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Cisco MWR 2941 Mobile Wireless Edge Router Software Configuration Guide, Release 12.2(33)MRB

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Chapter 1 Cisco MWR 2941 Router Overview

Features

RPF Check

When a multicast packet arrives at a router, the router performs an RPF check on the packet. If the RPF

check succeeds, the packet is forwarded. Otherwise, it is dropped.

For traffic flowing down a source tree, the RPF check procedure works as follows:

1. The router looks up the source address in the unicast routing table to determine if the packet has

arrived on the interface that is on the reverse path back to the source.

2. If the packet has arrived on the interface leading back to the source, the RPF check succeeds and the

packet is forwarded out the interfaces present in the outgoing interface list of a multicast routing

table entry.

3. If the RPF check in Step 2 fails, the packet is dropped.

Figure 9 shows an example of an unsuccessful RPF check.

Figure 9 RPF Check Fails

As Figure 9 illustrates, a multicast packet from source 151.10.3.21 is received on serial interface 0 (S0).

A check of the unicast route table shows that S1 is the interface this router would use to forward unicast

data to 151.10.3.21. Because the packet has arrived on interface S0, the packet is discarded.

Figure 10 shows an example of a successful RPF check.

Figure 10 RPF Check Succeeds

In this example, the multicast packet has arrived on interface S1. The router refers to the unicast routing

table and finds that S1 is the correct interface. The RPF check passes, and the packet is forwarded.

For more information about multicast routing, see the Cisco IOS IP Multicast Configuration Guide,

Release 12.2SR. For instructions on how to configure multicast routing, see the “Configuring IP

Multicast” section on page 4-64.

Multicast packet from

source 151.10.3.21

Multicast Route Table

RPF Check Fails

Network Interface

151.10.0.0/16 S1

198.14.32.0/24 S0

204.1.16.0/24 E0

Packet arrived on

wrong interface.

Discard packet.

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