Operation Manual – MulticastH3C S7500 Series Ethernet Switches Chapter 7 PIM Configuration7-4V. Assert mechanismIn the shared network such as Ethernet, the same packets may be sent repeatedly. Forexample, the LAN network segment contains many multicast routers, A, B, and C. Theyeach have their own receiving path to the multicast source S. As shown in Figure 7-2:EthernetRouter ARouter CRouter BReceiverMulticast packetsAssert messageAssert messageFigure 7-2 Diagram for assert mechanismWhen Router A and Router B receive a multicast packet sent from the multicast sourceS, they will all forward the multicast packet to the Ethernet. In this case, thedownstream node Router C will receive two copies of the same multicast packet.In order to avoid such cases, the Assert mechanism is needed to select one forwarder.Routers in the network select the best path through sending Assert messages. If two ormore paths have the same priority and metric to the multicast source, the router withthe highest IP address will be the upstream neighbor of the (S, G) entry, which isresponsible for forwarding the (S, G) multicast packets. The unselected routers willprune the corresponding interfaces to disable the information forwarding.7.1.3 Introduction to PIM-SMProtocol independent multicast sparse mode (PIM-SM) is a sparse mode multicastprotocol. It is generally used in the following occasions where:z Group members are sparsely distributedz The range is widez Large scaled networksIn PIM-SM, all hosts do not receive multicast packets by default. Multicast packets areforwarded to the hosts which need multicast packets explicitly.In order that the receiver can receive the multicast data streams of the specific IGMPgroups, PIM-SM adopts rendezvous points (RP) to forward multicast information to allPIM-SM routers with receivers. RP is adopted in multicast forwarding. As a result, the
