PIM-SM

PIM-SM, a shared distribution tree technology, designates a router as the rendezvous point (RP), which is the root of a shared tree for a particular group. All sources send packets to the group via the RP (that is, traffic flows from the sender to the RP, and from the RP to the receiver). By maintaining one RP-rooted tree instead of multiple source-rooted trees, bandwidth is conserved.

The following figure illustrates the PIM traffic flow.

1. The source‘s DR registers (encapsulates) and sends multicast data from the source directly to the RP via a unicast routing protocol (number 1 in figure). The RP de-encapsulates each register message and sends the resulting multicast packet down the shared tree.
2. The last-hop router (that is, the receiver‘s DR) sends a multicast group (*,G) join message upstream to the RP, indicating that the receiver wants to receive the multicast data (number 2 in figure). This builds the RP tree (RPT) between the last-hop router and the RP.
3. The RP sends an S,G join message to the source (number 3 in figure). It may send the join message immediately, or after the data rate exceeds a configured threshold. This allows the administrator to control how PIM-SM uses network resources.
4. The last-hop router joins the shortest path tree (SPT) and sends an S,G join message to the source. (number 4 in figure).This builds the SPT.
5. Native multicast packets (that is, non-registered packets) are sent from the source‘s DR to the receiver on its SPT (number 5 in figure), while registered multicast packets continue to be sent from the source‘s DR to the RP.
6. A prune message is sent from the last-hop router to the RP (number 6 in figure).
7. A prune message (register-stop) is sent from the RP to the source‘s DR (number 7 in figure). Once traffic is flowing down the SPT, the RPT is pruned for that given S,G.

When receivers go away, prunes are sent (S,G prune messages towards the source on the SPT, and *,G prune messages towards the RP on the RPT). When new receivers appear, the process begins again.