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PIM-SM is a type of sparse mode multicast protocol. It uses the pull mode for multicast forwarding,
and is suitable for large-sized and medium-sized networks with sparsely and widely distributed
multicast group members.
The basic implementation of PIM-SM is as follows:
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PIM-SM assumes that no hosts need to receive multicast data. In the PIM-SM mode, routers
must specifically request a particular multicast stream before the data is forwarded to them. The
core task for PIM-SM to implement multicast forwarding builds and maintains RPTs. An RPT is
rooted at a router in the PIM domain as the common node, or RP, through which the multicast
data travels along the RPT and reaches the receivers.
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After a receiver joins a multicast group, the router connected to this receiver sends a join
message to the RP associated with that multicast group. The path along which the message
goes hop by hop to the RP forms a branch of the RPT.
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When a multicast source sends multicast streams to a multicast group, the source-side DR first
registers the multicast source with the RP by sending register messages to the RP by unicast
until it receives a register-stop message from the RP. The arrival of a register message at the
RP triggers the establishment of an SPT. Then, the multicast source sends subsequent
multicast packets along the SPT to the RP. After reaching the RP, the multicast packet is
duplicated and delivered to the receivers along the RPT.
NOTE:
Multicast traffic is duplicated only where the distribution tree branches, and this process
automatically repeats until the multicast traffic reaches the receivers.
The operating mechanism of PIM-SM is summarized as follows:
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Neighbor discovery
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For more information, see
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DR election
PIM-SM also uses hello messages to elect a DR for a multi-access network (such as Ethernet). The
elected DR is the only multicast forwarder on this multi-access network.
A DR must be elected in a multi-access network, no matter this network connects to multicast
sources or to receivers. The receiver-side DR sends join messages to the RP. The source-side DR
sends register messages to the RP.
A DR is elected on a multi-access subnet by means of comparison of the priorities and IP addresses
carried in hello messages. An elected DR is substantially meaningful to PIM-SM. PIM-DM itself does
not require a DR. However, if IGMPv1 runs on any multi-access network in a PIM-DM domain, a DR
must be elected to act as the IGMPv1 querier on that multi-access network.
NOTE:
IGMP must be enabled on a device that acts as a receiver-side DR before receivers attached to this
device can join multicast groups through this DR. For more information about IGMP, see
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