R211x-HP Flexfabric 11900 IP Multicast Configuration Guide

Figure 15 RPF check process

As shown in Figure 15, assume that unicast routes are available in the network, and no static multicast

routes have been configured on Switch C. A multicast packet (S, G) travels along the SPT from the

multicast source to the receivers. The multicast forwarding table on Switch C contains the (S, G) entry,

with VLAN-interface 20 as the incoming interface.

• When VLAN-interface 20 of Switch C receives the 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 VLAN-interface 10 of Switch C receives the 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 VLAN-interface 20. 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

Switch A

Switch B

Switch C

Vlan-int20

Vlan-int10

Multicast packets

Destination/Mask

IP Routing Table on Switch C

192.168.0.0/24

Interface

Vlan-int20