section 12.1.2 for more details).


    12.4.  Originating link state advertisements

        Into any given OSPF area, a router will originate several link
        state advertisements.  Each router originates a router links
        advertisement.  If the router is also the Designated Router for
        any of the area's networks, it will originate network links
        advertisements for those networks.

        Area border routers originate a single summary link
        advertisement for each known inter-area destination.  AS
        boundary routers originate a single AS external link
        advertisement for each known AS external destination.
        Destinations are advertised one at a time so that the change in
        any single route can be flooded without reflooding the entire

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        collection of routes.  During the flooding procedure, many link
        state advertisements can be carried by a single Link State
        Update packet.

        As an example, consider Router RT4 in Figure 6.  It is an area
        border router, having a connection to Area 1 and the backbone.
        Router RT4 originates 5 distinct link state advertisements into
        the backbone (one router links, and one summary link for each of
        the networks N1-N4).  Router RT4 will also originate 8 distinct
        link state advertisements into Area 1 (one router links and
        seven summary link advertisements as pictured in Figure 7).  If
        RT4 has been selected as Designated Router for Network N3, it
        will also originate a network links advertisement for N3 into
        Area 1.

        In this same figure, Router RT5 will be originating 3 distinct
        AS external link advertisements (one for each of the networks
        N12-N14).  These will be flooded throughout the entire AS,
        assuming that none of the areas have been configured as stubs.
        However, if area 3 has been configured as a stub area, the
        external advertisements for networks N12-N14 will not be flooded
        into area 3 (see Section 3.6).  Instead, Router RT11 would
        originate a default summary link advertisement that would be
        flooded throughout area 3 (see Section 12.4.3).  This instructs
        all of area 3's internal routers to send their AS external
        traffic to RT11.

        Whenever a new instance of a link state advertisement is
        originated, its LS sequence number is incremented, its LS age is
        set to 0, its LS checksum is calculated, and the advertisement
        is added to the link state database and flooded out the
        appropriate interfaces.  See Section 13.2 for details concerning
        the installation of the advertisement into the link state
        database.  See Section 13.3 for details concerning the flooding
        of newly originated advertisements.


        The ten events that can cause a new instance of a link state
        advertisement to be originated are:


        (1) The LS age field of one of the router's self-originated
            advertisements reaches the value LSRefreshTime. In this
            case, a new instance of the link state advertisement is
            originated, even though the contents of the advertisement
            (apart from the link state advertisement header) will be the
            same.  This guarantees periodic originations of all link
            state advertisements. This periodic updating of link state

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            advertisements adds robustness to the link state algorithm.
            Link state advertisements that solely describe unreachable
            destinations should not be refreshed, but should instead be
            flushed from the routing domain (see Section 14.1).


        When whatever is being described by a link state advertisement
        changes, a new advertisement is originated.  However, two
        instances of the same link state advertisement may not be
        originated within the time period MinLSInterval.  This may
        require that the generation of the next instance be delayed by
        up to MinLSInterval.  The following events may cause the
        contents of a link state advertisement to change.  These events
        should cause new originations if and only if the contents of the
        new advertisement would be different:


        (2) An interface's state changes (see Section 9.1).  This may
            mean that it is necessary to produce a new instance of the
            router links advertisement.

        (3) An attached network's Designated Router changes.  A new
            router links advertisement should be originated.  Also, if
            the router itself is now the Designated Router, a new
            network links advertisement should be produced.  If the
            router itself is no longer the Designated Router, any
            network links advertisement that it might have originated
            for the network should be flushed from the routing domain
            (see Section 14.1).

        (4) One of the neighboring routers changes to/from the FULL
            state.  This may mean that it is necessary to produce a new
            instance of the router links advertisement.  Also, if the
            router is itself the Designated Router for the attached
            network, a new network links advertisement should be
            produced.


        The next four events concern area border routers only:


        (5) An intra-area route has been added/deleted/modified in the
            routing table.  This may cause a new instance of a summary
            links advertisement (for this route) to be originated in
            each attached area (possibly including the backbone).

        (6) An inter-area route has been added/deleted/modified in the
            routing table.  This may cause a new instance of a summary

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            links advertisement (for this route) to be originated in
            each attached area (but NEVER for the backbone).

        (7) The router becomes newly attached to an area.  The router
            must then originate summary link advertisements into the
            newly attached area for all pertinent intra-area and inter-
            area routes in the router's routing table.  See Section
            12.4.3 for more details.

        (8) When the state of one of the router's configured virtual
            links changes, it may be necessary to originate a new router
            links advertisement into the virtual link's transit area
            (see the discussion of the router links advertisement's bit
            V in Section 12.4.1), as well as originating a new router
            links advertisement into the backbone.


        The last two events concern AS boundary routers (and former AS
        boundary routers) only:


        (9) An external route gained through direct experience with an
            external routing protocol (like EGP) changes.  This will
            cause an AS boundary router to originate a new instance of
            an AS external link advertisement.

        (10)
            A router ceases to be an AS boundary router, perhaps after
            restarting. In this situation the router should flush all AS
            external link advertisements that it had previously
            originated.  These advertisements can be flushed via the
            premature aging procedure specified in Section 14.1.


        The construction of each type of link state advertisement is
        explained in detail below.  In general, these sections describe
        the contents of the advertisement body (i.e., the part coming
        after the 20-byte advertisement header).  For information
        concerning the building of the link state advertisement header,
        see Section 12.1.

        12.4.1.  Router links

            A router originates a router links advertisement for each
            area that it belongs to.  Such an advertisement describes
            the collected states of the router's links to the area.  The
            advertisement is flooded throughout the particular area, and
            no further.

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                  ....................................
                  . 192.1.2                   Area 1 .
                  .     +                            .
                  .     |                            .
                  .     | 3+---+1                    .
                  .  N1 |--|RT1|-----+               .
                  .     |  +---+                    .
                  .     |                _______N3  .
                  .     +               /          .  1+---+
                  .                     * 192.1.1 *------|RT4|
                  .     +               /_______/   .   +---+
                  .     |              /     |       .
                  .     | 3+---+1     /      |       .
                  .  N2 |--|RT2|-----+      1|       .
                  .     |  +---+           +---+8    .         6+---+
                  .     |                  |RT3|----------------|RT6|
                  .     +                  +---+     .          +---+
                  . 192.1.3                  |2      .   18.10.0.6|7
                  .                          |       .            |
                  .                   +------------+ .
                  .                     192.1.4 (N4) .
                  ....................................


                    Figure 15: Area 1 with IP addresses shown

            The format of a router links advertisement is shown in
            Appendix A (Section A.4.2).  The first 20 bytes of the
            advertisement consist of the generic link state
            advertisement header that was discussed in Section 12.1.
            Router links advertisements have LS type = 1.  The router
            indicates whether it is willing to calculate separate routes
            for each IP TOS by setting (or resetting) the T-bit of the
            link state advertisement's Options field.

            A router also indicates whether it is an area border router,
            or an AS boundary router, by setting the appropriate bits
            (bit B and bit E, respectively) in its router links
            advertisements. This enables paths to those types of routers
            to be saved in the routing table, for later processing of
            summary link advertisements and AS external link
            advertisements.  Bit B should be set whenever the router is
            actively attached to two or more areas, even if the router
            is not currently attached to the OSPF backbone area.  Bit E
            should never be set in a router links advertisement for a
            stub area (stub areas cannot contain AS boundary routers).
            In addition, the router sets bit V in its router links

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            advertisement for Area A if and only if it is the endpoint
            of an active virtual link using Area A as its Transit area.
            This enables the other routers attached to Area A to
            discover whether the area supports any virtual links (i.e.,
            is a transit area).

            The router links advertisement then describes the router's
            working connections (i.e., interfaces or links) to the area.
            Each link is typed according to the kind of attached
            network.  Each link is also labelled with its Link ID.  This
            Link ID gives a name to the entity that is on the other end
            of the link.  Table 18 summarizes the values used for the
            Type and Link ID fields.



                   Link type   Description       Link ID
                   __________________________________________________
                   1           Point-to-point    Neighbor Router ID
                               link
                   2           Link to transit   Interface address of
                               network           Designated Router
                   3           Link to stub      IP network number
                               network
                   4           Virtual link      Neighbor Router ID


                           Table 18: Link descriptions in the
                              router links advertisement.


            In addition, the Link Data field is specified for each link.
            This field gives 32 bits of extra information for the link.
            For links to transit networks, numbered links to routers and
            virtual links, this field specifies the IP interface address
            of the associated router interface (this is needed by the
            routing table calculation, see Section 16.1.1).  For links
            to stub networks, this field specifies the network's IP
            address mask.  For unnumbered point-to-point networks, the
            Link Data field should be set to the unnumbered interface's
            MIB-II [RFC 1213] ifIndex value.

            Finally, the cost of using the link for output (possibly
            specifying a different cost for each Type of Service) is
            specified.  The output cost of a link is configurable.  It
            must always be non-zero.

            To further describe the process of building the list of link

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            descriptions, suppose a router wishes to build a router
            links advertisement for Area A.  The router examines its
            collection of interface data structures.  For each
            interface, the following steps are taken:


            o   If the attached network does not belong to Area A, no
                links are added to the advertisement, and the next
                interface should be examined.

            o   Else, if the state of the interface is Down, no links
                are added.

            o   Else, if the state of the interface is Point-to-Point,
                then add links according to the following:

                -   If the neighboring router is fully adjacent, add a
                    Type 1 link (point-to-point) if this is an interface
                    to a point-to-point network, or add a Type 4 link
                    (virtual link) if this is a virtual link.  The Link
                    ID should be set to the Router ID of the neighboring
                    router. For virtual links and numbered point-to-
                    point networks, the Link Data should specify the IP
                    interface address. For unnumbered point-to-point
                    networks, the Link Data field should specify the
                    interface's MIB-II [RFC 1213] ifIndex value.

                -   If this is a numbered point-to-point network (i.e,
                    not a virtual link and not an unnumbered point-to-
                    point network) and the neighboring router's IP
                    address is known, add a Type 3 link (stub network)
                    whose Link ID is the neighbor's IP address, whose
                    Link Data is the mask 0xffffffff indicating a host
                    route, and whose cost is the interface's configured
                    output cost.

            o   Else if the state of the interface is Loopback, add a
                Type 3 link (stub network) as long as this is not an
                interface to an unnumbered serial line.  The Link ID
                should be set to the IP interface address, the Link Data
                set to the mask 0xffffffff (indicating a host route),
                and the cost set to 0.

            o   Else if the state of the interface is Waiting, add a
                Type 3 link (stub network) whose Link ID is the IP
                network number of the attached network and whose Link
                Data is the attached network's address mask.

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            o   Else, there has been a Designated Router selected for
                the attached network.  If the router is fully adjacent
                to the Designated Router, or if the router itself is
                Designated Router and is fully adjacent to at least one
                other router, add a single Type 2 link (transit network)
                whose Link ID is the IP interface address of the
                attached network's Designated Router (which may be the
                router itself) and whose Link Data is the router's own
                IP interface address.  Otherwise, add a link as if the
                interface state were Waiting (see above).


            Unless otherwise specified, the cost of each link generated
            by the above procedure is equal to the output cost of the
            associated interface.  Note that in the case of serial
            lines, multiple links may be generated by a single
            interface.

            After consideration of all the router interfaces, host links
            are added to the advertisement by examining the list of
            attached hosts.  A host route is represented as a Type 3
            link (stub network) whose Link ID is the host's IP address
            and whose Link Data is the mask of all ones (0xffffffff).

            As an example, consider the router links advertisements
            generated by Router RT3, as pictured in Figure 6.  The area
            containing Router RT3 (Area 1) has been redrawn, with actual
            network addresses, in Figure 15.  Assume that the last byte
            of all of RT3's interface addresses is 3, giving it the
            interface addresses 192.1.1.3 and 192.1.4.3, and that the
            other routers have similar addressing schemes.  In addition,
            assume that all links are functional, and that Router IDs
            are assigned as the smallest IP interface address.

            RT3 originates two router links advertisements, one for Area
            1 and one for the backbone.  Assume that Router RT4 has been
            selected as the Designated router for network 192.1.1.0.
            RT3's router links advertisement for Area 1 is then shown
            below.  It indicates that RT3 has two connections to Area 1,
            the first a link to the transit network 192.1.1.0 and the
            second a link to the stub network 192.1.4.0.  Note that the
            transit network is identified by the IP interface of its
            Designated Router (i.e., the Link ID = 192.1.1.4 which is
            the Designated Router RT4's IP interface to 192.1.1.0).
            Note also that RT3 has indicated that it is capable of
            calculating separate routes based on IP TOS, through setting
            the T-bit in the Options field.  It has also indicated that
            it is an area border router.

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              ; RT3's router links advertisement for Area 1

              LS age = 0                     ;always true on origination
              Options = (T-bit|E-bit)        ;TOS-capable
              LS type = 1                    ;indicates router links
              Link State ID = 192.1.1.3      ;RT3's Router ID
              Advertising Router = 192.1.1.3 ;RT3's Router ID
              bit E = 0                      ;not an AS boundary router
              bit B = 1                      ;area border router
              #links = 2
                     Link ID = 192.1.1.4     ;IP address of Desig. Rtr.
                     Link Data = 192.1.1.3   ;RT3's IP interface to net
                     Type = 2                ;connects to transit network
                     # other metrics = 0
                     TOS 0 metric = 1

                     Link ID = 192.1.4.0     ;IP Network number
                     Link Data = 0xffffff00  ;Network mask
                     Type = 3                ;connects to stub network
                     # other metrics = 0
                     TOS 0 metric = 2

            Next RT3's router links advertisement for the backbone is
            shown.  It indicates that RT3 has a single attachment to the
            backbone.  This attachment is via an unnumbered point-to-
            point link to Router RT6.  RT3 has again indicated that it
            is TOS-capable, and that it is an area border router.

              ; RT3's router links advertisement for the backbone

              LS age = 0                     ;always true on origination
              Options = (T-bit|E-bit)        ;TOS-capable
              LS type = 1                    ;indicates router links
              Link State ID = 192.1.1.3      ;RT3's router ID
              Advertising Router = 192.1.1.3 ;RT3's router ID
              bit E = 0                      ;not an AS boundary router
              bit B = 1                      ;area border router
              #links = 1
                     Link ID = 18.10.0.6     ;Neighbor's Router ID
                     Link Data = 0.0.0.3     ;MIB-II ifIndex of P-P link
                     Type = 1                ;connects to router
                     # other metrics = 0
                     TOS 0 metric = 8

            Even though Router RT3 has indicated that it is TOS-capable
            in the above examples, only a single metric (the TOS 0
            metric) has been specified for each interface.  Different
            metrics can be specified for each TOS.  The encoding of TOS

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            in OSPF link state advertisements is described in Section
            12.3.

            As an example, suppose the point-to-point link between
            Routers RT3 and RT6 in Figure 15 is a satellite link.  The
            AS administrator may want to encourage the use of the line
            for high bandwidth traffic.  This would be done by setting
            the metric artificially low for the appropriate TOS value.
            Router RT3 would then originate the following router links
            advertisement for the backbone (TOS 8 = maximize
            throughput):

              ; RT3's router links advertisement for the backbone

              LS age = 0                  ;always true on origination
              Options = (T-bit|E-bit)     ;TOS-capable
              LS type = 1                 ;indicates router links
              Link State ID = 192.1.1.3   ;RT3's Router ID
              Advertising Router = 192.1.1.3
              bit E = 0                   ;not an AS boundary router
              bit B = 1                   ;area border router
              #links = 1
                     Link ID = 18.10.0.6  ;Neighbor's Router ID
                     Link Data = 0.0.0.3  ;MIB-II ifIndex of P-P link
                     Type = 1             ;connects to router
                     # other metrics = 1
                     TOS 0 metric = 8
                             TOS = 8      ;maximize throughput
                             metric = 1   ;traffic preferred


        12.4.2.  Network links

            A network links advertisement is generated for every transit
            multi-access network.  (A transit network is a network
            having two or more attached routers).  The network links
            advertisement describes all the routers that are attached to
            the network.

            The Designated Router for the network originates the
            advertisement.  The Designated Router originates the
            advertisement only if it is fully adjacent to at least one
            other router on the network.  The network links
            advertisement is flooded throughout the area that contains
            the transit network, and no further.  The networks links
            advertisement lists those routers that are fully adjacent to
            the Designated Router; each fully adjacent router is
            identified by its OSPF Router ID.  The Designated Router

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            includes itself in this list.

            The Link State ID for a network links advertisement is the
            IP interface address of the Designated Router.  This value,
            masked by the network's address mask (which is also
            contained in the network links advertisement) yields the
            network's IP address.

            A router that has formerly been the Designated Router for a
            network, but is no longer, should flush the network links
            advertisement that it had previously originated.  This
            advertisement is no longer used in the routing table
            calculation.  It is flushed by prematurely incrementing the
            advertisement's age to MaxAge and reflooding (see Section
            14.1). In addition, in those rare cases where a router's
            Router ID has changed, any network links advertisements that
            were originated with the router's previous Router ID must be
            flushed. Since the router may have no idea what it's
            previous Router ID might have been, these network links
            advertisements are indicated by having their Link State ID
            equal to one of the router's IP interface addresses and
            their Advertising Router not equal to the router's current
            Router ID (see Section 13.4 for more details).

            As an example of a network links advertisement, again
            consider the area configuration in Figure 6.  Network links
            advertisements are originated for Network N3 in Area 1,
            Networks N6 and N8 in Area 2, and Network N9 in Area 3.
            Assuming that Router RT4 has been selected as the Designated
            Router for Network N3, the following network links
            advertisement is generated by RT4 on behalf of Network N3
            (see Figure 15 for the address assignments):

              ; network links advertisement for Network N3

              LS age = 0                     ;always true on origination
              Options = (T-bit|E-bit)        ;TOS-capable
              LS type = 2                    ;indicates network links
              Link State ID = 192.1.1.4      ;IP address of Desig. Rtr.
              Advertising Router = 192.1.1.4 ;RT4's Router ID
              Network Mask = 0xffffff00
                     Attached Router = 192.1.1.4    ;Router ID
                     Attached Router = 192.1.1.1    ;Router ID
                     Attached Router = 192.1.1.2    ;Router ID
                     Attached Router = 192.1.1.3    ;Router ID

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        12.4.3.  Summary links

            Each summary link advertisement describes a route to a
            single destination.  Summary link advertisements are flooded
            throughout a single area only.  The destination described is
            one that is external to the area, yet still belonging to the
            Autonomous System.

            Summary link advertisements are originated by area border
            routers.  The precise summary routes to advertise into an
            area are determined by examining the routing table structure
            (see Section 11) in accordance with the algorithm described
            below. Note that only intra-area routes are advertised into
            the backbone, while both intra-area and inter-area routes
            are advertised into the other areas.

            To determine which routes to advertise into an attached Area
            A, each routing table entry is processed as follows.
            Remember that each routing table entry describes a set of
            equal-cost best paths to a particular destination:


            o   Only Destination Types of network and AS boundary router
                are advertised in summary link advertisements.  If the
                routing table entry's Destination Type is area border
                router, examine the next routing table entry.

            o   AS external routes are never advertised in summary link
                advertisements.  If the routing table entry has Path-
                type of type 1 external or type 2 external, examine the
                next routing table entry.

            o   Else, if the area associated with this set of paths is
                the Area A itself, do not generate a summary link
                advertisement for the route.[14]

            o   Else, if the next hops associated with this set of paths
                belong to Area A itself, do not generate a summary link
                advertisement for the route.[15] This is the logical
                equivalent of a Distance Vector protocol's split horizon
                logic.

            o   Else, if the routing table cost equals or exceeds the
                value LSInfinity, a summary link advertisement cannot be
                generated for this route.

            o   Else, if the destination of this route is an AS boundary
                router, generate a Type 4 link state advertisement for

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                the destination, with Link State ID equal to the AS
                boundary router's Router ID and metric equal to the
                routing table entry's cost.  These advertisements should
                not be generated if Area A has been configured as a stub
                area.

            o   Else, the Destination type is network. If this is an
                inter-area route, generate a Type 3 advertisement for
                the destination, with Link State ID equal to the
                network's address (if necessary, the Link State ID can
                also have one or more of the network's host bits set;
                see Appendix F for details) and metric equal to the
                routing table cost.

            o   The one remaining case is an intra-area route to a
                network.  This means that the network is contained in
                one of the router's directly attached areas.  In
                general, this information must be condensed before
                appearing in summary link advertisements.  Remember that
                an area has been defined as a list of address ranges,
                each range consisting of an [address,mask] pair and a
                status indication of either Advertise or DoNotAdvertise.
                At most a single Type 3 advertisement is made for each
                range. When the range's status indicates Advertise, a
                Type 3 advertisement is generated with Link State ID
                equal to the range's address (if necessary, the Link
                State ID can also have one or more of the range's "host"
                bits set; see Appendix F for details) and cost equal to
                the smallest cost of any of the component networks. When
                the range's status indicates DoNotAdvertise, the Type 3
                advertisement is suppressed and the component networks
                remain hidden from other areas.

                By default, if a network is not contained in any
                explicitly configured address range, a Type 3
                advertisement is generated with Link State ID equal to
                the network's address (if necessary, the Link State ID
                can also have one or more of the network's "host" bits
                set; see Appendix F for details) and metric equal to the
                network's routing table cost.

                If virtual links are being used to provide/increase
                connectivity of the backbone, routing information
                concerning the backbone networks should not be condensed
                before being summarized into the virtual links' Transit
                areas. Nor should the advertisement of backbone networks
                into Transit areas be suppressed.  In other words, the
                backbone's configured ranges should be ignored when

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                originating summary links into Transit areas.  The
                existence of virtual links is determined during the
                shortest path calculation for the Transit areas (see
                Section 16.1).

            If a router advertises a summary advertisement for a
            destination which then becomes unreachable, the router must
            then flush the advertisement from the routing domain by
            setting its age to MaxAge and reflooding (see Section 14.1).
            Also, if the destination is still reachable, yet can no
            longer be advertised according to the above procedure (e.g.,
            it is now an inter-area route, when it used to be an intra-
            area route associated with some non-backbone area; it would
            thus no longer be advertisable to the backbone), the
            advertisement should also be flushed from the routing
            domain.

            For an example of summary link advertisements, consider
            again the area configuration in Figure 6.  Routers RT3, RT4,
            RT7, RT10 and RT11 are all area border routers, and
            therefore are originating summary link advertisements.
            Consider in particular Router RT4.  Its routing table was
            calculated as the example in Section 11.3.  RT4 originates
            summary link advertisements into both the backbone and Area
            1.  Into the backbone, Router RT4 originates separate
            advertisements for each of the networks N1-N4.  Into Area 1,
            Router RT4 originates separate advertisements for networks
            N6-N8 and the AS boundary routers RT5,RT7.  It also
            condenses host routes Ia and Ib into a single summary link
            advertisement.  Finally, the routes to networks N9,N10,N11
            and Host H1 are advertised by a single summary link
            advertisement.  This condensation was originally performed
            by the router RT11.

            These advertisements are illustrated graphically in Figures
            7 and 8.  Two of the summary link advertisements originated
            by Router RT4 follow.  The actual IP addresses for the
            networks and routers in question have been assigned in
            Figure 15.

              ; summary link advertisement for Network N1,
              ; originated by Router RT4 into the backbone

              LS age = 0                  ;always true on origination
              Options = (T-bit|E-bit)     ;TOS-capable
              LS type = 3                 ;summary link to IP net
              Link State ID = 192.1.2.0   ;N1's IP network number
              Advertising Router = 192.1.1.4       ;RT4's ID

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                     TOS = 0
                     metric = 4

              ; summary link advertisement for AS boundary router RT7
              ; originated by Router RT4 into Area 1

              LS age = 0                  ;always true on origination
              Options = (T-bit|E-bit)     ;TOS-capable
              LS type = 4                 ;summary link to ASBR
              Link State ID = Router RT7's ID
              Advertising Router = 192.1.1.4       ;RT4's ID
                     TOS = 0
                     metric = 14

            Summary link advertisements pertain to a single destination
            (IP network or AS boundary router).  However, for a single
            destination there may be separate sets of paths, and
            therefore separate routing table entries, for each Type of
            Service.  All these entries must be considered when building
            the summary link advertisement for the destination; a single
            advertisement must specify the separate costs (if they
            exist) for each TOS.  The encoding of TOS in OSPF link state
            advertisements is described in Section 12.3.

            Clearing the T-bit in the Options field of a summary link
            advertisement indicates that there is a TOS 0 path to the
            destination, but no paths for non-zero TOS.  This can happen
            when non-TOS-capable routers exist in the routing domain
            (see Section 2.4).

        12.4.4.  Originating summary links into stub areas

            The algorithm in Section 12.4.3 is optional when Area A is
            an OSPF stub area. Area border routers connecting to a stub
            area can originate summary link advertisements into the area
            according to the above Section's algorithm, or can choose to
            originate only a subset of the advertisements, possibly
            under configuration control.  The fewer advertisements
            originated, the smaller the stub area's link state database,
            further reducing the demands on its routers' resources.
            However, omitting advertisements may also lead to sub-
            optimal inter-area routing, although routing will continue
            to function.

            As specified in Section 12.4.3, Type 4 link state
            advertisements (ASBR summary links) are never originated
            into stub areas.

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            In a stub area, instead of importing external routes each
            area border router originates a "default summary link" into
            the area. The Link State ID for the default summary link is
            set to DefaultDestination, and the metric set to the (per-
            area) configurable parameter StubDefaultCost.  Note that
            StubDefaultCost need not be configured identically in all of
            the stub area's area border routers.

        12.4.5.  AS external links

            AS external link advertisements describe routes to
            destinations external to the Autonomous System.  Most AS
            external link advertisements describe routes to specific
            external destinations; in these cases the advertisement's
            Link State ID is set to the destination network's IP address
            (if necessary, the Link State ID can also have one or more
            of the network's "host" bits set; see Appendix F for
            details).  However, a default route for the Autonomous
            System can be described in an AS external link advertisement
            by setting the advertisement's Link State ID to
            DefaultDestination (0.0.0.0).  AS external link
            advertisements are originated by AS boundary routers.  An AS
            boundary router originates a single AS external link
            advertisement for each external route that it has learned,
            either through another routing protocol (such as EGP), or
            through configuration information.

            In general, AS external link advertisements are the only
            type of link state advertisements that are flooded
            throughout the entire Autonomous System; all other types of
            link state advertisements are specific to a single area.
            However, AS external link advertisements are not flooded
            into/throughout stub areas (see Section 3.6).  This enables
            a reduction in link state database size for routers internal
            to stub areas.

            The metric that is advertised for an external route can be
            one of two types.  Type 1 metrics are comparable to the link
            state metric.  Type 2 metrics are assumed to be larger than
            the cost of any intra-AS path.  As with summary link
            advertisements, if separate paths exist based on TOS,
            separate TOS costs can be included in the AS external link
            advertisement.  The encoding of TOS in OSPF link state
            advertisements is described in Section 12.3.  If the T-bit
            of the advertisement's Options field is clear, no non-zero
            TOS paths to the destination exist.

            If a router advertises an AS external link advertisement for

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            a destination which then becomes unreachable, the router
            must then flush the advertisement from the routing domain by
            setting its age to MaxAge and reflooding (see Section 14.1).

            For an example of AS external link advertisements, consider
            once again the AS pictured in Figure 6.  There are two AS
            boundary routers: RT5 and RT7.  Router RT5 originates three
            external link advertisements, for networks N12-N14.  Router
            RT7 originates two external link advertisements, for
            networks N12 and N15.  Assume that RT7 has learned its route
            to N12 via EGP, and that it wishes to advertise a Type 2
            metric to the AS.  RT7 would then originate the following
            advertisement for N12:

              ; AS external link advertisement for Network N12,
              ; originated by Router RT7

              LS age = 0                  ;always true on origination
              Options = (T-bit|E-bit)     ;TOS-capable
              LS type = 5                 ;indicates AS external link
              Link State ID = N12's IP network number
              Advertising Router = Router RT7's ID
                     bit E = 1            ;Type 2 metric
                     TOS = 0
                     metric = 2
                     Forwarding address = 0.0.0.0

            In the above example, the forwarding address field has been
            set to 0.0.0.0, indicating that packets for the external
            destination should be forwarded to the advertising OSPF
            router (RT7).  This is not always desirable.  Consider the
            example pictured in Figure 16.  There are three OSPF routers
            (RTA, RTB and RTC) connected to a common network.  Only one
            of these routers, RTA, is exchanging EGP information with
            the non-OSPF router RTX.  RTA must then originate AS
            external link advertisements for those destinations it has
            learned from RTX.  By using the AS external link
            advertisement's forwarding address field, RTA can specify
            that packets for these destinations be forwarded directly to
            RTX.  Without this feature, Routers RTB and RTC would take
            an extra hop to get to these destinations.

            Note that when the forwarding address field is non-zero, it
            should point to a router belonging to another Autonomous
            System.

            A forwarding address can also be specified for the default
            route.  For example, in figure 16 RTA may want to specify

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            that all externally-destined packets should by default be
            forwarded to its EGP peer RTX.  The resulting AS external
            link advertisement is pictured below.  Note that the Link
            State ID is set to DefaultDestination.

              ; Default route, originated by Router RTA
              ; Packets forwarded through RTX

              LS age = 0                  ;always true on origination
              Options = (T-bit|E-bit)          ;TOS-capable
              LS type = 5                 ;indicates AS external link
              Link State ID = DefaultDestination  ; default route
              Advertising Router = Router RTA's ID
                     bit E = 1            ;Type 2 metric
                     TOS = 0
                     metric = 1
                     Forwarding address = RTX's IP address

            In figure 16, suppose instead that both RTA and RTB exchange
            EGP information with RTX.  In this case, RTA and RTB would
            originate the same set of AS external link advertisements.
            These advertisements, if they specify the same metric, would
            be functionally equivalent since they would specify the same
            destination and forwarding address (RTX).  This leads to a
            clear duplication of effort.  If only one of RTA or RTB
            originated the set of external advertisements, the routing
            would remain the same, and the size of the link state
            database would decrease.  However, it must be unambiguously
            defined as to which router originates the advertisements
            (otherwise neither may, or the identity of the originator
            may oscillate).  The following rule is thereby established:
            if two routers, both reachable from one another, originate
            functionally equivalent AS external advertisements (i.e.,
            same destination, cost and non-zero forwarding address),
            then the advertisement originated by the router having the
            highest OSPF Router ID is used.  The router having the lower
            OSPF Router ID can then flush its advertisement.  Flushing a
            link state advertisement is discussed in Section 14.1.



(page 126 continued on part 6)

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