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Protocol Independent Multicast-Sparse Mode
(PIM-SM) is similar
to PIM-DM in that both protocols depend on the underlying unicast
routing protocol for determining RPF interfaces. A sparse mode
protocol is assumed to operate in an environment where the multicast
sources and multicast receivers are not closely located, so the
distribution of PIM-SM nodes is sparse. This does not imply that
PIM-SM cannot be used in a LAN environment but implies that sparse
mode protocols operate more efficiently over Wide Area Networks
(WAN). Dense mode
protocols, on the other hand, use a broadcast and prune methodology,
whereas multicast routers assume everyone wants to receive multicast
traffic. Under this model, traffic from a multicast source is sent
on all downstream interfaces until an interface is pruned from the
multicast tree. An interface has a limited prune time, after which
the interface is grafted back onto the multicast delivery tree and
multicast traffic is again flooded onto the network. |
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