Receiver Joining a Group

The following steps describe how a receiver joins a multicast group:

1. A receiver multicasts an IGMP host membership message to the group that it wants to join.

2. After the last-hop router (the DR), normally the PIM router with the highest IP address for that VLAN, receives the IGMP message for a new group join, the router looks up the associated elected RP with responsibility for the group.

3. After it determines the RP router for the group, the last-hop router creates a (*,G) route entry in the multicast forwarding table and sends a (*,G) join message to the RP. After the last-hop router receives data packets from the RP, if the multicast packet arrival rate exceeds the DR threshold, the last-hop router switches to the SPT by sending an (S,G) join message to the source. (S denotes the source unicast IP address, and G denotes the multicast group address.)

4. If the last-hop router switches to the SPT, the following actions occur:

   • All intermediate PIM routers along the path to the source create the (S,G) entry.

   • To trim the shared tree, the router sends an (S,G) prune message to the RP.

You can enable the PIM Infinite Threshold Policy feature to prevent the SPT switchover. Multicast traffic follows the shared tree path through a Rendezvous Point (RP) instead of switching over to SPT.