4000 Router Series IP Multicast Configuration Guide

Figure 14 RPF check process

As shown in Figure 14, assume that unicast routes are available in the network, and no static multicast

routes have been configured on Router C. Multicast packets travel along the SPT from the multicast source

to the receivers. The multicast forwarding table on Router C contains the (S, G) entry, with Ethernet 1/2

as the incoming interface.

• When Ethernet 1/2 of Router C receives a multicast packet, because the interface is the incoming

interface of the (S, G) entry, the router forwards the packet out of all outgoing interfaces.

• When Ethernet 1/1 of Router C receives a multicast packet, because the interface is not the

incoming interface of the (S, G) entry, the router performs an RPF check on the packet. The router

looks up its unicast routing table and finds that the outgoing interface to the source (the RPF

interface) is Ethernet 1/2. It means that the (S, G) entry is correct, but the packet traveled along a

wrong path. The RPF check fails and the router discards the packet.

Static multicast routes

Depending on the application environment, a static multicast route can change an RPF route or create an

RPF route.

Changing an RPF route

Typically, the topology structure of a multicast network is the same as that of a unicast network, and

multicast traffic follows the same transmission path as unicast traffic does. You can configure a static

multicast route for a given multicast source to change the RPF route, so that the router creates a

transmission path for multicast traffic that is different from the transmission path for unicast traffic.

Source

192.168.0.1/24

Receiver

Router A

Router B

Router C

Eth1/2

Eth1/1

Multicast packets

Destination/Mask

IP Routing Table on Router C

192.168.0.0/24

Interface

Eth1/2