Pim-Sm Overview - HP A5500 SI Switch Series Configuration Manual

Figure 37 Assert mechanism
As shown in
they both forward the packet to the local subnet. As a result, the downstream node Router C receives
two identical multicast packets, and both Router A and Router B, on their own local interface, receive a
duplicate packet forwarded by the other. Upon detecting this condition, both routers send an assert
message to all PIM routers (224.0.0.13) on the local subnet through the interface that received the
packet. The assert message contains 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 following describes the comparison process:
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
2.
to the source wins.
If a tie occurs in route metric to the source, the router with a higher IP address of the local interface
3.
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-sized 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
•
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
•
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.
Figure 37, after Router A and Router B receive an (S, G) packet from the upstream node,
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