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Protocol Independent Multicast-Sparse Mode (PIM-SM), as defined in RFC 2362, was designed to support multicast groups spread out across large areas of a company or the Internet. PIM-SM routes multicast packets to multicast groups, and establishes distribution trees across wide area networks.
Unlike dense mode protocols, such as DVMRP, that initially flood multicast traffic to all routers over an entire internetwork, PIM-SM sends multicast traffic only to routers that have specifically joined a multicast group. This technique reduces traffic flow over WAN links and overhead costs for processing unwanted multicast packets.
Dense-mode protocols use a "flood-and-prune" technique, which is efficient where receivers are densely populated. However, for sparsely populated networks, PIM-SM is more efficient because it sends multicast traffic only to those routers that belong to a specific multicast group and that choose to receive the traffic. PIM-SM is independent of any specific unicast routing protocol, but it does require the presence of a unicast routing protocol, such as RIP or OSPF. PIM-SM uses the information from the unicast routing table to create and maintain multicast trees that enables PIM-enabled routers to communicate.
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