IP multicast transmits a data stream to multiple hosts simultaneously. Protocol-Independent Multicast (PIM) is one of several protocols designed for IP multicast. PIM does not rely on a specific routing protocol to create its network topology state. Instead, PIM uses routing information supplied by other traditional routing protocols, such as Open Shortest Path First, Border Gateway Protocol, and Multicast Source Discovery Protocol.
PIM messages are sent encapsulated in an IP packet where the IP protocol is 103. Depending on the type of message, packets are sent to the PIM All-Router-Multicast address (224.0.0.13) or sent as unicast to a specific host.
With PIM, a source sends the same information to multiple receivers by using one stream of traffic. With its processing load minimized, the source needs to maintain only one session irrespective of the number of actual receivers. The load on the IP network is also minimized, because packets are sent only on links that lead to an interested receiver.
Extreme Networks supports PIM-SM (Sparse Mode) and PIM-SSM (Source Specific Multicast). PIM-SM explicitly builds unidirectional shared trees that are rooted at a rendezvous point (RP) for a group, and, optionally, creates shortest-path trees per source.