3. PIM-SM Protocol Overview

This section provides an overview of PIM-SM behavior. It is intended as an introduction to how PIM-SM works, and it is NOT definitive. For the definitive specification, see Section 4.

PIM relies on an underlying topology-gathering protocol to populate a routing table with routes. This routing table is called the Multicast Routing Information Base (MRIB). The MRIB may be taken directly from the unicast routing table or provided by a separate routing protocol such as MBGP. The primary role of the MRIB is to provide the next-hop router along a multicast-capable path to each destination subnet. Data flows along the reverse path of the Join messages.

Like all multicast routing protocols implementing the RFC 1112 service model, PIM-SM must be able to route data packets from sources to receivers without sources or receivers knowing of each other's existence. This is accomplished in three phases, although these phases may occur simultaneously as senders and receivers can come and go at any time.