Pim-Sm Overview - HP 6125G Configuration Manual

If a tie exists in route metric to the source, the router with a higher IP address of the downstream
3.
interface wins.

PIM-SM overview

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-sized and medium-sized networks with sparsely and widely distributed multicast group
members.
The basic implementation of PIM-SM is as follows:
PIM-SM assumes that no hosts need to receive multicast data. In the PIM-SM mode, routers must
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specifically request a particular multicast stream before the data is forwarded to them. The core task
for PIM-SM to implement multicast forwarding will 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.
When a receiver is interested in the multicast data addressed to a specific multicast group, the
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router connected to this receiver sends a join message to the RP that corresponds to that multicast
group. The path along which the message goes hop by hop to the RP forms a branch of the RPT.
When a multicast source sends multicast streams to a multicast group, the source-side designated
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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. 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 working mechanism of PIM-SM is summarized as follows:
• Neighbor discovery
DR election
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RP discovery
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• RPT building
Multicast source registration
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Switchover to SPT
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Assert
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Neighbor discovery
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For more information, see
"Neighbor discovery."
DR election
PIM-SM also uses hello messages to elect a DR for a multi-access network (such as Ethernet). The elected
DR will be the only multicast forwarder on this multi-access network.
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