1-13using the RPF interface as the incoming interface, and installs the entry into the multicastforwarding table.z If the interface on which the packet actually arrived is the RPF interface, the RPF check issuccessful and the router forwards the packet to all the outgoing interfaces.z If the interface on which the packet actually arrived is not the RPF interface, the RPF check failsand the router discards the packet.RPF CheckThe basis for an RPF check is a unicast route. A unicast routing table contains the shortest path to eachdestination subnet. A multicast routing protocol does not independently maintain any type of unicastroute; instead, it relies on the existing unicast routing information in creating multicast routing entries.When performing an RPF check, a router searches its unicast routing table. The specific process is asfollows: The router automatically chooses an optimal unicast route by searching its unicast routing table,using the IP address of the “packet source” as the destination address. The outgoing interface in thecorresponding routing entry is the RPF interface and the next hop is the RPF neighbor. The routerconsiders the path along 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 1-10. Multicast packets travel alongthe SPT from the multicast source to the receivers.Figure 1-10 RPF check processSource192.168.0.1/24ReceiverReceiverRouter ASwitch BSwitch CVlan-int2Vlan-int1Vlan-int1Vlan-int2Multicast packetsDestination/MaskIP Routing Table on Switch C192.168.0.0/24InterfaceVlan-int2z A multicast packet from Source arrives to VLAN-interface 1 of Switch C, and the correspondingforwarding entry does not exist in the multicast forwarding table of Switch C. Switch C performs anRPF check, and finds in its unicast routing table that the outgoing interface to 192.168.0.0/24 isVLAN-interface 2. This means that the interface on which the packet actually arrived is not the RPFinterface. The RPF check fails and the packet is discarded.z A multicast packet from Source arrives to VLAN-interface 2 of Switch C, and the correspondingforwarding entry does not exist in the multicast forwarding table of Switch C. The router performsan RPF check, and finds in its unicast routing table that the outgoing interface to 192.168.0.0/24 is
