z In the network, multicast packet transmission is based on the guidance of the multicast forwardingtable derived from the unicast routing table or the multicast routing table specially provided formulticast.z To process the same multicast information from different peers received on different interfaces ofthe same device, every multicast packet is subject to a Reverse Path Forwarding (RPF) check onthe incoming interface. The result of the RPF check determines whether the packet will beforwarded or discarded. The RPF check mechanism is the basis for most multicast routingprotocols to implement multicast forwarding.The RPF mechanism enables multicast devices to forward multicast packets correctly based on themulticast route configuration. In addition, the RPF mechanism also helps avoid data loops caused byvarious reasons.Implementation of the RPF MechanismUpon receiving a multicast packet that a multicast source S sends to a multicast group G, the multicastdevice first searches its multicast forwarding table:1) If the corresponding (S, G) entry exists, and the interface on which the packet actually arrived is theincoming interface in the multicast forwarding table, the router forwards the packet to all theoutgoing interfaces.2) If the corresponding (S, G) entry exists, but the interface on which the packet actually arrived is notthe incoming interface in the multicast forwarding table, the multicast packet is subject to an RPFcheck.z If the result of the RPF check shows that the RPF interface is the incoming interface of the existing(S, G) entry, this means that the (S, G) entry is correct but the packet arrived from a wrong path andis to be discarded.z If the result of the RPF check shows that the RPF interface is not the incoming interface of theexisting (S, G) entry, this means that the (S, G) entry is no longer valid. The router replaces theincoming interface of the (S, G) entry with the interface on which the packet actually arrived andforwards the packet to all the outgoing interfaces.3) If no corresponding (S, G) entry exists in the multicast forwarding table, the packet is also subject toan RPF check. The router creates an (S, G) entry based on the relevant routing information andusing 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 router25-11
