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RFC 1584


Multicast Extensions to OSPF

Part 4 of 4, p. 76 to 102
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13.  Maintaining the forwarding cache

    A MOSPF router may, for resource reasons, limit the size of its
    forwarding cache. At any time cache entries can be purged to make
    room for newer entries, since the purged entries can always be
    rebuilt when necessary. This memo does not specify an algorithm to
    select which entries to purge. However, care should be taken to
    ensure that any particular entry is not continually rebuilt and then
    purged again (i.e., thrashing should be avoided).

    The building of the forwarding cache has been previously described
    in Section 12. There are events that force one or more forwarding
    cache entries to be deleted; these events are described below. Note
    that deleted cache entries will be rebuilt on an as-needed basis.

    o   When the internal topology of the MOSPF system changes, all
        forwarding cache entries must be deleted. This is because
        internal topology changes may invalidate the previously
        calculated datagram shortest-path trees. Since the multicast
        routing calculation depends on the result of the unicast routing
        calculations, the forwarding cache should be cleared after the
        unicast routing table is rebuilt.  Internal topology changes are
        indicated when both a) a new instance of either a router-LSA or
        a network-LSA is received and b) the contents of the new
        advertisement (other than the LS age, LS sequence number and LS
        checksum fields) are different from the previous instance. This
        covers routers and links going up or down, routers that change
        from being multicast-incapable to being multicast-capable, etc.

    o   When a Type 3 summary-link-LSA (network summary) changes, those
        forwarding cache entries specifying datagram sources belonging
        to the range of addresses described by the updated summary-
        link-LSA must be deleted. See Sections 12.2.3 and 12.2.5.

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    o   Suppose that the content of an AS external-link-LSA changes. If
        the AS external-link-LSA describes an external network N, then
        all forwarding cache entries specifying an external source
        network that is contained in N or that contains N (i.e.,
        external sources that are a subset or a superset of N) must be

    o   When membership in a multicast group changes, all forwarding
        cache entries for the particular group must be deleted. Group
        membership changes are indicated when either a) the content of a
        group-membership-LSA changes or b) an entry in the local group
        database (see Section 8.4) changes.

    o   When the cost to an AS boundary router or to a forwarding
        address specified by one or more AS external-link-LSAs changes,
        all forwarding cache entries specifying an external network as
        datagram source must be deleted. In this case, potentially all
        inter-AS datagram shortest-path trees have been invalidated. The
        forwarding cache entries should be deleted after the new best
        cost to the AS boundary router/forwarding address has been

14.  Other additions to the OSPF specification

    MOSPF requires some modifications to the base OSPF protocol. All
    these modifications are backward-compatible. A router running MOSPF
    will still interoperate with an OSPF router when forwarding unicast
    traffic. Most of the modifications have been described earlier in
    this document. This section collects together those changes which
    have yet to be mentioned, organizing them by the affected Section of

    14.1.  The Designated Router

        This functionality is described in Section 7.3 of [OSPF]. In
        OSPF, a network's Designated Router has two specialized roles.
        First, it originates the network's network-LSA. Second, it
        controls the flooding on the network, in that all of the routers
        on the network synchronize with the Designated Router (and the
        Backup Designated Router) only.  For these reasons[32], when one
        or more of the network's routers are running MOSPF, the
        Designated Router should be running MOSPF also.  This can be
        ensured by assigning all non-multicast routers the Router
        Priority of 0.

        In MOSPF, the Designated Router also has the additional
        responsibility of monitoring the network's multicast group
        membership. This is done by periodically sending Host Membership

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        Queries, and receiving Host Membership Reports in response (see
        Section 9). This is yet another reason why the Designated Router
        must be multicast-capable.

    14.2.  Sending Hello packets

        This functionality is described in Section 9.5 of [OSPF]. A
        MOSPF router sets the MC-bit in the Options field of its Hello
        packets. This indicates that the router is multicast-capable; it
        does not necessarily indicate the state of the sending
        interface's IPMulticastForwarding parameter (see Section B.2).
        Setting the MC-bit in Hellos is done strictly for informational
        purposes. Neighbors receiving the router's Hello packets do not
        act on the state of the MC-bit. A neighbor's multicast-
        capability is learned instead during the Database Exchange
        Process (see Section 14.4).

    14.3.  The Neighbor state machine

        This functionality is described in Section 10.3 of [OSPF]. When
        a neighbor enters state Exchange, the neighbor Database summary
        list is initialized (see the OSPF neighbor FSM entry for State:
        ExStart and Event: NegotiationDone). This list describes of the
        portion of the router's link state database that needs to be
        synchronized with the neighbor.  Group-membership-LSAs are
        included in the neighbor Database summary list if and only if
        the neighbor is multicast-capable. The neighbor's multicast
        capability is learned by examining the neighbor's Database
        Description packets (see Section 14.4).

    14.4.  Receiving Database Description packets

        This functionality is described in Section 10.6 of [OSPF]. A
        neighbor's multicast-capability is learned through received
        Database Description packets. When the Database Description
        packet is received that transitions the neighbor from ExStart to
        Exchange, the state of the MC-bit in the packet's Options field
        is examined. The neighbor is multicast-capable if and only if
        the MC-bit is set.

        The neighbor's multicast capability controls whether group-
        membership-LSAs are summarized to the neighbor during the
        Database Exchange process (see Section 14.3), and whether
        group-membership-LSAs are flooded to the neighbor during the
        flooding process (see Section 10.2).

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    14.5.  Sending Database Description packets

        This functionality is described in Section 10.8 of [OSPF]. A
        MOSPF router sets the MC-bit in the Options field of its
        Database Description packets. This indicates to its adjacent
        neighbors that the router is multicast-capable; it does not
        necessarily indicate the state of the sending interface's
        IPMulticastForwarding parameter (see Section B.2).

        When a router goes from being multicast-capable to multicast-
        incapable, or vice-versa, it must indicate this fact to its
        adjacent neighbors by restarting the Database Description
        process (i.e., rolling back the state of all adjacent neighbors
        to Exstart).

    14.6.  Originating Router-LSAs

        This functionality is described in Section 12.4.1 of [OSPF]. A
        MOSPF router sets the MC-bit in the Options field of its
        router-LSA. This allows the router to be included in datagram
        shortest-path trees (see Step 5a of Section 12.2).

        In addition, MOSPF has introduced a new flag in the router-LSA's
        rtype field: the W-bit. When the W-bit is set, the router is
        included on all datagram shortest-path trees, regardless of
        multicast group (see Section 12.2.6). Such a router is called a
        wild-card multicast receiver. The router sets the W-bit when it
        wishes to receive all multicast datagrams, regardless of
        destination. This will sometimes be true of inter-area multicast
        forwarders (see Section 3.1), and inter-AS multicast forwarders
        (see Section 4).

        A router must originate a new instance of its router-LSA
        whenever an event occurs that would invalidate the LSA's current
        contents. In particular, if the router's multicast capability or
        its ability to function as either an inter-area or inter-AS
        multicast forwarder changes, its router-LSA must be

    14.7.  Originating Network-LSAs

        This functionality is described in Section 12.4.2 of [OSPF]. In
        OSPF, a transit network's network-LSA is originated by the
        network's Designated Router. The Designated Router sets the MC-
        bit in the Options field of the network-LSA if and only if both
        a) the Designated Router is multicast-capable (i.e., running
        MOSPF) and b) the Designated Router's interface's
        IPMulticastForwarding parameter has been set to a value other

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        than disabled (see Section B.2). When the network-LSA has the
        MC-bit set, the network can be included in datagram shortest-
        path trees (see Section 12.2.6).

        It is intended that all routers attached to a common network
        agree on the network's IPMulticastForwarding capability.
        However, this agreement is not enforced. When there are
        disagreements, incorrect routing of multicast datagrams can

    14.8.  Originating Summary-link-LSAs

        This functionality is described in Section 12.4.3 of [OSPF].
        Inter-area multicast forwarders always set the MC-bit in the
        Options field of their summary-link-LSAs, regardless of whether
        the path described by the summary-link-LSA is actually
        multicast-capable. Indeed, it is possible that there is no
        multicast-capable path to the described destination. All other
        area border routers (ones that are not inter-area multicast
        forwarders) clear the MC-bit in the Options field of their

        If its MC-bit is clear, the summary-link-LSA will not be used
        when initializing the candidate list in Sections 12.2.2, 12.2.3
        and 12.2.5.

    14.9.  Originating AS external-link-LSAs

        This functionality is described in Section 12.4.4 of [OSPF].
        Unlike in summary-link-LSAs, an inter-AS multicast forwarder
        should clear the MC-bit in the Options field of one of its AS
        external-link-LSAs if it is known that there is no multicast-
        capable path from the described destination to the router
        itself. This knowledge may possibly be obtained, for example,
        from an inter-AS multicast routing algorithm (see Section 4).
        If the inter-AS multicast forwarder is unsure of whether a
        multicast-capable path exists between the described destination
        and the router itself, the MC-bit should be set in the AS
        external-link-LSA.  All other AS boundary routers (ones that are
        not inter-AS multicast forwarders) clear the MC-bit in the
        Options field of their AS external-link-LSAs.

        If its MC-bit is clear, the AS external-link-LSA will not be
        used when initializing the candidate list in Section 12.2.4.

        When multicast connectivity to an external destination exists,
        but no unicast connectivity, an AS external-link-LSA can be
        originated having its MC-bit set and specifying a cost of

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        LSInfinity. Such an AS external-link-LSA will still be used by
        the multicast routing calculation (see Section 12.2.4). As a
        result, when a MOSPF router wishes to stop advertising an AS
        external destination, it must use the premature aging procedure
        specified in Section 14.1 of [OSPF], rather than simply setting
        the AS external-link-LSA's cost to LSInfinity.

    14.10.  Next step in the flooding procedure

        This functionality is described in Section 13.3 of [OSPF].
        Group-membership-LSAs are specific to a OSPF single area, and
        are flooded to multicast-capable routers only. When flooding a
        group-membership-LSA, Section 13.3 of the OSPF specification is
        modified as follows: 1) The list of interfaces examined during
        flooding (called the eligible interfaces in Section 13.3 of
        [OSPF]) is the set of all interfaces attaching to Area A (the
        area that the group-membership-LSA is received from), just as
        for router-LSAs, network-LSAs and summary-link-LSAs. 2) When
        examining each interface, a group-membership-LSA is added to a
        neighbor's link state retransmission list if and only if both a)
        Step 1d of [OSPF]'s Section 13.3 is reached for the neighbor and
        b) the neighbor is multicast-capable. The neighbor's multicast
        capability is discovered during the Database Exchange process
        (see Section 14.4).

        Note that, since on broadcast networks Link State Update packets
        are sent initially as multicasts, non-multicast routers may
        receive group-membership-LSAs. However, non-multicast routers
        will simply drop the group-membership-LSAs, for reasons of
        unrecognized LS type (see Step 2 of [OSPF]'s Section 13). Link
        State acknowledgments for group-membership-LSAs are not expected
        from non-multicast routers, and group-membership-LSAs will never
        be retransmitted to non-multicast routers, since the LSAs are
        not added to these routers' link state retransmission lists (see
        above paragraph).

        For more information on flooding group-membership-LSAs, see
        Section 10.2.

    14.11.  Virtual links

        This functionality is described in Section 15 of [OSPF]. When a
        MOSPF router (i.e., multicast-capable router) is both an area
        border router and an endpoint of a virtual link whose other
        endpoint is also multicast capable, the router must then also be
        an inter-area multicast forwarder. This is necessary to ensure
        that multicast datagrams will flow through the virtual link's
        transit area, from one endpoint to the other. When the

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        backbone's datagram shortest-path tree is constructed in Section
        12.1, it is assumed that virtual links are capable of forwarding
        multicast datagrams whenever both endpoints are multicast-

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15.  References

    [Bharath-Kumar] Bharath-Kumar, K. and J. Jaffe, "Routing to Multiple
                    Destinations in Computer Networks", IEEE
                    Transactions on Communications, COM-31[3], March

    [Deering]       Deering, S., "Multicast Routing in Internetworks and
                    Extended LANs", SIGCOMM Summer 1988 Proceedings,
                    August 1988.

    [Deering2]      Deering, S., "Multicast Routing in a Datagram
                    Internetwork", Stanford Technical Report, STAN-CS-
                    92-1415, Department of Computer Science, Stanford
                    University, December 1991.

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

    [RFC 1075]      Waitzman, D., Partridge, C., and S. Deering,
                    "Distance Vector Multicast Routing Protocol", RFC
                    1075, BBN STC, Stanford University, November 1988.

    [RFC 1112]      Deering, S., "Host Extensions for IP Multicasting",
                    STD 5, RFC 1112, Stanford University, May 1988.

    [RFC 1209]      Piscitello, D., and J. Lawrence, "Transmission of IP
                    Datagrams over the SMDS Service", RFC 1209, Bell
                    Communications Research, March 1991.

    [RFC 1340]      Reynolds, J. and J. Postel, "Assigned Numbers", STD
                    2, RFC 1340, USC/Information Sciences Institute,
                    July 1992.

    [RFC 1390]      Katz, D., "Transmission of IP and ARP over FDDI
                    Networks", STD 36, RFC 1390, cisco Systems, Inc.,
                    January 1993.

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    [1]Actually, OSPF allows a separate link cost to be configured for
    each TOS. MOSPF then potentially calculates separate paths for each
    TOS. For details, see Section 6.2.

    [2]We also assume in this section that the pictured multi-access
    networks provide data-link multicast/broadcast services.

    [3]Note that if N3 were a non-broadcast network, Router RT3 would
    send separate copies of the datagram to routers RT1 and RT2. Since
    the IGMP protocol is not defined on non-broadcast networks, there
    could in this case be no Group B member attached to Network N3.
    However the multicast datagram would still be delivered to the Group
    B members attached to networks N1 and N2.

    [4]Actually, in MOSPF there is a separate forwarding cache entry for
    each combination of source, destination and TOS. For a discussion of
    TOS-based multicast routing, see Section 6.2.

    [5]The discussion in this section omits mention of the Backup
    Designated Router's role in the IGMP protocol. While the Backup
    Designated Router does not send IGMP Host Membership Queries, it
    does listen to IGMP Host Membership Reports, building "shadow" local
    group database entries in the process. These entries do not lead to
    group-membership-LSAs, nor do they influence delivery of multicast
    datagrams, but are merely maintained to ease the transition from
    Backup Designated Router to Designated Router, should the Designated
    Router fail. See Sections 2.3.4, 9 and 10 for details.

    [6]One might imagine building all possible datagram shortest-path
    trees up front. However, this might be expensive, both in router CPU
    time and in router memory. It is hoped that building the datagram
    shortest-path trees on demand and caching the results will ease
    demands on router resources by spreading out the calculations over a
    longer period of time.

    [7]It is possible that, due to the existence of alternate paths,
    several different shortest-path trees are available. MOSPF depends
    on all routers constructing the exact same shortest path tree. For
    that reason, tie-breaking schemes have been implemented during tree
    construction to ensure that identical trees result. See Section 12
    for more details.

    [8]Note that the expanding ring search yields the nearest server in
    terms of hop count, but not necessarily in terms of the OSPF metric.

    [9]This means that in MOSPF, just as in OSPF, the only kind of link

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    state advertisement that can be flooded between areas is the AS

    [10]A router indicates that it is a wild-card multicast receiver by
    setting the appropriate flag in its router-LSA. See Section 14.6 for

    [11]This is not quite true. As we shall see, any inter-AS multicast
    forwarders belonging to the backbone are designated as wild-card
    multicast receivers. See Section 4.

    [12]It is possible that through the operation of an inter-AS
    multicast routing protocol, Router RT7 knows that it does not have
    multicast connectivity to Network N15 (even though it has unicast
    connectivity). In this case, RT7 would not advertise the external
    link to N15 as being multicast capable.

    [13]Synchronization of the IPMulticastForwarding interface parameter
    is not enforced by the MOSPF protocol, since it is not included in
    the contents of a MOSPF router's Hello packets.

    [14]Actually, when multiple IP networks have been assigned to the
    same physical network, the first thing that needs to be done is to
    associate an IP network with the received Host Membership Report.
    This is done in the same way that a receiving interface is
    associated with a received multicast datagram; see Section 11.1.

    [15]For this reason when a transit network has both MOSPF routers
    and non-multicast OSPF routers attached, care should be taken to
    ensure that a MOSPF router is elected Designated Router. This can be
    accomplished through proper setting of the routers' configured
    Router Priority.

    [16]Note that just because these advertisements exist in the link
    state database, it does not mean that the Group G members are
    reachable.  Reachability does not enter into the building of the
    transit vertex list, in order to simplify the calculation. This is a
    trade-off. As a result, some multicast datagrams may be forwarded
    further than necessary, when the described Group G members actually
    are unreachable.

    [17]Since the Designated Router controls flooding on the network,
    this is another reason to ensure that a MOSPF router is elected as
    Designated Router.

    [18]In other words, group-membership-LSAs will never be
    retransmitted to non-multicast routers.

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    [19]This last step will not be necessary if the configuration
    guidelines presented in Section 6.5 are followed.

    [20]The TOS 0 routing table entry is examined regardless of the TOS
    specified by the multicast datagram.

    [21]It is assumed that a MOSPF router that wants to stop advertising
    a route to an external destination will use the premature aging
    procedure specified in Section 14.1 of [OSPF], rather than setting
    the AS external-link-LSA's cost to LSInfinity.

    [22]This preference ordering is used in Step 5c of Section 12.2.

    [23]No attempt is made to match the links' two halves. See Step 5d.

    [24]However, a summary-link-LSA is eligible for matching even if the
    MC-bit in its Options field is clear.

    [25]Costs may have both a Type 2 and a Type 1 component; the Type 2
    component is always most significant.

    [26]This case mirrors the SourceIntraArea candidate list
    initialization in Section 12.2.1.

    [27]This case mirrors the SourceInterArea1 candidate list
    initialization in Section 12.2.2.

    [28]This case mirrors the SourceInterArea2 candidate list
    initialization in Section 12.2.3.

    [29]Note that selecting the upstream node in this manner enforces
    the inter-area routing architecture outlined in Section 3.1. Namely,
    the multicast datagram is forwarded from the source area, over the
    backbone and then into the non-backbone areas. This is similar to
    the "hub and spoke" architecture for unicast forwarding described in
    Section 3.2 of [OSPF].

    [30]This procedure seems backwards. One would expect that the TOS X
    datagram tree would be built first. However, the SPF calculation
    must ensure that all routers participating in the forwarding of that
    datagram, both TOS-capable and non-TOS-capable, build the same tree.
    Since it is known that the non-TOS-capable routers will use the TOS
    0 tree, the only safe way to use the TOS X tree is when you are
    guaranteed that the non-TOS-capable routers will decline to forward
    the datagram. This guarantee is clearly met when there are only
    TOS-capable routers on the TOS 0 datagram tree.

    [31]Indeed, there will also be those cases where the router, not

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    being on a particular datagram shortest-path tree, will never have
    to calculate the particular tree, since the router will not receive
    the datagram in the first place.

    [32]Group-membership-LSAs are not processed by non-multicast routers
    (see Section 10.2). Also, if the Designated Router was not running
    the multicast extensions, multicast datagrams would not be forwarded
    over the network because its network-LSA would have its MC-bit clear
    (see Step 5a in Section 12.2).

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A. Data Formats

    This section documents the format of MOSPF protocol packets and link
    state advertisements (LSAs). All changes and additions made to the
    OSPF Version 2 data formats have been made in a backward-compatible
    manner. In other words, multicast routers running MOSPF can
    interoperate with (non-multicast) OSPF Version 2 routers when
    forwarding regular (unicast) IP data traffic.

    The MOSPF packet formats are the same as for OSPF Version 2
    (described in Appendix A of [OSPF]). One additional option has been
    added to the Options field that appears in OSPF Hello packets,
    Database Description packets and all link state advertisements. This
    new option indicates a router's/network's multicast capability, and
    is documented in Section A.1.  The presence of this new option is
    ignored by all non-multicast routers.

    To support MOSPF, one of OSPF's link state advertisements has been
    modified, and a new link state advertisement has been added. The
    format of the router-LSA has been modified (see Section A.2) to
    include a new flag indicating whether the router is a wild-card
    multicast receiver. A new link state advertisement, called the
    group-membership-LSA, has been added to pinpoint multicast group
    members in the link state database. This new advertisement is
    neither flooded nor processed by non-multicast routers. The group-
    membership-LSA is documented in Section A.3.

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A.1 The Options field

    The OSPF Options field is present in OSPF Hello packets, Database
    Description packets and all link state advertisements. 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 LSA types to a neighbor
    because of its reduced functionality. Lastly, listing capabilities
    in LSAs allows routers to route traffic around reduced functionality
    routers, by excluding them from parts of the routing table

    Three capabilities are currently defined. For each capability, the
    effect of the capability's appearance (or lack of appearance) in
    Hello packets, Database Description packets and link state
    advertisements is specified below. For example, the
    ExternalRoutingCapability (below called the E-bit) has meaning only
    in OSPF Hello packets.

                     | * | * | * | * | * |MC | E | T |

                          The OSPF Options field

    o   T-bit. This describes the router's TOS capability. If the T-bit
        is reset, then the router supports only a single TOS (TOS 0).
        Such a router is also said to be incapable of TOS-routing. The
        absence of the T-bit in a router links advertisement causes the
        router to be skipped when building a non-zero TOS shortest-path
        tree. In other words, routers incapable of TOS routing will be
        avoided as much as possible when forwarding data traffic
        requesting a non-zero TOS. The absence of the T-bit in a summary
        link advertisement or an AS external link advertisement
        indicates that the advertisement is describing a TOS 0 route
        only (and not routes for non-zero TOS).

    o   E-bit. AS external link advertisements are not flooded
        into/through OSPF stub areas. The E-bit ensures that all members
        of a stub area agree on that area's configuration. The E-bit is
        meaningful only in OSPF Hello packets. When the E-bit is reset

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        in the Hello packet sent out a particular interface, it means
        that the router will neither send nor receive AS external link
        state advertisements on that interface (in other words, the
        interface connects to a stub area). Two routers will not become
        neighbors unless they agree on the state of the E-bit.

    o   MC-bit. The MC-bit describes the multicast capability of the
        various pieces of the OSPF routing domain. When calculating the
        path of multicast datagrams, only those link state
        advertisements having their MC-bit set are used. In addition, a
        router uses the MC-bit in its Database Description packets to
        tell adjacent neighbors whether the router will participate in
        the flooding of the new group-membership-LSAs.

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A.2 Router-LSA

    An OSPF router originates a router-LSA into each of its attached
    areas. The router-LSA describes the state and cost of the router's
    interfaces to the area. The contents of the router-LSA are described
    in detail in Section A.4.2 of [OSPF]. There are flags in the
    router-LSA that indicate whether the router is either a) an area
    border router or b) an AS boundary router or c) the endpoint of a
    virtual link. One more flag has been added to the router-LSA for
    MOSPF; it is called bit W below. This flag indicates whether the
    router wishes to receive all multicast datagrams regardless of
    destination (i.e., is a wild-card multicast receiver).

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |            LS age             |     Options   |       1       |
       |                        Link State ID                          |
       |                     Advertising Router                        |
       |                     LS sequence number                        |
       |         LS checksum           |             length            |
       |    rtype      |        0      |            # links            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
       |                          Link ID                              | P
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ E
       |                         Link Data                             | R
       |     Type      |     # TOS     |        TOS 0 metric           | #
     + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L
     # |      TOS      |        0      |            metric             | I
     T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
     O |                              ...                              | K
     S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ S
     | |      TOS      |        0      |            metric             | |
     + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
       |                              ...                              |

                                The router LSA

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                     | * | * | * | * | W | V | E | B |

                                The rtype field

    The following defines the flags found in the rtype field. Each flag
    classifies the router by function:

    o   bit B. When set, the router is an area border router (B is for
        border). These routers forward unicast data traffic between OSPF

    o   bit E. When set, the router is an AS boundary router (E is for
        external). These routers forward unicast data traffic between
        Autonomous Systems.

    o   bit V. When set, the router is an endpoint of an active virtual
        link (V is for virtual) which uses the described area as its
        Transit area.

    o   bit W. When set, the router is a wild-card multicast receiver.
        These routers receive all multicast datagrams, regardless of
        destination.  Inter-area multicast forwarders and inter-AS
        multicast forwarders are sometimes wild-card multicast receivers
        (see Sections 3 and 4).

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A.3 Group-membership-LSA

    Group-membership-LSAs are the Type 6 link state advertisements.
    Group-membership-LSAs are specific to a particular OSPF area. They
    are never flooded beyond their area of origination. A router's
    group-membership-LSA for Area A indicates its directly attached
    networks which belong to Area A and contain members of a particular
    multicast group. A router originates a group-membership-LSA for
    multicast group D when the following conditions are met for at least
    one directly attached network: 1) the router has been elected
    Designated Router for the network and 2) at least one host on the
    network has joined Group D via the IGMP protocol.

    A router may also originate a group-membership-LSA for Group D if
    the router itself has internal applications belonging to Group D. In
    addition, area border routers originate group-membership-LSAs into
    the backbone area when there are group members in the router's
    attached non-backbone areas. See Section 10 for more information
    concerning the origination of group-membership-LSAs.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |            LS age             |     Options   |       6       |
       |              Link State ID = Destination Group                |
       |                     Advertising Router                        |
       |                     LS sequence number                        |
       |         LS checksum           |             length            |
       |                        Vertex type                            |
       |                         Vertex ID                             |
       |                              ...                              |

                           The group-membership-LSA

    The group-membership-LSA consists of the standard 20-byte link state
    header (see Section A.4.1 of [OSPF]) followed by a list of transit
    vertices to label with the multicast destination. The
    advertisement's Link State ID is set to the destination multicast
    group address. There is no metric associated with the advertisement.
    Each transit vertex is specified by its Vertex type and Vertex ID

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    (see Section 12.1 for an explanation of this terminology):

    o   Vertex type. Set equal to 1 for a router, and 2 for a transit
        network.  Note that the only router that may be included in the
        list is the Advertising Router itself.

    o   Vertex ID. For router vertices, this field indicates the
        router's OSPF Router ID. For transit network vertices, this
        field indicates the IP address of the network's Designated
        Router. Note that the link state advertisement associated with
        the transit vertex is the LSA whose LS type = Vertex type, Link
        State ID = Vertex ID and Advertising Router = the group-
        membership-LSA's Advertising Router.

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B. Configurable Constants

    This section documents the configurable parameters used by OSPF's
    multicast routing extensions. These parameters are in addition to
    the configurable constants used by the base OSPF protocol
    (documented in Appendix C of [OSPF]). An implementation of MOSPF
    must provide the ability to set these parameters, either through
    network management or some other means.

    B.1 Global parameters

        The following parameters apply to the router as a whole.

        o   Multicast capability. An indication of whether the router is
            running MOSPF. If the router is running MOSPF, it will
            perform the algorithms as set forth in this specification.
            Otherwise, the router is still able to run the basic OSPF
            algorithm (as set forth in [OSPF]), and will be able to
            interoperate with multicast capable routers (see Section
            6.1) when forwarding regular (unicast) IP data traffic.

        o   Inter-area multicast forwarder. This parameter indicates
            whether the router will forward multicast datagrams between
            OSPF areas. Such a router summarizes group membership
            information to the backbone, and acts as a wild-card
            multicast receiver in all its attached non-backbone areas
            (see Section 3.1). Not all multicast-capable area border
            routers need be configured as inter-area multicast
            forwarders.  However, whenever both ends of a virtual link
            are multicast-capable, they must both be configured as
            inter-area multicast forwarders (see Section 14.11). By
            default, all multicast-capable area border routers are
            configured as inter-area multicast forwarders.

        o   Inter-AS multicast forwarder. This parameter indicates
            whether the router forwards multicast datagrams between
            Autonomous Systems. Such a router acts as a wild-card
            multicast receiver in all attached areas (see Section 4). It
            is also assumed that an inter-AS multicast forwarder runs
            some kind of inter-AS multicast routing algorithm.

    B.2 Router interface parameters

        The following parameters can be configured separately for each
        of the router's OSPF interfaces. Remember that an OSPF interface
        is the connection between the router and one of its attached IP
        networks.  Note that the IPMulticastForwarding parameter is
        really a description of the attached network. As such, it should

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        be configured identically on all routers attached to a common
        network; otherwise incorrect routing of multicast datagrams may

        o   IPMulticastForwarding. This configurable parameter indicates
            whether IP multicasts should be forwarded over the attached
            network, and if so, how the forwarding should be done. The
            parameter can assume one of three possible values: disabled,
            data-link multicast and data-link unicast. When set to
            disabled, IP multicast datagrams will not be forwarded out
            the interface. When set to data-link multicast, IP multicast
            datagrams will be forwarded as data-link multicasts. When
            set to data-link unicast, IP multicast datagrams will be
            forwarded as data-link unicasts. The default value for this
            parameter is data-link multicast. The other two settings are
            for use in the special circumstances described in Sections
            6.3 and 6.4. When set to disabled or to data-link unicast,
            IGMP group membership is not monitored on the attached

        o   IGMPPollingInterval. The number of seconds between IGMP Host
            Membership Queries sent out this interface. A multicast-
            capable router sends IGMP Host Membership Queries only when
            it has been elected Designated Router for the attached
            network. See [RFC 1112] for a discussion of this parameter's

        o   IGMP timeout. If no IGMP Host Membership Reports have been
            heard on an attached network for a particular multicast
            group A after this period of time, the entry [Group A,
            attached network] is deleted from the router's local group
            database. See Section 9 for more information.

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C. Sample datagram shortest-path trees

    In MOSPF, all routers must calculate exactly the same datagram
    shortest-path trees. In order to ensure this in internetworks having
    redundant links, a number of tie-breakers were defined in the MOSPF
    routing table calculation (see Steps 4 and 5c of Section 12.2, and
    Sections 12.2.4 and 12.2.7). This section illustrates the use of
    these tie-breakers on a sample topology.

    Three different examples are given. All examples use the same
    physical topology and the same set of OSPF interface costs (see the
    left side of Figure 14). The source of the datagram is always Host
    H1 on the network at the top of the figure (, and the
    destination group members are the two hosts labelled with Group Ma
    at the bottom of the figure. The first case shows an example of
    intra-area multicast, while the remaining two cases show the
    influence of OSPF areas on the path of a multicast datagram.

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C.1 An intra-area tree

    The datagram shortest-path tree resulting from the intra-area case
    is shown on the right of Figure 14. The root of the tree is the
    source network (, and the leaves are the two routers (RT4
    and RT3) directly attached to the stub networks containing Group Ma

    There are equal-cost paths available to both group members. For the
    group member on the left, the path could go either through network or through network By the tie-breaking rules, the
    path through is chosen since it has the larger IP network
    number (see Step 5c of Section 12.2).

    For the group member on the right, the path could go either over
    Network or over the serial line connecting routers RT2 and
    RT3. The path over Network is chosen after executing two
    tie-breaking rules. First, Network is placed on the
    shortest-path tree before Router RT3 since networks are always
    chosen over routers (see Step 4 of Section 12.2). Then, given a

                    Net  |
                            |            |
        +----------+        |1           |1
        |  Network |     8+---+        +---+            o
        | |------|RT1|        |RT2|            |
        +----------+      +---+        +---+           0|
             |              |8          8|              |
            8|         +----------+      |8             o RT1
           +---+10     | Network  |  10+---+            |
           |RT4|-------| |----|RT3|           8|
           +---+       +----------+    +---+            |
             |3                          |3             o
             |                           |             / \
        +---------+                  +-------+       0/   \0
             |                           |           /     \
           +--+                        +--+         o       o
           |Ma|                        |Ma|        RT4      RT3
           +--+                        +--+

                        Figure 14: An intra-area tree

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    choice of either Network or Router RT2 for RT3's parent on
    the tree, Net is again preferred since it is a network (see
    Step 5c of Section 12.2)

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C.2 The effect of areas

    In Figure 15 below, the previous diagram has been modified by the
    inclusion of OSPF areas. The datagram source is now part of the OSPF
    backbone (Area 0), while the rest of the topology is in Area 1. In
    this case, since the datagram source and the group members belong to
    different areas, reverse costs are used when building the tree (see
    Step 5b of Section 12.2). This actually eliminates the equal cost
    paths from the diagram, and leads to the Area 1 datagram shortest-
    path tree on the right of Figure 15.

                    Net  |
      ..................... |            |
      . +----------+      . |1           |1  
      . |  Network |     8+---+        +---+                o
      . | |------|RT1|........|RT2|...            / \
      . +----------+      +---+        +---+  .          1/   \1
      .      |              |8          8|    .          /     \
      .     8|         +----------+      |8   .         o RT1   o RT2
      .    +---+10     | Network  |  10+---+  .         |        \
      .    |RT4|-------| |----|RT3|  .        0|         \8
      .    +---+       +----------+    +---+  .         |          \
      .      |3                          |3   .         o  o
      .      |                           |    .         |          RT3
      . +---------+                  +-------+.        8|
      .      |                           |    .         |
      .    +--+                        +--+   .         o
      .    |Ma|                        |Ma|   .        RT4
      .    +--+     Area 1             +--+   .

                        Figure 15: The effect of areas

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C.3 The effect of virtual links

    In Figure 16 below, Network has been configured as a
    separate area (Area 1), while everything else belongs to the OSPF
    backbone (Area 0). In addition, a virtual link has been configured
    through Area 1, enhancing the backbone connectivity. In this case,
    both the source and the group members belong to the same area, so
    forward costs are used. However, since virtual links are preferred
    over regular links (see Step 5c of Section 12.2), the backbone
    datagram shortest-path tree uses Network instead of on the path to the left group member. This leads to the
    tree on the right of Figure 16.

                    Net  |
      ................   +------------------+
      . +----------+ .     /1            |
      . |  Network |8.    /              |1
      . | |-+---+             +---+            o
      . +----------+*|RT1|             |RT2|            |
      .     8|*******+---+             +---+           0|
      .Area1 |*VL    .    \8            8|              |
      .....+---+...... +----------+      |8             o RT1
           |RT4|10     | Network  |  10+---+           / \
           +---+-------| |----|RT3|          /8  \8
             |         +----------+    +---+         /     \
             |3                          |3         o 10.1  o
             |                           |          |       |
        +---------+                  +-------+      |0      |0
             |                           |          |       |
           +--+                        +--+         o       o
           |Ma|                        |Ma|        RT4      RT3
           +--+                        +--+

                   Figure 16: The effect of virtual links

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Security Considerations

    Security issues are not discussed in this memo.

Author's Address

    John Moy
    Proteon, Inc.
    9 Technology Drive
    Westborough, MA 01581
    Phone: (508) 898-2800