3Com S7902E, Configuration Manual

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2) If both routers have the same unicast route preference to the source, the router with a smaller
metric to the source wins;
3) If there is a tie in route metric to the source, the router with a higher IP address of the local interface
wins.
Introduction to PIM-SM
PIM-DM uses the “flood and prune” principle to build SPTs for multicast data distribution. Although an
SPT has the shortest path, it is built with a low efficiency. Therefore the PIM-DM mode is not suitable for
large- and medium-sized networks.
PIM-SM is a type of sparse mode multicast protocol. It uses the “pull mode” for multicast forwarding,
and is suitable for large- 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 is to build and maintain rendezvous point trees
(RPTs). An RPT is rooted at a router in the PIM domain as the common node, or rendezvous point
(RP), through which the multicast data travels along the RPT and reaches the receivers.
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When a receiver is interested in the multicast data addressed to a specific multicast group, the
router connected to this receiver sends a join message to the RP corresponding to 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 designated
router (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. Upon reaching the RP, the multicast packet is
duplicated and delivered to the receivers along the RPT.
Multicast traffic is duplicated only where the distribution tree branches, and this process automatically
repeats until the multicast traffic reaches the receivers.
How PIM-SM Works
The working mechanism of PIM-SM is summarized as follows:
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Neighbor discovery
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DR election
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RP discovery
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RPT building
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Multicast source registration
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Switchover to SPT
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Assert