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Figure 22 RPF check process
As shown in Figure 22, assume that unicast routes are available in the network and no static multicast
routes have been configured on Switch C. Multicast packets travel 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.
• If a multicast packet arrives at Switch C on VLAN-interface 20, Switch C forwards the packet out of
all outgoing interfaces.
• If a multicast packet arrives at Switch C on VLAN-interface 10, Switch C performs an RPF check on
the packet. Switch C looks up its unicast routing table and finds that the outgoing interface to the
source (the RPF interface) is VLAN-interface 20. In this case, the (S, G) entry is correct, but the
packet traveled along a wrong path. The packet fails the RPF check and Switch C 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 back to a multicast source to change the RPF route. The multicast traffic will travel along
a different transmission path than that for unicast traffic.
Source
192.168.0.1/24
Receiver
Receiver
Switch A
Switch B
Switch C
Vlan-int20
Vlan-int10
Vlan-int10
Multicast packets
Destination/Mask
IP Routing Table on Switch C
192.168.0.0/24
Interface
Vlan-int20
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