H3C Technologies H3C S12500 Series Switches - Manual (Page 77)

61
NOTE:
The switch allows you to configure special processing of packets that have failed an RPF check instead of
simply dropping them. For more information, see "
Configuring RPF check failure processing ."
As shown in Figure 23 , assume that unicast routes are available in the network, MBGP is not configured,
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 5 as the incoming interface.
Figure 23 RPF check process
• When a multicast packet arrives on interface VLAN-interface 5 of Switch C, because the interface
is the incoming interface of the (S, G) entry, the router forwards the packet out of all outgoing
interfaces.
• When a multicast packet arrives on interface VLAN-interface 2 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 5. 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. By configuring a static
multicast route for a given multicast source, you can 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-int5
Vlan-int2
Vlan-int2
Multicast packets
Destination/Mask
IP Routing Table on Switch C
192.168.0.0/24
Interface
Vlan-int5