Overview PIM-SM

PIM-SM (Protocol Independent Multicast, Sparse Mode) is applicable when the group members are relatively dispersive and their range is relatively broad or the network bandwidth resource is relatively limited.

PIM-SM does not depend on any particular unicast route protocol. The device announces the multicast information to all PIM-SM routers by actively sending the packets to request to set up multicast distributing tree (MDT) and set RP (Rendezvous Point) and BSR (Bootstrap Router). When the receiver adds to one multicast group, the receiving end DR (Designated Router) sends the PIM adding packet to RP, constructing the sharing tree-RPT with RP as root, while the source DR registers the multicast source to RP, constructing the source tree with the multicast source as root. The multicast service packets are transmitted to the receiver along the source tree and sharing tree; the receiving end DR sends the PIM adding packet to the multicast source. At last, switch from RPT to source-based SPT (Shortest-path Tree), so as to reduce the network delay.

PIM SSM is short for Protocol Independent Multicast ---- Source Specific Multicast. PIM-SSM is the subset of the PIM-SM protocol and should run on the basis of PIM-SM. The PIM-SSM protocol set the IPv4 address 232.0.0.0-232.255.255.255 to be reserved for SSM. PIM-SSM should work with IGMPv3, because IGMPv3 can send the IGMP membership report packet of the specified source and group.