The following steps describe how a receiver joins a multicast group:
A receiver multicasts an IGMP host membership message to the group that it wants to join.
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.
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.)
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.