A designated router is selected in multiaccess segments running PIM. The PIM DR is responsible for sending join, prune, and register messages to the RP. The PIM router with the highest IP address is selected as the DR.

Joining PIM-SM

With PIM-SM, DRs on end segments receive IGMP query messages from hosts wanting to join a multicast group. The router checks whether it is already receiving the group for another interface. If it is receiving the group, the router adds the new interface to the table and queries periodically on the new interface.

If the multicast group is not in the multicast table, the router adds the interface to the multicast table and sends a join message to the RP with multicast address 224.0.0.13 (all PIM routers) requesting the multicast group.

Pruning PIM-SM

When there are no more multicast receiving hosts or receiving routers, a prune message is sent to the RP. The prune message includes the group to be pruned or removed.

Auto-RP

Another way to configure the RP for a network is to have the RP announce its services to the PIM network. This process is called auto-RP. Candidate RPs send their announcements to RP mapping agents with multicast address 224.0.1.39 (using the command cisco-rp-announce). RP mapping agents are also configured. In a small network, the RP can be the mapping agent. The 224.0.1.40 address used in auto-RP discovery is the destination address for messages from the RP mapping agent to discover candidates. Configured RP mapping agents listen to the announcements. The RP mapping agent selects the RP for a group based on the highest IP address of all the candidate RPs. The RP mapping agents then send RP discovery messages to the rest of the PIM-SM routers in the internetwork with the selected RP-to-group mappings.

BIDIR-PIM

Bidirectional PIM (BIDIR-PIM), which is defined in RFC 5015, is a variant of PIM-SM that builds bidirectional shared trees connecting multicast sources and receivers. It never builds a shortest path tree toward the source, so it scales well because it does not need a source-specific state.

BIDIR-PIM eliminates the need for a first-hop route to encapsulate data packets being sent to the RP. BIDIR-PIM dispenses with both encapsulation and source state by allowing packets to be natively forwarded from a source to the RP using shared tree state. BIDIR-PM is recommended for many-to-many deployment models.