3Com S7902E, Руководство по настройке

Страница: 973 / 2621

1-4
z
When a multicast packet arrives on Vlan-interface20 of Router C, as the interface is the incoming
interface of the (S, G) entry, the router forwards the packet to all outgoing interfaces.
z
When a multicast packet arrives on Vlan-interface10 of Router C, as 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-interface20. This means the (S, G) entry is correct and packet arrived along a wrong path.
The RPF check fails and the packet is discarded.
Multicast Static Routes
A multicast static route is an important basis for RPF check. Depending on the application environment,
a multicast static route has the following two functions:
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 multicast
static route for a given multicast source, you can change the RPF route so as to create a transmission
path for multicast traffic different from that for unicast traffic.
Figure 1-2 Changing an RPF route
As shown in Figure 1-2 , when no multicast static route is configured, Router C’s RPF neighbor on the
path back to Source is Router A and the multicast information from Source travels along the path from
Router A to Router C, which is the unicast route between the two routers; with a static route configured
on Router C and with Router B as Router C’s RPF neighbor on the path back to Source, the multicast
information from Source travels from Router A to Router B and then to Router C.
Creating an RPF route
When a unicast route is blocked, multicast traffic forwarding is stopped due to lack of an RPF route. By
configuring a multicast static route for a given multicast source, you can create an RPF route so that a
multicast routing entry is created to guide multicast traffic forwarding.