PIM-DM is short for Protocol Independent Multicast Dense Mode. In this mode, the flood-prune mode is used to forward multicast data. PIM-DM is applicable to the small LAN in which group members densely reside. Rendezvous Points (RPs) and Designated Routers (DRs) are not involved in PIM-DM. Through the periodical flooding-pruning, PIM-DM creates and maintains a unidirectional and loop-free SPT connecting the multicast source and group members.


PIM-SM is short for Protocol Independent Multicast Sparse Mode. In this mode, an MDT is set up when receivers actively join a multicast group. PIM-SM is applicable to the network on which group members sparsely reside. RP maintenance, RPT construction, and multicast source registration are involved in PIM-SM.


PIM-SSM is similar to PIM-SM but maintaining RPs, constructing RPTs, and registering multicast sources are not required. In PIM-SSM, an SPT can be directly set up between a multicast source and group members as long as the source is routable. Therefore, PIM-SSM has a distinct advantage in inter-AS multicast data forwarding.

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