PIM-SM requires a unicast protocol to forward multicast traffic within the network to perform the Reverse Path Forwarding (RPF) check. PIM-SM uses the information from the unicast routing table to create and maintain the shared and shortest multicast tree that PIM-enabled routers use to communicate. The unicast routing table must contain a route to every multicast source in the network as well as routes to PIM entities like the RPs and BSR.
SSM uses only a subset of the PIM-SM features such as the SPT, DR, and some messages (hello, join, prune, and assert). However, some features are unique to SSM. These features, described in the following sections, are extensions of the IGMP and PIM protocols.