3com WX3000 Manuel D’Utilisation

 

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2)  If the corresponding (S, G) entry exists, but the interface on which the packet actually arrived is not 

the incoming interface in the multicast forwarding table, the multicast packet is subject to an RPF 

check.  

z 

If the result of the RPF check shows that the RPF interface is the incoming interface of the existing 

(S, G) entry, this means that the (S, G) entry is correct but the packet arrived from a wrong path and 

is to be discarded.  

z 

If the result of the RPF check shows that the RPF interface is not the incoming interface of the 

existing (S, G) entry, this means that the (S, G) entry is no longer valid. The router replaces the 

incoming interface of the (S, G) entry with the interface on which the packet actually arrived and 

forwards the packet to all the outgoing interfaces.  

3)  If no corresponding (S, G) entry exists in the multicast forwarding table, the packet is also subject to 

an RPF check. The router creates an (S, G) entry based on the relevant routing information and 

using the RPF interface as the incoming interface, and installs the entry into the multicast 

forwarding table.  

z 

If the interface on which the packet actually arrived is the RPF interface, the RPF check is 

successful and the router forwards the packet to all the outgoing interfaces.  

z 

If the interface on which the packet actually arrived is not the RPF interface, the RPF check fails 

and the router discards the packet.  

The basis for an RPF check is a unicast route. A unicast routing table contains the shortest path to each 

destination subnet. A multicast routing protocol does not independently maintain any type of unicast 

route; instead, it relies on the existing unicast routing information in creating multicast routing entries.  

When performing an RPF check, a router searches its unicast routing table. The specific process is as 

follows: The router automatically chooses an optimal unicast route by searching its unicast routing table, 

using the IP address of the “packet source” as the destination address. The outgoing interface in the 

corresponding routing entry is the RPF interface and the next hop is the RPF neighbor. The router 

considers the path along which the packet from the RPF neighbor arrived on the RPF interface to be the 

shortest path that leads back to the source.  

Assume that unicast routes exist in the network, as shown in 

. Multicast packets travel along 

the SPT from the multicast source to the receivers.  

Figure 1-7 RPF check process 

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Router A

Switch B

Switch C

Vlan -int2

Vlan -int1

Vlan -int1

Vlan -int2

Multicast packets

Destination/Mask

IP Routing Table on Switch C

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Interface

Vlan -int2