Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 299
Inter-Area MOSPF Routing

The basic algorithm for MOSPF works in a single area. Inter-area routing for multicast involves a source and one or more destination groups in different areas. The forwarding of multicast datagrams between areas is still decided by the information contained in the forwarding cache. The problem is the ability to accurately build a complete shortest-path tree because detailed information about the other area’s topology is not known (in OSPF it is summarized, in MOSPF it is not known). Furthermore, since LSAs are not flooded to different areas, the group LSA is not propagated to other areas. To compensate for this “unknown” information, estimates are made by using the wildcard feature of the Area Border Router (new to MOSPF) and the summary link advertisements provided by the ABR. After a few introductions, how MOSPF overcomes these limitations is explained.

In a multicast topology that is represented by multiple areas, a new function within the ABR, called inter-area multicast forwarders, is implemented. These forwarders pass group information and allow for the ability of multicast datagrams to cross areas. It is the ABR that is configured to perform this function. Forwarders runs as a separate function of the ABR and are used only for multicast datagrams.

ABRs implementing the multicast forwarder summarize their area’s group information (how is explained in a moment) and send it to the backbone area through the use of group-membership LSAs, which are “injected” into the backbone area. The backbone routers receive this information and include it in their link-state database by group and the router it received it from. The backbone routers process this information but do not forward any multicast information on to any other multicast ABRs. Furthermore, no information regarding the backbone’s group membership is forwarded to any area. It is asymmetrical. Information flows into the backbone, but the backbone does not flow the information into other areas. So how does information flow between areas, or from the backbone to an area? So how does the multicast forwarder know of the groups in its area and know when to pass information from the backbone to an area? This involves the concept known as the wildcard receiver.

MOSPF routers may indicate they want to receive all multicast datagrams regardless of the destination. They can indicate this through their router-LSA using a newly defined bit in the rtype field known as the W bit, or wildcard bit. This bit is used with inter-area multicast forwarders (ABRs) and permits a MOSPF router to receive all multicast traffic in an area regardless of the group. MOSPF routers that employ this function ensure that they remain on all pruned multicast trees, thus allowing them to receive all multicast datagrams regardless of group membership. By default, all multicast forwarder ABRs are wildcard receivers.