Operation of Protocol Independent Multicast Sparse Mode
PIM-SM works with a completely opposite strategy from that of PIM-DM, although the
mechanics of the protocol are not exactly opposite. PIM-SM assumes that no hosts want to receive
multicast packets until they specifically ask to receive them. As a result, until a host in a subnet
asks to receive multicasts for a particular group, multicasts are never delivered to that subnet. With
PIM-SM, downstream routers must request to receive multicasts using PIM Join messages. Also,
once they are receiving those messages, the downstream router must continually send Join messages
to the upstream router—otherwise, the upstream router stops forwarding, putting the link in a
pruned state. This process is opposite to that used by PIM-DM, in which the default is to flood
multicasts, with downstream routers needing to continually send Prunes or State Refresh messages
to keep a link in a pruned state.
PM-SM makes the most sense with a small percentage of subnets that need to receive packets sent
to any multicast group.