How Pim-Sm Works - 3Com 5500-EI PWR Install Manual

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:
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.
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.
When a multicast source sends a multicast packet to a multicast group, the router directly
connected with the multicast source first registers the multicast source with the RP by sending a
register message to the RP by unicast. The arrival of this 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:
Neighbor discovery
DR election
RP discovery
RPT building
Multicast source registration
Switchover from RPT to SPT
Assert
Neighbor discovery
PIM-SM uses exactly the same neighbor discovery mechanism as PIM-DM does. Refer to
discovery.
DR election
PIM-SM also uses hello messages to elect a designated router (DR) for a multi-access network. The
elected DR will be the only multicast forwarder on this multi-access network.
5
Neighbor