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

Chapter 302
Inter-Autonomous System Multicast

This type of multicast routing involves a source and destination that reside in different autonomous systems. This includes the ability of routing between an MOSPF domain and a DVMRP domain, but within the same AS. This is similar in the way OSPF treats RIP. Both OSPF and RIP can be used in the same AS, but they are treated as separate routing domains.

Just as in the inter-area multicast routing, configured Autonomous System Boundary Routers are configured to perform the inter-AS multicast forwarder function. This works under the assumption that each inter-AS multicast forwarder employs a multicast forwarding protocol that uses Reverse Path Forwarding, such as DVMRP or PIM. The multicast ASBRs use the wildcard capability. With this, each ASBR acts as a multicast wildcard receiver for each of its attached areas. This ensures that the inter-AS multicast forwarders are included in all multicast trees. They are not pruned.

Most of the operation of this router is similar to the inter-area multicast forwarder. However, there is one case in which it is different: If the source of the multicast datagram and the router making the calculation are in different ASs. Again, the details of each AS topology will not be known. To compensate for this, information can be assumed by using the Summary-ASBR link and the AS External links, which describe the source subnet. After the calculation is done, the multicast tree begins at the inter-AS multicast forwarder, with all branches stemming from this router.