Operation Manual – MulticastH3C S3100 Series Ethernet Switches Chapter 1 Multicast Overview1-133) If no corresponding (S, G) entry exists in the multicast forwarding table, the packetis also subject to an RPF check. The router creates an (S, G) entry based on therelevant routing information and using the RPF interface as the incoming interface,and installs the entry into the multicast forwarding table.z If the interface on which the packet actually arrived is the RPF interface, the RPFcheck is successful and the router forwards the packet to all the outgoinginterfaces.z If the interface on which the packet actually arrived is not the RPF interface, theRPF check fails and the router discards the packet.1.4.2 RPF CheckThe basis for an RPF check is a unicast route. A unicast routing table contains theshortest path to each destination subnet. A multicast routing protocol does notindependently maintain any type of unicast route; instead, it relies on the existingunicast routing information in creating multicast routing entries.When performing an RPF check, a router searches its unicast routing table. Thespecific process is as follows: The router automatically chooses an optimal unicastroute by searching its unicast routing table, using the IP address of the “packet source”as the destination address. The outgoing interface in the corresponding routing entry isthe RPF interface and the next hop is the RPF neighbor. The router considers the pathalong which the packet from the RPF neighbor arrived on the RPF interface to be theshortest path that leads back to the source.Assume that unicast routes exist in the network, as shown in Figure 1-7. Multicastpackets travel along the SPT from the multicast source to the receivers.Source192.168.0.1/24ReceiverReceiverRouter ASwitch BSwitch CVlan-int2Vlan-int1Vlan-int1Vlan-int2Multicast packetsDestination/MaskIP Routing Table on Switch C192.168.0.0/24InterfaceVlan-int2Figure 1-7 RPF check process
