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If a multicast packet arrives at Router C on GigabitEthernet 1/0/1, the receiving interface is not
the incoming interface of the (S, G) entry. Router C searches its unicast routing table and finds
that the outgoing interface to the source (the RPF interface) is GigabitEthernet 1/0/2. In this
case, the (S, G) entry is correct, but the packet traveled along a wrong path. The packet fails the
RPF check and Router 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 for a multicast source to change the RPF route. As a result, the router creates a
transmission path for multicast traffic that is different from the transmission path for unicast traffic.
Figure 16 Changing an RPF route
, when no static multicast route is configured, Router C's RPF neighbor on the
path back to the source is Router A. The multicast data from the source travels through Router A to
Router C. You can configure a static multicast route on Router C and specify Router B as Router C's
RPF neighbor on the path back to the source. The multicast data from the source travels along the
path: Router A to Router B and then to Router C.
Creating an RPF route
When a unicast route is blocked, multicast forwarding might be stopped due to lack of an RPF route.
You can configure a static multicast route to create an RPF route. In this way, a multicast routing
entry is created to guide multicast forwarding.
Receiver
Router A
Router B
Router C
GE1/0/2
GE1/0/1
1.1.1.1/24
Multicast packets
Source
192.168.0.1/24
Receiver
GE1/0/1
1.1.1.2/24
Multicast static route
Source/Mask
Multicast Routing Table Static on Router C
192.168.0.0/24
Interface
GE1/0/1
RPF neighbor/Mask
1.1.1.1/24
