H3C Technologies H3C SecPath F1000-E - Manual (Page 372)

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Figure 2 Assert mechanism
As shown in
Figure 2 , after Device A and Device B receive an (S, G) packet from the upstream node, they
both forward the packet to the local subnet. As a result, the downstream node Device C receives two
identical multicast packets, and both Device A and Device B, on their own local interface, receive a
duplicate packet forwarded by the other. Upon detecting this condition, both devices send an assert
message to all PIM routers (224.0.0.13) through the 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 Device A or Device B becomes the unique forwarder of the subsequent (S, G) packets
on the multi-access subnet. The comparison process is as follows:
1. The device with a higher unicast route preference to the source wins;
2. 8If both devices have the same unicast route preference to the source, the device with a smaller
metric to the source wins;
3. If there is a tie in route metric to the source, the device 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:
• 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.