How does PIM-SM work?
PIM-SM is a multicast routing protocol belonging to the shared-tree family. It uses a rendezvous point (RP), which multicast sources use to register their sessions. The RP maintains a table with this source and group information. When a host wishes to join a multicast session, it sends a join request to its gateway router for a multicast group. Since the gateway router doesn’t have information about the source address, it will build a path back to the RP, which does contain the source information. The path that is chosen is determined by the unicast routing protocol (such as IS-IS, OSPF, IGRP, EIGRP, or RIP) running on the router. The RP will continue to build a tree from the destination back to the source, and forward multicast packets to the destination. Once traffic has been received from the source of the session, the gateway router can then build a tree back to the source directly, which may be a more optimal path than going through the RP. PIM-SM is defined in RFC 2362.
