H3C Technologies H3C S7500E Series Switches - Manual (Page 142)

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interface on which the packet was received. The assert message contains the following
information: the multicast source address (S), the multicast group address (G), and the
preference and metric of the unicast route to the source. By comparing these parameters, either
Router A or Router B becomes the unique forwarder of the subsequent (S, G) packets on the
multi-access subnet. The comparison process is as follows:
The router with a higher unicast route preference to the source wins;
1) If both routers have the same unicast route preference to the source, the router with a
smaller metric to the source wins;
2) 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:
z 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.
z 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.
z 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.