multicast packet, the router or multilayer device uses a reverse path forwarding (RPF) check on the packet as
follows:
1
The router or multilayer device examines the source address of the arriving multicast packet to decide
whether the packet arrived on an interface that is on the reverse path back to the source.
2
If the packet arrives on the interface leading back to the source, the RPF check is successful and the packet
is forwarded to all interfaces in the outgoing interface list (which might not be all interfaces on the router).
3
If the RPF check fails, the packet is discarded.
Some multicast routing protocols, such as DVMRP, maintain a separate multicast routing table and use it for
the RPF check. However, PIM uses the unicast routing table to perform the RPF check.
DVMRP is not supported on the device.Note
The following figure shows port 2 receiving a multicast packet from source 151.10.3.21. The following table
shows that the port on the reverse path to the source is port 1, not port 2. Because the RPF check fails, the
multilayer device discards the packet. Another multicast packet from source 151.10.3.21 is received on port
1, and the routing table shows this port is on the reverse path to the source. Because the RPF check passes,
the device forwards the packet to all port in the outgoing port list
Figure 5: RPF Check
Table 13: Routing Table Example for an RPF Check
PortNetwork
Gigabit Ethernet 1/0/1151.10.0.0/16
Gigabit Ethernet 1/0/3198.14.32.0/32
Gigabit Ethernet 1/0/4204.1.16.0/24
PIM uses both source trees and RP-rooted shared trees to forward datagrams. The RPF check is performed
differently for each:
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
OL-29890-01 121
Configuring PIM
Multicast Forwarding and Reverse Path Check
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