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packet arrived is the RPF interface, the router forwards the packet to all the outgoing interfaces.
Otherwise, it discards the packet.
Assume that unicast routes are available in the network, MBGP is not configured, and no static multicast
routes have been configured on Switch C, as shown in Figure 22. Multi
cast 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.
Figure 22 RPF check process
• When a multicast packet arrives on interface VLAN-interface 20 of Switch C, because the interface
is the incoming interface of the (S, G) entry, the router forwards the packet to all outgoing
interfaces.
• When a multicast packet arrives on interface VLAN-interface 10 of Switch C, because the interface
is not the incoming interface of the (S, G) entry, the router performs an RPF check on the packet. The
router searches its unicast routing table and finds that the outgoing interface to Source (the RPF
interface) is VLAN-interface 20. This means the (S, G) entry is correct, and packet arrived along a
wrong path. The RPF check fails and the packet is discarded.
Static multicast routes
A static multicast route is an important basis for RPF check. Depending on the application environment,
a static multicast route can change an RPF route and 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 to create a transmission path for
multicast traffic that is different from 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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