1164 CHAPTER 71: PIM C ONFIGURATIONFigure 341 Assert mechanismAs shown in Figure 341, after Router A and Router B receive an (S, G) packet fromthe 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 bothRouter A and Router B, on their own local interface, receive a duplicate packetforwarded by the other. Upon detecting this condition, both routers send an assertmessage to all PIM routers (224.0.0.13) through the interface on which the packetwas received. The assert message contains the following information: themulticast source address (S), the multicast group address (G), and the preferenceand 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 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.Introduction to PIM-SM PIM-DM uses the “flood and prune” principle to build SPTs for multicast datadistribution. 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” formulticast forwarding, and is suitable for large- and medium-sized networks withsparsely 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-SMmode, routers must specifically request a particular multicast stream before thedata is forwarded to them. The core task for PIM-SM to implement multicastforwarding is to build and maintain rendezvous point trees (RPTs). An RPT isrooted at a router in the PIM domain as the common node, or rendezvouspoint (RP), through which the multicast data travels along the RPT and reachesthe receivers.EthernetRouter A Router BRouter C ReceiverMulticast packetsAssert message
