1-12When 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 1-7. Multicast packets travel alongthe SPT from the multicast source to the receivers.Figure 1-7 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 isthe interface on which the packet actually arrived. The RPF check succeeds and the packet isforwarded.
