Operation Manual – MulticastH3C S5600 Series Ethernet Switches Chapter 1 Multicast Overview1-14unicast 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 processz A multicast packet from Source arrives to VLAN-interface 1 of Switch C, and thecorresponding forwarding entry does not exist in the multicast forwarding table ofSwitch C. Switch C performs an RPF check, and finds in its unicast routing tablethat the outgoing interface to 192.168.0.0/24 is VLAN-interface 2. This means thatthe interface on which the packet actually arrived is not the RPF interface. TheRPF check fails and the packet is discarded.z A multicast packet from Source arrives to VLAN-interface 2 of Switch C, and thecorresponding forwarding entry does not exist in the multicast forwarding table ofSwitch C. The router performs an RPF check, and finds in its unicast routing tablethat the outgoing interface to 192.168.0.0/24 is the interface on which the packetactually arrived. The RPF check succeeds and the packet is forwarded.
