Instead of flooding multicast traffic as is done with PIM-DM, PIM-SM uses a set of multiple routers to operate as RPs. Each RP controls multicast traffic forwarding for one or more multicast groups as follows:
  • Receives traffic from multicast sources (S) via a DR.

  • Receives multicast joins from routers requesting multicast traffic.

  • Forwards the requested multicast traffic to the requesting routers.

Note that the routers requesting multicast traffic are either edge routers or intermediate routers. Edge routers are directly connected to specific multicast receivers using ICMP to request traffic. Intermediate routers are on the path between edge routers and the RP. This is known as a RP Tree (RPT) where only the multicast address appears in the routing table. For example:

( *, G ), where:

* = a variable (wildcard) representing the IP address of any multicast source

G = a particular multicast group address.


The software supports up to 100 RPs in a given PIM-SM domain.