rfc1584.txt
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the area configuration pictured in Figure 4. The result of adding the group-membership-LSAs to the area databases yields the databases pictured in Figures 6 and 7. Figure 6 shows Area 1's MOSPF database. Figure 7 shows the backbone's MOSPF database. Superscripts indicate which transit vertices have been advertised as requesting particular multicast destinations. A superscript of "w" indicates that the router is advertising itself as a wild-card multicast receiver (see below). The dashed lines are OSPF summary-link-LSAs or AS external-link-LSAs. Note in Figure 7 that Router RT11 has condensed its routes to Networks N9-N11 and Host H1 into a single summary-link-LSA.Moy [Page 19] RFC 1584 Multicast Extensions to OSPF March 1994 .................................. . + . . | 3+---+ +--+ +--+ . N12 N14 . N1|--|RT1|\1 |Mb| |H4| . \ N13 / . _| +---+ \ +--+ /+--+ . 8\ |8/8 . | + \ _|__/ . \|/ . +--+ +--+ / \ 1+---+8. 8+---+6 . |Mb| |Mb| * N3 *---|RT4|------|RT5|--------+ . +--+ /+--+ \____/ +---+ . +---+ | . + / | . |7 | . | 3+---+ / | . | | . N2|--|RT2|/1 |1 . |6 | . __| +---+ +---+8 . 6+---+ | . | + |RT3|--------------|RT6| | . +--+ +--+ +---+ +--+. +---+ | . |Ma| |H3|_ |2 _|H2|. Ia|7 | . +--+ +--+ \ | / +--+. | | . +---------+ . | | .Area 1 N4 . | | .................................. | | ................................ | | . N11 . | | . +---------+ . | | . | \ . | | N12 . |3 +--+ . | |6 2/ . +---+ |Ma| . | +---+/ . |RT9| +--+ . | |RT7|---N15 . +---+ ....... | +---+ 9 . |1 .. + ...|..........|1........ . _|__ .. | Ib|5 __|_ +--+. . / \ 1+----+2.. | 3+----+1 / \--|Ma|. . * N9 *------|RT11|----|---|RT10|---* N6 * +--+. . \____/ +----+ .. | +----+ \____/ . . | !*******|*****! | . . |1 Virtual + Link |1 . . +--+ 10+----+ ..N8 +---+ . . |H1|-----|RT12| .. |RT8| . . +--+SLIP +----+ .. +---+ +--+. . |2 .. |4 _|H5|. . | .. | / +--+. . +---------+ .. +--------+ . . N10 Area 3..Area 2 N7 . ............................................................. Figure 4: A sample MOSPF area configurationMoy [Page 20] RFC 1584 Multicast Extensions to OSPF March 1994 Suppose an OSPF router has a local group database entry for [Group Y, Network X]. The router then originates a group- membership-LSA for Group Y into the area containing Network X. For example, in the area configuration pictured in Figure 4, Router RT1 originates a group-membership-LSA for Group B. This group-membership-LSA is flooded throughout Area 1, and no further. Likewise, assuming that Router RT3 has been elected Designated Router for Network N3, RT3 originates a group- membership-LSA into Area 1 listing the transit Network N3 as having group members. Note that in the link state database for Area 1 (Figure 6) both Router RT1 and Network N3 have accordingly been labelled with Group B. In OSPF, the area border routers forward routing information and data traffic between areas. In MOSPF. a subset of the area border routers, called the inter-area multicast forwarders, forward group membership information and multicast datagrams between areas. Whether a given OSPF area border router is also a MOSPF inter-area multicast forwarder is configuration dependent (see Section B.1). In Figure 4 we assume that all area border routers are also inter-area multicast forwarders. In order to convey group membership information between areas, inter-area multicast forwarders "summarize" their attached areas' group membership to the backbone. This is very similar functionality to the summary-link-LSAs that are generated in the base OSPF protocol. An inter-area multicast forwarder calculates which groups have members in its attached non- backbone areas. Then, for each of these groups, the inter-area multicast forwarder injects a group-membership-LSA into the backbone area. For example, in Figure 4 there are two groups having members in Area 1: Group A and Group B. For that reason, both of Area 1's inter-area multicast forwarders (Routers RT3 and RT4) inject group-membership-LSAs for these two groups into the backbone. As a result both of these routers are labelled membership +------------------+ datagrams + > > > >>| Backbone |< < < < + ^ +------------------+ ^ ^ / | \ ^ ^ / | \ ^ +----^-----+/ +----------+ \+----^-----+ | Area 1 | | Area 2 | | Area 3 | +----------+ +----------+ +----------+ Figure 5: Inter-area routing architectureMoy [Page 21] RFC 1584 Multicast Extensions to OSPF March 1994 with Groups A and B in the backbone link state database (see Figure 7). However, unlike the summarization of unicast destinations in the base OSPF protocol, the summarization of group membership in MOSPF is asymmetric. While a non-backbone area's group membership is summarized to the backbone, this information is not then readvertised into other non-backbone areas. Nor is the backbone's group membership summarized for the non-backbone areas. Going back to the example in Figure 4, while the presence of Area 3's group (Group A) is advertised to the backbone, this information is not then redistributed to Area 1. In other words, routers internal to Area 1 have no idea of Area 3's group membership. At this point, if no extra functionality was added to MOSPF, multicast traffic originating in Area 1 destined for Multicast Group A would never be forwarded to those Group A members in Area 3. To accomplish this, the notion of wild-card multicast receivers is introduced. A wild-card multicast receiver is a router to which all multicast traffic, regardless of multicast destination, should be forwarded. A router's wild-card multicast reception status is per-area. In non-backbone areas, all inter- area multicast forwarders[10] are wild-card multicast receivers. This ensures that all multicast traffic originating in a non- backbone area will be forwarded to its inter-area multicast forwarders, and hence to the backbone area. Since the backbone has complete knowledge of all areas' group membership, the datagram can then be forwarded to all group members. Note that in the backbone itself there is no need for wild-card multicast receivers[11]. As an example, note that Routers RT3 and RT4 are wild-card multicast receivers in Area 1 (see Figure 6), while there are none in the backbone (see Figure 7). This yields the inter-area routing architecture pictured in Figure 5. All group membership is advertised by the non- backbone areas into the backbone. Likewise, all IP multicast traffic arising in the non-backbone areas is forwarded to the backbone. Since at this point group membership information meets the multicast datagram traffic, delivery of the multicast datagrams becomes possible. 3.2. Building inter-area datagram shortest-path trees When building datagram shortest-path trees in the presence of areas, it is often the case that the source of the datagram and (at least some of) the destination group members are in separate areas. Since detailed topological information concerning oneMoy [Page 22] RFC 1584 Multicast Extensions to OSPF March 1994 **FROM** |RT|RT|RT|RT|RT|RT| |1 |2 |3 |4 |5 |7 |N3| ----- ------------------- RT1| | | | | | |0 | RT2| | | | | | |0 | RT3| | | | | | |0 | * RT4| | | | | | |0 | * RT5| | |14|8 | | | | T RT7| | |20|14| | | | O N1|3 | | | | | | | * N2| |3 | | | | | | * N3|1 |1 |1 |1 | | | | N4| | |2 | | | | | Ia,Ib| | |15|22| | | | N6| | |16|15| | | | N7| | |20|19| | | | N8| | |18|18| | | | N9-N11,H1| | |19|16| | | | N12| | | | |8 |2 | | N13| | | | |8 | | | N14| | | | |8 | | | N15| | | | | |9 | | Figure 6: Area 1's MOSPF database. Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X. In addition, RT1, RT2 and N3 are labelled with multicast group A, RT1 is labelled with multicast group B, and both RT3 and RT4 are labelled as wild-card multicast receivers.Moy [Page 23] RFC 1584 Multicast Extensions to OSPF March 1994 **FROM** |RT|RT|RT|RT|RT|RT|RT |3 |4 |5 |6 |7 |10|11| ------------------------ RT3| | | |6 | | | | RT4| | |8 | | | | | RT5| |8 | |6 |6 | | | RT6|8 | |7 | | |5 | | RT7| | |6 | | | | | * RT10| | | |7 | | |2 | * RT11| | | | | |3 | | T N1|4 |4 | | | | | | O ⌨️ 快捷键说明
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