Operation Manual – MulticastH3C S5500-EI Series Ethernet Switches Chapter 7 PIM Configuration7-5IV. AssertIf multiple multicast routers exist on a multi-access subnet, duplicate packets may flowto the same subnet. To shut off duplicate flows, the assert mechanism is used forelection of a single multicast forwarder on a multi-access network.Figure 7-2 Assert mechanismAs shown in Figure 7-2, after Router A and Router B receive an (S, G) packet from theupstream node, they both forward the packet to the local subnet. As a result, thedownstream node Router C receives two identical multicast packets, and both Router Aand Router B, on their own local interface, receive a duplicate packet forwarded by theother. Upon detecting this condition, both routers send an assert message to all PIMrouters (224.0.0.13) through the interface on which the packet was received. Theassert message contains the following information: the multicast source address (S),the multicast group address (G), and the preference and metric of the unicast route tothe source. By comparing these parameters, either Router A or Router B becomes theunique forwarder of the subsequent (S, G) packets on the multi-access subnet. Thecomparison 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 routerwith 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 ofthe local interface wins.7.1.3 Introduction to PIM-SMPIM-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 thePIM-DM mode is not suitable for large- and medium-sized networks.
