3Com WX3000 operation manual RPF Check, 7RPF check process

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.

zIf 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.

zIf 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that the RPF interface is not the incoming interface of the existing (S, G) entry, this means that the (S, G) entry is no longer valid.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.

zIf 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.

zIf the interface on which the packet actually arrived is not the RPF interface, the RPF check fails and the router discards the packet.

RPF Check

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 Figure 1-7. Multicast packets travel along the SPT from the multicast source to the receivers.

Figure 1-7RPF check process

Switch B Receiver

Source

192 .168.0.1/24

Multicast packets

IP Routing Table on Switch C

Destination/Mask Interface

192.168.0.0/24 Vlan -int2

Vlan -int2

Vlan -int1

Router A

Vlan -int1

Vlan -int2

Switch C

Receiver

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