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

Chapter 300
Inter-Area Multicast Example

Having the multicast forwarders as wildcards enables them to receive multicast datagrams from the backbone and any multicast forwarder to be included in any multicast tree built in their area. When a multicast datagram is to be forwarded, it is received by the ABR (multicast forwarder) for forwarding to the backbone. The backbone routers know which groups are active in which areas, and since the ABR is part of the backbone they can receive the information from the backbone to be forwarded to their area.

The backbone routers do not implement the wildcard function, for these routers inherently know about all multicast groups through the multicast ABRs flowing summary information into the backbone, which is received by the backbone routers.

How is the forwarding cache built based on these assumptions? It depends on whether the source and a router building the tree are in the same area or not. The forwarding of multicast information is still accomplished using the forwarding cache, but an accurate picture cannot be drawn.

If the source and the router performing the calculation are in the same area, the wildcard feature of the ABR comes into play. This forces the router to be included in all multicast computations, which allows for the branches of the ABR to be included in the shortest-path tree. The ABR will not be pruned.

If the source and the router performing the calculation are in different areas, then the summary link advertisements are used. This forces the inter-area multicast forwarders of the ABR to be included in the calculated tree.

A final note: Area Border Routers have separate link-state databases for each area they attach to—this is a normal OSPF process. For multicast, however, this means that each multicast forwarder must build a separate forwarding tree for each area they attach to. But all of the area’s forwarding information is contained in one forwarding cache and as soon as this is built, the shortest-path trees for each area are dismantled. There is no need for them once the forwarding cache contains all the information needed for forwarding multicast datagrams.

Inter-Area Multicast Example