Rpf Check - H3C S3100 Series Operation Manual

H3c s3100 series ethernet switches operation manual

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

forwarding table.

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.

If 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

the SPT from the multicast source to the receivers.

Figure 1-10 RPF check process

192.168.0.1/24

Multicast packets

IP Routing Table on Switch C

Destination/Mask

192.168.0.0/24

A multicast packet from Source arrives to VLAN-interface 1 of Switch C, and the corresponding

forwarding entry does not exist in the multicast forwarding table of Switch C. Switch C performs an

RPF check, and finds in its unicast routing table that the outgoing interface to 192.168.0.0/24 is

VLAN-interface 2. This means that the interface on which the packet actually arrived is not the RPF

interface. The RPF check fails and the packet is discarded.

A multicast packet from Source arrives to VLAN-interface 2 of Switch C, and the corresponding

forwarding entry does not exist in the multicast forwarding table of Switch C. The router performs

an RPF check, and finds in its unicast routing table that the outgoing interface to 192.168.0.0/24 is

1-10. Multicast packets travel along

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