Introduction To Pim-Sm - H3C S7500E Series Operation Manual

Operation Manual – Multicast
H3C S7500E Series Ethernet Switches
IV. Assert
If multiple multicast routers exist on a multi-access subnet, duplicate packets may flow
to the same subnet. To shut off duplicate flows, the assert mechanism is used for
election of a single multicast forwarder on a multi-access network.
Figure 5-2 Assert mechanism
As shown in
upstream node, 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) 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 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 unicast route preference to the source wins;
2) If both routers have the same unicast 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 local interface wins.

5.1.3 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.
Figure 5-2, after Router A and Router B receive an (S, G) packet from the
5-5
Chapter 5 PIM Configuration