source, rather than to the receiver, is called Reverse Path Forwarding (RPF). RPF is an algorithm used for
forwarding multicast datagrams.
Protocol Independent Multicast (PIM) uses the unicast routing information to create a distribution tree along
the reverse path from the receivers towards the source. The multicast routers then forward packets along the
distribution tree from the source to the receivers. RPF is a key concept in multicast forwarding. It enables
routers to correctly forward multicast traffic down the distribution tree. RPF makes use of the existing unicast
routing table to determine the upstream and downstream neighbors. A router will forward a multicast packet
only if it is received on the upstream interface. This RPF check helps to guarantee that the distribution tree
will be loop-free.
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.
The figure shows an example of an unsuccessful RPF check.
Figure 8: RPF Check Fails
As the figure 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.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
OL-32598-01 139
Configuring PIM
Multicast Forwarding
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