3com S7906E 설치 설명서

 

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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.  

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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.  

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:  

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 

, 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.  

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.