Unlike PIM-DM, Protocol-Independent Multicast - Sparse Mode (PIM-SM) is an explicit join protocol, which means that multicast receivers, and the routers that support them, must join multicast groups before they receive multicast traffic. When all receivers on a network branch leave a multicast group, that branch is pruned so that the multicast traffic does not continue to consume bandwidth on that branch. PIM-SM supports shared trees as well as shortest path trees (SPTs). PIM-SM is beneficial for large networks that have group members that are sparsely distributed.
Note
This feature is supported at and above the license level listed for this feature in the license tables in the Switch Engine v33.1.1 Licensing Guide document.Using PIM-SM, the router sends a join message to the rendezvous point (RP). The RP is a central multicast router that is responsible for receiving and distributing the initial multicast packets. You can configure a dynamic or static RP.
When a router has a multicast packet to distribute, it encapsulates the packet in a unicast message and sends it to the RP. The RP decapsulates the multicast packet and distributes it among all member routers.
When a router determines that the multicast rate has exceeded a configured threshold, that router can send an explicit join to the originating router. When this occurs, the receiving router gets the multicast directly from the sending router and bypasses the RP.
Note
You can run either PIM-DM or PIM-SM per VLAN.