Overview of PIM

Definition

Protocol Independent Multicast (PIM) is a multicast routing solution that performs a reverse path forwarding (RPF) check on multicast packets based on the unicast routing table, creates multicast routing entries if multicast packets pass the RPF check, and forwards the multicast packets using multicast routing entries. PIM is termed protocol-independent because it is not dependent on any particular unicast routing protocol for topology discovery.

Currently, the device supports PIM-DM and PIM-SM.

Purpose

In 1992, the multicast backbone (Mbone) was established as a virtual backbone for IP multicast traffic to support network video and audio conferencing. Mbone uses multicast open shortest path first (MOSPF) and the Distance Vector Multicast Routing Protocol (DVMRP) as multicast routing protocols.

Mbone promotes the use and development of multicast technologies and routing protocols. However, new problems are created because various multicast routing protocols are widely used. If multicast routes like unicast are dynamically generated using different routing algorithms, complex operations such as route import are required. Network devices will then need to maintain both unicast and multicast routing information.

PIM was developed to simplify operations and reduce network device load. This solution maintains only the information about multicast group members and the multicast source status. PIM reduces protocol implementation complexity because there is no need to maintain a large amount of routing information. Instead, routing information is obtained from only the unicast routing table. As a result, PIM became the most widely used intra-domain multicast protocol.

A PIM network is comprised of multiple PIM routers. A large PIM network can be partitioned into multiple PIM domains to manage and control multicast packet forwarding.