PIM-SM uses a shared distribution tree built rooted at an RP to determine paths from source to receiver groups. The RP must be administratively configured on the network.
The multicast source registers with the RP and sends the traffic to the RP. Receivers join the group and are added to the tree that is rooted at the RP. The RP distributes the traffic down the tree to the hosts that have joined the multicast group.
By using a shared tree, PIM-SM reduces the amount of information the router needs to maintain. Only information about multicast groups is maintained; state information about sources is not. Unlike PIM-DM, a PIM-SM router with no receivers need not maintain pruning information.
In addition, unlike PIM-DM, multicast data is not flooded to the network. Multicast traffic is not sent to a network segment unless the downstream router specifically requests it. This can significantly reduce the amount of traffic on the network.
Because of these advantages, PIM-SM is currently the multicast protocol of choice in wide-area, inter-domain networks.