Pim-Sm Overview - HP A8800 Configuration Manual

Figure 38 Assert mechanism
As shown in
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 downstream 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 downstream 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/MBGP
route/multicast static 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:
1. The router with a higher route preference to the source wins;
2. If both routers have the same 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 downstream
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- 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 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
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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.
Figure 38, after Router A and Router B receive an (S, G) packet from the upstream node, they
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