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 routes in this table may be taken directly from the unicast routing table, or they may be different and provided by a separate routing protocol such as MBGP [10]. Regardless of how it is created, the primary role of the MRIB in the PIM protocol is to provide the next-hop router along a multicast-capable path to each destination subnet. The MRIB is used to determine the next-hop neighbor to which any PIM Join/Prune message is sent. Data flows along the reverse path of the Join messages. Thus, in contrast to the unicast RIB, which specifies the next hop that a data packet would take to get to some subnet, the MRIB gives reverse-path information and indicates the path that a multicast data packet would take from its origin subnet to the router that has the MRIB.

Like all multicast routing protocols that implement the service model from RFC 1112 [3], PIM-SM must be able to route data packets from sources to receivers without either the sources or receivers knowing a priori of the existence of the others. This is essentially done in three phases, although as senders and receivers may come and go at any time, all three phases may occur simultaneously.