InterAs Mospf
RFC 1584 provides for the routing of multicast packets into and out of an MOSPF domain You know from Chapter 9 of Volume I that a router redistributing routes into an OSPF domain from some other routing protocol is called an Autonomous System Boundary Router (ASBR) An ASBR uses AS-External (type 5) LSAs to advertise destinations outside of the OSPF domain and ASBR Summary (type 4) LSAs to advertise their own location These LSAs are flooded into all areas of the OSPF domain, with the exception of stub areas
A router that connects an MOSPF domain to some other multicast routing domain (most likely DVMRP presently, and possibly some multicast EGP in the future) is called an inter-AS multicast forwarder These routers behave very similarly to inter-area multicast forwarders To forward multicast packets to destinations outside of the MOSPF domain, inter-AS multicast forwarders set the W bit in their Router LSAs and become wildcard multicast forwarders When the routers are forwarding packets into the MOSPF domain from external sources, they become "proxy sources," with their external link serving as the root for the group's SPF tree
Like ASBRs, inter-AS multicast forwarders can be located in any area Notice, however, that wildcard multicast forwarding capability is signaled by the W bit of type 1 LSAs, and type 1 LSAs are not flooded outside of an area If the inter-AS forwarder is located in area 0, this is not a problem, the inter-area multicast forwarders already pull all multicast traffic to the backbone If the inter-AS forwarder is located in a nonbackbone area, however, that area's inter-area forwarder also must become a wildcard forwarder for the backbone area Therefore, it is recommended that inter-AS multicast forwarders be located only in area 0
It is also recommended that inter-AS forwarders be placed carefully within the MOSPF domain Because all multicast traffic within the domain is forwarded to these routers, links leading to the routers can easily become congested
Continue reading here: Operation of Protocol Independent Multicast Dense Mode PIMDM
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