Operation Manual – Multicast ProtocolH3C S3610&S5510 Series Ethernet Switches Chapter 8 Multicast Policy Configuration8-3Source192.168.0.1/ 24ReceiverReceiverMulticast packetRouter ARouter CRouter BIP Routing Table on Router CDestination / Mask192 .168.0.0/ 24InterfacePOS 5/1POS5/ 0POS5/1POS 5/0POS 5/1×Figure 8-1 RPF check processz A multicast packet from Source arrives on POS 5/0 of Router C, and the multicastforwarding table does not contain the corresponding forwarding entry. The routerperforms an RPF check, and finds in its unicast routing table that the outgoinginterface on the shortest path to the subnet 192.168.0.0/24 is POS 5/1, so therouter knows that the interface on which the packet has actually arrived is not theRPF interface. The packet fails the RPF check and is discarded.z A multicast packet from Source arrives on POS 5/1 of Router C, and the multicastforwarding table does not contain the corresponding forwarding entry. The routerperforms an RPF check, and finds in its unicast routing table that the outgoinginterface on the shortest path to the subnet 192.168.0.0/24 is the interface onwhich the packet has actually arrived. The packet passes the RPF check and isforwarded.The RPF mechanism enables routers to correctly forward multicast packets based onthe multicast route configuration. In addition, the RPF mechanism also helps avoid dataloops caused by various reasons.8.1.2 How a Multicast Policy WorksI. Multicast static routeIf the topology structure of a multicast network is the same as that of a unicast network,receivers can receive multicast data via unicast routes. However, the topology structurea multicast network may differ from that of a unicast network, and some routers maysupport only unicast but not multicast. In this case, you can configure multicast staticroutes to provide multicast transmission paths that are different from those for unicasttraffic. Note the following two points:z A multicast static route is used only for RPF check, rather than for guidingmulticast forwarding.z A multicast static route is effective on the multicast router on which it is configured,and will not be broadcast throughout the network or injected to other routers.
