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RFC 1584


Multicast Extensions to OSPF

Part 2 of 4, p. 27 to 52
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4.  Inter-AS multicasting

    This section explains how MOSPF deals with the forwarding of
    multicast datagrams between Autonomous Systems. Certain AS boundary
    routers in a MOSPF system will be configured as inter-AS multicast
    forwarders. It is assumed that these routers will also be running an
    inter-AS multicast routing protocol. This specification does not
    dictate the operation of such an inter-AS multicast routing
    protocol. However, the following interactions between MOSPF and the
    inter-AS routing protocol are assumed:

    (1) MOSPF guarantees that the inter-AS multicast forwarders will
        receive all multicast datagrams; but it is up to each router so
        designated to determine whether the datagram should be forwarded
        to other Autonomous Systems. This determination will probably be
        made via the inter-AS routing protocol.

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    (2) MOSPF assumes that the inter-AS routing protocol is forwarding
        multicast datagrams in an RPF (reverse path forwarding; see
        [Deering] for an explanation of this terminology) fashion. In
        other words, it is assumed that a multicast datagram whose
        source (call it X) lies outside the MOSPF domain will enter the
        MOSPF domain at those points that are advertising (into OSPF)
        the best routes back to X. MOSPF calculates the path of the
        datagram through the MOSPF domain based on this assumption.

    MOSPF designates an inter-AS multicast forwarder as a wild-card
    multicast receiver in all of its attached areas. As in the inter-
    area case, this ensures that the routers remain on all pruned
    shortest-path trees and thereby receive all multicast datagrams,
    regardless of destination.

    As an example, suppose that in Figure 1 both RT5 and RT7 were
    configured as inter-AS multicast forwarders. Then the link state
    database would look like the one pictured in Figure 2, with the
    addition of a) wild-card status for RT5 and RT7 (they would appear
    with superscripts of "w") and b) the external links originated by
    RT5 and RT7 being labelled as multicast-capable[12].

    As another example, consider the area configuration in Figure 4.
    Again suppose RT5 and RT7 are configured as inter-AS multicast
    forwarders. Then in Area 1's link state database (Figure 6), the
    external links originated by RT5 and RT7 would again be labelled as
    multicast-capable. However, note that in Area 1's database RT5 and
    RT7 are not labelled as wild-card multicast receivers. This is
    unnecessary; since Area 1's inter-area multicast forwarders (RT3 and
    RT4) are wild-cards, all multicast datagrams will be forwarded to
    the backbone. And in the backbone's link state database (Figure 7),
    RT5 and RT7 will be labelled as wild-cards.

    4.1.  Building inter-AS datagram shortest-path trees.

        When multicast datagrams are to be forwarded between Autonomous
        Systems, the datagram shortest-path tree is built as follows.
        Remember that the router builds a separate tree for each area to
        which it is attached; these trees are then merged into a single
        forwarding cache entry. Suppose that the router is building the
        tree for Area A. We break up the tree building into three cases.
        This first two cases have already been described earlier in this
        memo: Case 1 (the source of the datagram belongs to Area A)
        having been described in Section 2.3.2 and Case 2 (the source of
        the datagram belongs to another OSPF area) having been described
        in Section 3.2. The only modification to these cases is that
        inter-AS multicast forwarders, as well as group members and
        inter-area multicast forwarders, must remain on the pruned

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        trees.  The new case is as follows:

        o   Case 3: The source of the datagram belongs to another
            Autonomous System. The immediate neighborhood of the source
            is then unknown. In this case the multicast-capable AS
            external links are used to approximate the neighborhood of
            the source; the tree begins with links directly attaching
            the source to one or more inter-AS multicast forwarders. The
            approximating AS external links point in the reverse
            direction (i.e., towards the source), just as with the
            approximating summary links in Case 2. Also, as in Case 2,
            all links included in the tree must point in the reverse
            direction. The final datagram shortest-path tree is then
            produced (as always) by pruning those branches having no
            group members nor wild-card multicast receivers.

            As an example, suppose that a host on Network N12 (see
            Figure 4) originates a multicast datagram for Destination
            Group B. Assume that all external costs pictured are OSPF
            external type 1 metrics. Then any routers in Area 1
            receiving the datagram would build the datagram shortest-
            path tree pictured in Figure 10. Note that all links in the
            tree point in the reverse direction, towards the source. The
            tree indicates that the routers expect the datagram to enter
            the Autonomous System at Router RT7, and then to enter the
            area at Router RT4.

            Note that in those cases where the "best" inter-AS multicast
            forwarder is not directly attached to the area, the
            neighborhood of the source is actually approximated by the
            concatenation of a summary link and a multicast-capable AS
            external link. This is in fact the case in Figure 10.

        In Case 3 (datagram source in another AS) the requirement that
        all tree links point in the reverse direction (towards the
        source) accommodates the fact that summary links and AS external
        links already point in the reverse direction. This also leads to
        the requirement that the inter-AS multicast routing protocol
        operate in a reverse path forwarding fashion (see condition 2 of
        Section 4). Note that Reverse path forwarding can lead to sub-
        optimal routing when costs are configured asymmetrically. And it
        can even lead to non-delivery of multicast datagrams in the case
        of asymmetric reachability.

        Inter-AS multicast forwarders may end up calculating a
        forwarding cache entry's upstream node as being external to the
        AS. As an example, Router RT7 in Figure 10 will end up
        calculating an external router (via its external link to Network

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                                     o N12
                                     o RT7
                                     o RT4 (W)
                                     o N3 (Mb)
                                   / | \
                                 1/  | 1\
                                 /  1|   \
                                /    |    \
                      RT1 (Mb) o     |     o RT3 (W)
                                RT2 (Ma,Mb)

               Figure 10: Datagram shortest-path tree: Area 1,
                 source N12, destination Group B. Note that
                  reverse costs (i.e., toward origin) are
                             used throughout.

        N12) as the upstream node for the datagram. This means that RT7
        must receive the datagram from a router in another AS before
        injecting the datagram into the MOSPF system.

    4.2.  Stub area behavior

        AS external links are not imported into stub areas. Suppose that
        the source of a particular datagram lies outside of the
        Autonomous System, and that the datagram is forwarded into a
        stub area. In the stub area's datagram shortest-path tree the
        neighborhood of the datagram's source cannot be approximated by
        AS external links. Instead the neighborhood of the source is
        approximated by the default summary links (see Section 3.6 of
        [OSPF]) that are originated by the stub area's intra-area
        multicast forwarders.

        Except for this small change to the construction of a stub
        area's datagram shortest-path trees, all other MOSPF algorithms
        (e.g., merging with other areas' datagram shortest-path trees to

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        form the forwarding cache) function the same for stub areas as
        they do for non-stub areas.

    4.3.  Inter-AS multicasting in a core Autonomous System

        It may be the case that the MOSPF routing domain connects
        together many different Autonomous Systems, thereby serving as a
        "core Autonomous System" (e.g, the old NSFNet backbone). In this
        case, it could very well be that the majority of the MOSPF
        routers are also inter-AS multicast forwarders. Having each
        inter-AS multicast forwarder then declare itself a wild-card
        multicast receiver could very well waste considerable network
        bandwidth. However, as an alternative to declaring themselves
        wild-card multicast receivers, the inter-AS multicast routers
        could instead explicitly advertise all groups that they were
        interested in forwarding (to other "client" Autonomous Systems)
        in group-membership-LSAs. These advertised groups would have to
        be learned through an inter-AS multicast routing protocol (or
        possibly even statically configured).

        This in essence allows the clients of the core Autonomous System
        to advertise their group membership into the core. However,
        since any client MOSPF domains will still have their inter-AS
        multicast forwarders configured as wild-card multicast
        receivers, this advertisement will be asymmetric: the core will
        not advertise its or others' group membership to the clients.
        The achieves the same inter-AS multicast routing architecture
        that MOSPF uses for inter-area multicast routing (see Figure 5).

5.  Modelling internal group membership

    A MOSPF router may itself contain multicast applications. A typical
    example of this is a UNIX workstation that doubles as a multicast
    router. This section concerns two alternative ways of representing
    the group membership of the MOSPF router's internal applications.
    Both representations have advantages. For maximum flexibility, the
    MOSPF forwarding algorithm (see Section 11) has been specified so
    that either representation can be used in a MOSPF router (and in
    fact, both representations can be used at once, depending on the

    The first representation is based on the paradigm presented in RFC
    1112. In this case, an application joins a multicast group on one or
    more specific physical interfaces. The application then receives a
    multicast datagram if and only if it is received on one of the
    specified interfaces. If a datagram is received on multiple
    specified interfaces, the application receives multiple copies.
    Figure 11 shows this algorithm as it is implemented in (modified)

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    BSD UNIX kernels.  The figure shows the processing of a multicast
    datagram, starting with its reception on a particular interface.
    First copies of the datagram are given to those applications that
    have joined on the receiving interface. Then the forwarding decision
    (pictured as a box containing a question mark) is made, and the
    packet is (possibly) forwarded out certain interfaces. If these
    interfaces are not capable of receiving their own multicasts, a copy
    of the datagram must be internally looped back to appropriately
    joined applications.

    The advantages to the RFC 1112 representation are as follows:

    o   It is the standard for the way an IP host joins multicast
        groups. It is simplest to use the same membership model for
        hosts and routers; most would consider an IP router to be a
        special case of an IP host anyway.

    o   It is the way group membership has been implemented in BSD UNIX.
        Existing multicast applications are written to join multicast
        groups on specific interfaces.

    o   The possibility of receiving multiple datagram copies may
        improve fault tolerance. If the datagram is dropped due to an

                                |---> To application
                      |forwarding decision|
                               / \
                              /---\----> To application
                             /     \------> To application
                            /       \
                           /         \
                     +--------+  +--------+
                     |transmit|  |transmit|
                     +--------+  +--------+

              Figure 11: RFC 1112 representation of internal
                          group membership

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        error on the path to some interface, another interface may still
        receive a copy.

    o   The ability to specify a particular receiving interface may
        improve the accuracy of IP multicast's expanding ring search
        mechanism (see Section 2.3.4).

    o   Membership in the non-routable multicast groups ( - must be on a per-interface basis. An OSPF router
        always belongs to (AllSPFRouters) on its OSPF
        interfaces, and may belong to (AllDRouters) on one or
        more of its OSPF interfaces.

    The second representation is MOSPF-specific. In this case, an
    application joins a multicast group on an interface-independent
    basis.  In other words, group membership is associated with the
    router as a whole, not separately on each interface. The application
    then receives a copy of a multicast datagram if and only if the
    datagram would actually be forwarded by the MOSPF router. Figure 12
    shows how this algorithm would be implemented. The datagram is
    received on a particular interface. If the datagram is validated for
    forwarding (i.e., the receiving interface connects to the matching
    forwarding cache entry's upstream node), a copy of the datagram is
    also given to appropriately joined applications. Note that this
    model of group membership is not as general as the RFC 1112 model,
    in that it can only be implemented in MOSPF routers and not in
    arbitrary IP hosts.  However, it has the following advantages:

    o   The application does not need to have knowledge of the router
        interfaces. It does not need to know what kind or how many
        interfaces there are; this will be taken care of by the MOSPF
        protocol itself.

    o   As long as any interface is operational, the application will
        continue to receive multicast datagrams. This happens
        automatically, without the application modifying its group

    o   The application receives only one copy of the datagram. Using
        the RFC1112 representation, whenever an application joins on
        more than one interface (which must be done if the application
        does not want to rely on a single interface), multiple datagram
        copies will be received during normal operation.

6.  Additional capabilities

    This section describes the MOSPF configuration options that allow
    routers of differing capabilities to be mixed together in the same

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                      |forwarding decision|---> to application
                               / \
                              /   \
                             /     \
                            /       \
                           /         \
                     +--------+  +--------+
                     |transmit|  |transmit|
                     +--------+  +--------+

              Figure 12: MOSPF-specific representation of internal
                             group membership

    routing domain. Note that these options handle special circumstances
    that may not be encountered in normal operation. Default values for
    the configuration settings are specified in Appendix B.

    6.1.  Mixing with non-multicast routers

        MOSPF routers can be mixed freely with routers that are running
        only the base OSPF algorithm (called non-multicast routers in
        the following). This allows MOSPF to be deployed in a piecemeal
        fashion, thereby speeding deployment and allowing
        experimentation with multicast routing on a limited scale.

        When a MOSPF router builds a datagram shortest-path tree, it
        omits all non-multicast routers. For example, in Figure 1, if
        Router RT6 was not a multicast router, the datagram shortest-
        path tree in Figure 3 would be built with a more circuitous
        branch through Router RT5, instead of through Router RT6. In
        addition, non-multicast routers do not participate in the
        flooding of the new group-membership-LSAs. This adheres to the
        general principle that a router should not have to handle those
        link state advertisements whose format (or contents) the router
        does not understand.

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        Mixing MOSPF routers with non-multicast routers creates a number
        of potential problems. Certain mixings of MOSPF and non-
        multicast routers can cause multicast datagrams to take
        suboptimal paths, or in other cases can lead to the non-delivery
        of multicast datagrams. In addition, mixing MOSPF routers and
        non-multicast routers can cause the paths of multicast datagrams
        to diverge radically from the path of unicast datagrams. Such
        divergences can make routing problems harder to debug.

        In particular, the following specific difficulties may arise
        when mixing MOSPF routers with non-multicast routers:

        o   Even though there is unicast connectivity to a destination,
            there may not be multicast connectivity. For example, if
            Router RT10 in Figure 1 becomes a non-multicast router, the
            group member connected to Network N11 will no longer be able
            to receive multicasts sourced by Host H2.  But the two hosts
            will be able to exchange unicasts (e.g., ICMP pings).

        o   When the Designated Router for a multi-access network is a
            non-multicast router, the network will not be used for
            forwarding multicast datagrams. For example, if in Figure 1
            Router RT4 is Designated Router for Network N3, and RT4 is
            non-multicast, Network N3 will not be used to forward IP
            multicasts. This would mean that multicast datagrams
            originated by Hosts H2 and H3 would not be forwarded beyond
            their local network (N4), even though it seems that the
            needed multicast connectivity exists.

        o   When forwarding multicast datagrams between areas, mixing of
            MOSPF routers and non-multicast routers in the source area
            may cause unexpected loss of multicast connectivity. This is
            because in the inter-area routing of multicast datagrams the
            neighborhood of the datagram's source is approximated by
            OSPF summary links, and OSPF summary-link-LSAs do not carry
            indications/guarantees of the summarized path's multicast
            routing capability.

    6.2.  TOS-based multicast

        MOSPF allows a separate datagram shortest-path tree to be built
        for each IP Type of Service. This means that the path of a
        multicast datagram can vary depending on the datagram's TOS
        classification, as well as its source and destination.

        For each router interface, OSPF allows a separate metric to be
        configured for each IP TOS. When building the shortest path tree
        for TOS X, the cost of a path is the sum of the component

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        interfaces' TOS X metrics. Note that OSPF requires that a TOS 0
        metric be specified for each interface. However, as a form of
        data compression, metrics need only be specified for non-zero
        TOS if they are different than the TOS 0 metric.

        Additionally, OSPF routers can be configured to ignore TOS when
        forwarding packets. Such routers, called TOS-incapable, build
        only the TOS 0 portion of the routing table. TOS-incapable
        routers can be mixed freely with TOS-capable routers when
        forwarding unicast packets. The way this is handled for unicast
        packets is that the unicast is forwarded along the TOS 0 route
        whenever the TOS X route does not exist. However, MOSPF must
        treat this situation somewhat differently, since each router
        must build the exact same tree rooted at the datagram's source.

        Like OSPF, MOSPF allows TOS-based routing to be optional. TOS-
        capable and TOS-incapable multicast routers can be mixed freely
        in the routing domain. TOS-incapable routers will only ever
        build TOS 0 datagram shortest-path trees. TOS-capable routers
        will first build TOS 0 datagram shortest-path trees. If these
        trees contain only TOS-capable routers, datagram shortest-path
        trees are then built separately for non-zero TOS values.
        Otherwise, the TOS 0 datagram shortest-path tree is used to
        forward all traffic, regardless of its TOS designation.  Using
        this logic, all routers in essence continue to utilize identical
        datagram shortest-path trees. See Section 12.2.8 for more

    6.3.  Assigning multiple IP networks to a physical network

        Assigning multiple IP networks/subnets to a single physical
        network causes some confusion in MOSPF. This is because the
        underlying OSPF protocol treats these IP networks/subnets as
        entirely separate entities, originating separate network-LSAs
        for each and forming separate adjacencies for each, while IGMP
        recognizes only the single underlying physical network. Adding
        to the problem is the fact that when a multicast datagram is
        received from such a multiply-addressed physical wire, there is
        no good way to choose the datagram's upstream node (which must
        be done in order to make the forwarding decision; see Section 11
        for details). As a result, unless this situation is dealt with
        through configuration, unwanted replication of multicast
        datagrams may occur when they are forwarded over multiply-
        addressed wires.

        As a remedy, MOSPF allows multicast forwarding to be disabled on
        certain IP networks/subnets. When multicast forwarding is
        disabled on the wire's "extra" subnets (i.e., all but one), the

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        extra subnets will not appear in datagram shortest-path trees,
        nor will they appear in local group database or forwarding cache
        entries. As a result, the possibility of unwanted datagram
        replication is eliminated. The actual disabling of multicast
        forwarding on a subnet is done through setting the
        IPMulticastForwarding parameter to disabled on all router
        interfaces connecting to the subnet (see Section B.2).

    6.4.  Networks on Autonomous System boundaries

        Another complication can arise on IP networks/subnets that lie
        on the boundary of a MOSPF Autonomous System. Similar to the
        unicast situation where these networks may be running multiple
        IGPs (Interior Gateway Protocols), these networks may also be
        running multiple multicast routing protocols. It may then become
        impossible for a MOSPF router to determine whether a multicast
        datagram is being forwarded along the datagram shortest-path
        tree, or whether it has been inadvertently received from the
        other Autonomous System. Guessing wrong can lead to either
        unwanted replication or non-delivery of the multicast datagram.
        In addition, in order to prevent receiving duplicate multicast
        datagrams, group members on these boundary networks will
        probably want to declare their membership to one Autonomous
        System and not another.

        For example, consider the two Autonomous Systems pictured in
        Figure 13. Network X is on the boundary of both ASes. One
        possible multicast datagram path is shown; the datagram
        originates in a third Autonomous System, and is then delivered
        to both AS #1 and AS #2 separately. The paths through the two
        Autonomous Systems may end up having certain boundary networks
        as common segments. In Figure 13, Network X is common to both
        paths. In this case, if both Autonomous Systems were running
        (separate copies of) MOSPF, the same datagram would appear twice
        on Network X as a data-link multicast. This would cause
        duplicate datagrams to be received by any group members on
        Network X or downstream from Network X.

        MOSPF has two mechanisms to eliminate this replication of
        multicast datagrams. First, a system administrator can configure
        certain networks to forward multicast datagrams as data-link
        unicasts instead of data-link multicasts. This is done by
        setting the IPMulticastForwarding parameter to data-link unicast
        on those router interfaces attaching to the network (see Section
        B.2). As an example, in Figure 13 the routers in AS #2 could be
        configured so that Router C would send the multicast datagram
        out onto Network X as a data-link unicast addressed directly to
        Router D. Router D would accept this data-link unicast, but

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                              <-Datagram path->*
                             *                 *
                             *                 *
                             *            .....*.........
                    .........*.....   |   .    *    AS #2
                    AS #1    *    .   |*****+---+
                            |RTA|----*|*  . +---+
                            +---+ .  *|*  .
                                  .  *|*  .
                                  .  *|*  . +---+
                            +---+ .  *|*----|RTD|
                            +---+*****|   .....*..........
                    .........*....    |        *
                             *        |        *
                             *    Network X    *

                     Figure 13: Networks on AS boundaries

        would reject any data-link multicast forwarded by Router A. This
        would eliminate replication of multicast datagrams downstream
        from Network X. In addition, if the IPMulticastForwarding
        parameter is set to data-link unicast on Network X, group
        membership will not be monitored on the network. This will
        prevent group members attached directly to Network X from
        receiving multiple datagram copies, since group membership on
        the boundary network will be monitored from only one AS (AS #1
        in our example).

        It should be noted that forwarding IP multicasts as data-link
        unicasts has some disadvantages when three or more MOSPF routers
        are attached to the network. First of all, it is more work for a
        router to send multiple unicasts than a single multicast.
        Second, the multiple unicasts consume more network bandwidth
        than a single multicast. And last, it increases the delay for
        some group members since multiple unicasts also take longer to
        send than a single multicast.

    6.5.   Recommended system configuration

        In order to make MOSPF's selection of routes more predictable,
        it is recommended that all routers in any particular OSPF area
        have the same multicast and TOS capabilities.Keeping areas
        homogeneous ensures that IP multicast packets will follow
        relatively the same path as IP unicasts. In contrast, while

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        heterogeneous areas will function, and will probably be
        necessary at least during the initial introduction of multicast
        routing, such areas may produce seemingly sub-optimal and
        unexpected routes. For example, see Section 6.1 above for a
        detailed description of the possible pitfalls when mixing
        multicast and non-multicast routers.

        As for the other options presented above, to achieve the most
        predictable results it is recommended that a router interface's
        IPMulticastForwarding parameter be set to a value other than
        data-link multicast only when either a) multiple IP networks
        have been assigned to a single physical wire or b) multiple
        multicast routing protocols are running on the attached network.

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7.  Basic implementation requirements

    An implementation of MOSPF requires the following pieces of system
    support. Note that this support is in addition to that required for
    the base OSPF implementation as outlined in Section 4.4 of [OSPF].

    o   Promiscuous multicast reception. In a multicast router, it is
        necessary to receive all IP multicasts at the data-link level.
        On those interfaces where IP multicast datagrams are
        encapsulated by a wide range of data-link multicast destination
        addresses (e.g, ethernet and FDDI), this is most easily
        accomplished by disabling any hardware filtering of multicast
        destinations (i.e., by "opening up" the interface's multicast

    o   Data-link multicast/broadcast detection. To avoid unwanted
        replication of multicast datagrams in certain exceptional
        conditions, it is necessary for the multicast router to
        determine whether a datagram was received as a data-link
        multicast/broadcast or as a data-link unicast, for later use by
        the MOSPF forwarding mechanism.  See Section 6.4 for more

    o   An implementation of IGMP. MOSPF uses the Internet Group
        Management Protocol (IGMP, documented in [RFC 1112]) to monitor
        multicast group membership. See Section 9 for details.

8.  Protocol data structures

    The MOSPF protocol is described herein in terms of its operation on
    various protocol data structures. These data structures are included
    for explanatory uses only, and are not intended to constrain a MOSPF
    implementation. Besides the data structures listed below, this
    specification will also reference the various data structures (e.g.,
    OSPF interfaces and neighbors) defined in [OSPF].

    In a MOSPF router, the following items are added to the list of
    global OSPF data structures described in Section 5 of [OSPF]:

    o   Local group database. This database describes the group
        membership on all attached networks for which the router is
        either Designated Router or Backup Designated Router. This in
        turn determines the group-membership-LSAs that the router will
        originate, and the local delivery of multicast datagrams (see
        Sections 2.3.1 and 10).

    o   Forwarding cache. Each entry in the forwarding cache describes
        the path of a multicast datagram having a particular [source

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        net, multicast destination, TOS] combination. These cache
        entries are calculated when building the datagram shortest-path
        trees. See Sections 2.3.4 and 11 for more details.

    o   Multicast routing capability. Indicates whether the router is
        running the multicast extensions defined in this memo. A router
        running the multicast extensions must still run the base OSPF
        algorithm as set forth in [OSPF]. Such a router will continue to
        interoperate with non-multicast-capable OSPF routers when
        forwarding IP unicast traffic.

    o   Inter-area multicast forwarder. Indicates whether the router
        will forward IP multicasts from one OSPF area to another. Such a
        router declares itself a wild-card multicast receiver in its
        non-backbone area router-LSAs (see Section 14.6), and also
        summarizes its attached areas' group membership to the backbone
        in group-membership-LSAs. When building inter-area datagram
        shortest-path trees, it is these routers that appear immediately
        adjacent to the datagram source at the root of the tree (see
        Section 3.2). 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).

    o   Inter-AS multicast forwarder. Indicates whether the router will
        forward IP multicasts between Autonomous Systems. Such a router
        declares itself a wild-card multicast receiver in its router-
        LSAs (see Section 14.6). These routers are also assumed to be
        running some kind of inter-AS multicast protocol. They mark all
        external routes that they import into the OSPF domain as to
        whether they provide multicast connectivity (see Section 14.9).
        When building inter-AS multicast datagram trees, it is these
        routers that appear immediately adjacent to the datagram source
        at the root of the tree.

    8.1.  Additions to the OSPF area structure

        The OSPF area data structure is described in Section 6 of
        [OSPF]. In a MOSPF router, the following item is added to the
        OSPF area structure:

        o   List of group-membership-LSAs. These link state
            advertisements describe the location of the area's multicast
            group members.  Group-membership-LSAs are flooded throughout
            a single area only. Area border routers also summarize their
            attached areas' membership by originating group-membership-
            LSAs into the backbone area. For more information, see

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            Sections 3.1 and 10.

    8.2.  Additions to the OSPF interface structure

        The OSPF interface structure is described in Section 9 of
        [OSPF]. In a MOSPF router, the following items are added to the
        OSPF interface structure. Note that the IPMulticastForwarding
        parameter is really a description of the attached network. As
        such, it should be configured identically on all routers
        attached to a common network; otherwise incorrect routing of
        multicast datagrams may result[13].

        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. When the router is actively monitoring
            group membership on the attached network, it periodically
            sends IGMP Host Membership Queries. IGMPPollingInterval is a
            configurable parameter indicating the number of seconds
            between IGMP Host Membership Queries.  The router actively
            monitors group membership on the attached network when both
            a) the interface's IPMulticastForwarding parameter is set to
            data-link multicast and b) the router has been elected
            Designated Router on the attached network. See Section 9 for

        o   IGMPTimeout. This configurable parameter indicates the
            length of time (in seconds) that a local group database
            entry associated with this interface will persist without
            another matching IGMP Host Membership Report being received.
            See Section 9 for details.

        o   IGMP polling timer. The firing of this interval timer causes
            an IGMP Host Membership Query to be sent out the interface.
            The length of this timer is the configurable parameter

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            IGMPPollingInterval. See Section 9 for details.

    8.3.  Additions to the OSPF neighbor structure

        The OSPF neighbor structure is defined in Section 10 of [OSPF].
        In a MOSPF router, the following items are added to the OSPF
        neighbor structure:

        o   Neighbor Options. This field was already defined in the OSPF
            specification. However, in MOSPF there is a new option which
            indicates the neighbor's multicast capability. This new
            option is learned in the Database Exchange process through
            reception of the neighbor's Database Description packets,
            and determines whether group-membership-LSAs are flooded to
            the neighbor. See the items concerning flooding in Section
            14 for a more detailed explanation.

    8.4.  The local group database

        The local group database has already been introduced in Section
        2.3.1.  The current section attempts a more precise definition.
        The local group database tracks the group membership of the
        router's directly attached networks. Database entries are
        created and maintained by the IGMP protocol. Database entries
        can cause group-membership-LSAs to be originated, which in turn
        enable the pruning of datagram shortest-path trees. The local
        group database also dictates the router's responsibility for the
        delivery of multicast datagrams to directly attached group

        Each entry in the local group database has three components: the
        multicast group, the attached network and the entry's age. A
        database entry is indexed by the first two components: multicast
        group and attached network. A database lookup function is
        assumed to exist, so that given a [multicast group, attached
        network] pair, the matching database entry (if any) can be
        discovered. A database entry for [Group A, Network N1] exists if
        and only if there are Group A members currently located on
        Network N1.

        The three components of a local group database entry are defined
        as follows:

        o   MulticastGroup. The multicast group whose members are being
            tracked by this entry. Each multicast group is represented
            as a class D IP address. For the semantics of multicast
            group membership, see [RFC 1112].

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        o   AttachedNetwork. Each database entry is concerned with the
            group members belonging to a single attached network. To get
            a complete picture of the local group membership (when for
            example building a group-membership-LSA), it may be
            necessary to consult multiple database entries, one for each
            attached network. Note that a router is only required to
            maintain entries for those attached networks on which the
            router has been elected Designated Router or Backup
            Designated Router (see Section 9).

        o   Age. Indicates the number of seconds since an IGMP Host
            Membership Report for multicast Group A has been seen on
            Network N1. If the age field hits Network N1's configured
            IGMPTimeout value, the local group database entry is removed
            (i.e., the entry has "aged out"). See Sections 9.2 and 9.3
            for more information.

    8.5.  The forwarding cache

        The forwarding cache has already been defined in Section 2.3.
        The current section attempts a more precise definition. Each
        entry in the forwarding cache indicates how a multicast datagram
        having a particular [source network, destination multicast
        group, IP TOS] will be forwarded. A forwarding cache entry is
        built on demand from the local group database and the datagram's
        shortest-path tree. For more details, consult Sections 2.3.4 and

        Each entry in the forwarding cache has six components: the
        multicast datagram's source network, the destination multicast
        group, the IP TOS, the upstream node, the list of downstream
        interfaces and (possibly) a list of downstream neighbors. A
        forwarding cache entry is indexed by source network, destination
        multicast group and IP TOS. A lookup function is assumed to
        exist, so that given a multicast datagram with a particular [IP
        source, destination multicast group, IP TOS], a matching cache
        entry (if any) can be found.

        The six components of a forwarding cache entry are defined as

        o   Source network. The datagram's source network is described
            by a network/subnet/supernet number and its corresponding
            mask. The source network for a datagram is discovered via a
            routing table/database lookup of the datagram's IP source
            address, as described in Section 11.2.

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        o   Destination multicast group. The destination group to which
            matching datagrams are being forwarded. For the semantics of
            multicast group membership, see [RFC 1112].

        o   IP TOS. The IP Type of Service specified by matching
            datagrams. Note that this means that the path of the
            multicast datagram depends on its TOS classification.

        o   Upstream node. The attached network/neighboring router from
            which the datagram must be received. If received from a
            different attached network/neighboring router, the matching
            datagram is dropped instead of forwarded. This prevents
            unwanted replication of multicast datagrams. It is possible
            that the upstream node is unspecified (i.e., set to NULL).
            In this case, matching datagrams will always be dropped, no
            matter where they are received from. It is also possible
            that the upstream node is specified as the placeholder
            EXTERNAL. This means that the datagram must be received on a
            non-MOSPF interface in order to be forwarded.

        o   List of downstream interfaces. These are the router
            interfaces that the matching datagram should be forwarded
            out of (assuming that the datagram was received from
            upstream node). Each interface is also listed with a TTL
            value. The TTL value is the minimum number of hops necessary
            to reach the closest (in terms of router hops) group member.
            This allows the router to drop datagrams that have no chance
            of reaching a destination group member.

        o   List of downstream neighbors. When the datagram is to be
            forwarded out a non-broadcast multi-access network, or if
            the interface's IPMulticastForwarding parameter is set to
            data-link unicast, the datagram must be forwarded separately
            to each downstream neighbor (see Sections 2.3.3 and 6.4). As
            done for downstream interfaces, each downstream neighbor is
            specified together with the smallest TTL that will actually
            reach a group member.

9.  Interaction with the IGMP protocol

    MOSPF uses the IGMP protocol (see [RFC 1112]) to monitor multicast
    group membership. In short, the Designated Router on a network
    periodically sends IGMP Host Membership Queries (see Section 9.1),
    which in turn elicit IGMP Host Membership Reports from the network's
    multicast group members. These Host Membership Reports are then
    recorded in the Designated Router's and Backup Designated Router's
    local group databases (see Section 9.2).

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    9.1.  Sending IGMP Host Membership Queries

        Only the network's Designated Router sends Host Membership
        Queries.  This minimizes the amount of group membership
        information on the network, both in terms of queries and

        When a MOSPF router becomes Designated Router on a network, it
        checks to see that the network's IPMulticastForwarding parameter
        is set to data-link multicast (see Section B.2). If so, it
        starts the interface's IGMP polling timer. Then, whenever the
        timer fires (every IGMPPollingInterval seconds), the MOSPF
        router sends a Host Membership Query out the interface. The
        destination of the query is the IP address For the
        format of the query, see [RFC 1112].  If/when the MOSPF router
        ceases to be the network's Designated Router, the IGMP polling
        timer is disabled and no more Hosts Membership Queries are sent.

        Unusual behavior can result when multiple IP networks are
        assigned to a single physical network. MOSPF treats each such IP
        network separately, electing (possibly) a different Designated
        Router for each network.  However, IGMP operates on a physical
        network basis only: when a Host Membership Query is sent, all
        group members on the physical network respond, regardless of
        their IP addresses. So unless the IPMulticastForwarding
        parameter is set to a value other than data-link multicast on
        all but one of the physical network's IP networks, excess
        multicast membership reporting will result.

    9.2.  Receiving IGMP Host Membership Reports

        Received Host Membership Reports are processed by both the
        network's Designated Router and Backup Designated Router. It is
        the Designated Router's responsibility to distribute the
        network's group membership information throughout the routing
        domain, by originating group-membership-LSAs (see Section 10).
        The Backup Designated Router processes Reports so that it too
        has a complete picture of the network's group membership,
        enabling a quick cutover upon Designated Router failure.

        An IGMP Host Membership Report concerns membership in a single
        IP multicast group (call it Group A). The Report is sent to the
        Group A address so that other group members may see the Report
        and avoid sending duplicates (see [RFC 1112] for details). When
        an IGMP Host Membership Report, sent on Network N[14], is
        received by a MOSPF router, the following steps are executed:

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        (1) If the router is neither the Designated Router nor the
            Backup Designated Router on the network, the Report is
            discarded and processing stops.

        (2) If the Report concerns a multicast group in the range
   -, the Report is discarded and
            processing stops. This range of multicast groups are for
            local use (single hop) only, and datagrams sent to these
            destinations are never forwarded by multicast routers.

        (3) Locate the entry for [Group A, Network N] in the local group
            database.  If no such entry exists, create one. In any case,
            set the age of the entry to 0. Note that even if multiple
            hosts attached to Network N report membership in the same
            group, only a single local group database entry will be
            formed. See Section 8.4 for more details concerning the
            local group database.

        (4) If the router is the network's Designated Router, and a
            local group database entry was created in the previous step,
            it may be necessary to originate a new group-membership-LSA.
            See Section 10 for details.

    9.3.  Aging local group database entries

        Every local database entry has an age field. Suppose that there
        is a database entry for [Group A, Network N1]. The age field
        then indicates the length of time (in seconds) since the last
        Host Membership Report for Group A was received on Network N1.
        If the age of the entry reaches Network N1's configured
        IGMPTimeout value (see Section B.2), the entry is considered
        invalid and is removed from the database.

        Note that when a router, after having been either Network N1's
        Designated Router or Backup Designated Router, but now being
        neither, will (after IGMPTimeout seconds) automatically age out
        all of its local group database entries associated with Network
        N1. For this reason, it is not necessary to purge local group
        database entries on OSPF interface state changes.

    9.4.  Receiving IGMP Host Membership Queries

        If a MOSPF router has internal multicast applications, and if
        the applications have bound themselves to certain interfaces
        (using the RFC 1112 representation described in Section 5), then
        the MOSPF router responds to received Host Membership Queries by
        issuing Host Membership Reports. Identical to the operation of
        any IP host supporting multicast applications, the exact

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        procedure for issuing these Host Membership Reports is specified
        in [RFC 1112]. Note that in this case, if the router has been
        elected Designated Router on a network, it must receive its own
        Host Membership Reports and Host Membership Queries.

        If instead all of its applications have joined groups in an
        interface-independent fashion (using the MOSPF-specific
        representation described in Section 5), the MOSPF router does
        not respond to Host Membership Queries. Instead, the MOSPF
        router communicates this membership information by originating
        appropriate group-membership-LSAs (see Section 10.1).

10.  Group-membership-LSAs

    Group-membership-LSAs provide the means of distributing membership
    information throughout the MOSPF routing domain. Group-membership-
    LSAs are specific to a single OSPF area (see Section 3.1). Each
    group-membership-LSA concerns a single multicast group. Essentially,
    the group-membership-LSA lists those networks which are directly
    connected to the LSA's originator and which contain one or more
    group members. For more details on how the group-membership-LSA
    augments the OSPF link state database, see Section 2.3.1.

    The creation of group-membership-LSAs is discussed in Section 10.1.
    The format of the group-membership-LSA is described in Section A.3.
    A router will originate a group membership-LSA for multicast group A
    when one or more of the following conditions hold:

    (1) The router is Designated Router on a network (call it Network
        X), the interface to Network X has its IPMulticastForwarding
        parameter set to data-link multicast (see Section B.2), and
        Network X contains one or more members of Group A.

    (2) The router is an inter-area multicast forwarder (see Section
        B.1), and one or more of the router's attached non-backbone
        areas contain Group A members. In this case, the router will
        originate a group-membership-LSA for Group A into the backbone.
        This is the way group membership is conveyed between areas (see
        Section 3.1).

    (3) The router itself has applications that are requesting
        membership in Group A, in an interface-independent fashion (see
        Section 5).

    As for all other types of OSPF link state advertisements (e.g,
    router-LSAs, network-LSAs, etc.), group-membership-LSAs are aged as
    they are held in a router's link state database. To prevent valid
    advertisements from "aging out", a router must refresh its self-

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    originated group-membership-LSAs every LSRefreshTime interval, by
    incrementing their LS sequence numbers and reissuing them. In
    addition, when an event occurs that would alter one of the router's
    self-originated group-membership-LSAs, a new instance of the LSA is
    issued with an updated (i.e., incremented by 1) LS sequence number.
    Note however that a router is not allowed to originate two new
    instances of the same advertisement within MinLSInterval seconds.
    For that reason, occasionally advertisement originations will need
    to be deferred. Also, an event may occur that makes it inappropriate
    for the router to continue to originate a particular LSA. In that
    case, the router flushes the advertisement from the routing domain
    by "premature aging". For more information concerning the
    maintenance of LSAs, see Sections 12, 12.4, 14 and 14.1 of [OSPF].

    When one of the following events occurs, it may be necessary for a
    router to (re)issue one or more group-membership-LSAs:

    (1) One of the router's interfaces changes state. For example, the
        router may have become Designated Router on a particular
        network, causing the router to start advertising the network's
        group membership to the rest of the MOSPF system in group-

    (2) The router receives an IGMP Host Membership Report, causing a
        new local group database entry to be formed (see Section 9.2).

    (3) One of the router's local group database entries "ages out",
        because it is no longer being refreshed by received IGMP Host
        Membership Reports (see Section 9.3).

    (4) The router is an inter-area multicast forwarder, and the group
        membership of one of the router's attached non-backbone areas
        changes.  This is detected by the reception of a new, or the
        flushing of an old, group-membership-LSA into/from the non-
        backbone area's link state database.

    (5) The group membership of one of the router's internal
        applications changes.

    10.1.  Constructing group-membership-LSAs

        This section details how to build a group-membership-LSA. The
        format of a group-membership-LSA is described in Section A.3.
        Each group-membership-LSA concerns a single multicast group. The
        body of the advertisement is a list of the local transit nodes
        (the router itself and directly attached transit networks) that
        contain group members. Section 10 listed the conditions
        requiring the (re)origination of a group-membership-LSA. Note

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        that if the router is an area border router, it may be necessary
        to originate a separate group-membership-LSA for each attached

        The following defines the contents of a group-membership-LSA, as
        originated by Router X into Area A. It is assumed that the
        group-membership-LSA is to report membership in multicast group

        o   The advertisement fields that are not type-specific (LS age,
            LS sequence number, LS checksum and length) are set
            according to Section 12.1 of [OSPF].

        o   The Options field of a group-membership-LSA is not processed
            on receipt. However, for consistency, the Option field in
            these advertisements should have its MC-bit set, T-bit
            clear, and the E-bit should match the configuration of Area
            A (i.e., set if and only if Area A is not a stub area). The
            rest of the Options field is set to 0.

        o   The Link State ID is set to the group whose membership is
            being reported (Group G).

        o   The Advertising Router is set to the OSPF Router ID of the
            router originating the advertisement (Router X).

        o   The body of the advertisement is a list of local transit
            vertices that should be labelled with Group G membership
            (see Section 2.3.1). This list may include the advertising
            router itself, and any of the transit networks that are
            directly attached to said router. The following steps
            determine which of these transit vertices are actually
            included in the group-membership-LSA. Note that any
            particular vertex should be listed at most once, even though
            the following may indicate multiple reasons for a particular
            vertex to be listed. Also note that if no transit vertices
            are listed by the advertisement, the advertisement should
            not be (re)originated; if an instance of the advertisement
            already exists, it should then be flushed from the link
            state database using the premature aging procedure specified
            in Section 14.1 of [OSPF].

            a.  Consider those entries in the local group database that
                describe Group G membership (see Section 8.4). Consider
                each such entry in turn. Each entry references one of
                Router X's attached networks (call it Network N). If
                either Network N does not belong to Area A, or if Router
                X is not Network N's Designated Router[15], Network N

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                should not be added to the group-membership-LSA, and the
                next local group database entry should be examined.
                Otherwise, if N is a stub network (e.g., Router X is the
                only OSPF router attached to N), Router X adds itself to
                the advertisement by adding a vertex with Vertex type
                set to 1 (router) and Vertex ID set to Router X's OSPF
                Router ID. Otherwise, N is a transit network. In this
                case, Network N should be added to the advertisement by
                adding a vertex with Vertex type set to 2 (network) and
                Vertex ID set to the IP address of Network N's
                Designated Router (i.e., Router X's IP interface address
                on Network N).

            b.  If Router X itself has applications requesting Group G
                membership on an interface-independent basis (see
                Section 5), it should add itself to the advertisement by
                adding a vertex with Vertex type set to 1 (router) and
                Vertex ID set to Router X's OSPF Router ID.

            c.  If Router X is an inter-area multicast forwarder (see
                Section 3.1), Area A is the backbone area (Area ID
      , and at least one of Router X's attached non-
                backbone areas has Group G members (indicated by the
                presence of one or more advertisements in the areas'
                link state databases having Link State ID set to Group G
                and LS age set to a value other than MaxAge[16]), then
                Router X should add itself to the advertisement by
                adding a vertex with Vertex type set to 1 (router) and
                Vertex ID set to Router X's OSPF Router ID.

        Consider as an example the network configuration in Figure 4.
        Suppose that Router RT2 has been elected Designated Router for
        Network N3.  Router RT2 would then originate (into Area 1) the
        following group-membership-LSA for Group B:

          ; RT2's group-membership-LSA for Group B

          LS age = 0                     ;always true on origination
          Options = (E-bit|MC-bit)
          LS type = 6                    ;group-membership-LSA
          Link State ID = Group B
          Advertising Router = RT2's Router ID
                 Vertex type = 1         ;RT2 itself (for stub N2)
                 Vertex ID = RT2's Router ID
                 Vertex type = 2         ;Network N3 (since RT2 is DR)
                 Vertex ID = RT2's IP interface address on N3

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    10.2.  Flooding group-membership-LSAs

        When MOSPF routers and non-multicast OSPF routers are mixed
        together in a routing domain, the group-membership-LSAs are not
        flooded to the non-multicast routers[17].  As a general design
        principle, optional OSPF advertisements are only flooded to
        those routers that understand them.

        A MOSPF router learns of its neighbor's multicast-capability at
        the beginning of the "Database Exchange Process" (see Section
        10.6 of [OSPF], receiving Database Description packets from a
        neighbor in state Exstart). A neighbor is multicast-capable if
        and only if it sets the MC-bit in the Options field of its
        Database Description packets.  Then, in the next step of the
        Database Exchange process, group-membership-LSAs are included in
        the Database summary list sent to the neighbor (see Sections 7.2
        and 10.3 of [OSPF]) if and only if the neighbor is multicast-

        When flooding group-membership-LSAs to adjacent neighbors, a
        MOSPF router looks at the neighbor's multicast-capability.
        Group-membership-LSAs are only flooded to multicast-capable
        neighbors. To be more precise, in Section 13.3 of [OSPF],
        group-membership-LSAs are only placed on the Link state
        retransmission lists of multicast-capable neighbors[18].  Note
        however that when sending Link State Update packets as
        multicasts, a non-multicast neighbor may (inadvertently) receive
        group-membership-LSAs. The non-multicast router will then simply
        discard the LSA (see Section 13 of [OSPF], receiving LSAs having
        unknown LS types).

(page 52 continued on part 3)

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