CCIE RS – L3 Multicast – Troubleshooting RPF

Troubleshooting Reverse Path Forwarding

PIM uses the unicast routing information when creating the multicast tree.

Ensure loop-free forwarding of multicast packets in multicast routing

Help prevent IP spoofing in unicast routing

Incoming multicast packet will not be accepted/forwarded unless it is received on an interface that is the outgoing interface for unicast route to the source of the packet.

RPF Check

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

RPF Failure

RPF failures occur in 2 situations:

  1. Packets are flooded out of the wrong interface (looping prevention)
  2. Unicast routing does not match multicast distribution trees

You can fix the situation in a couple ways

  1. Static mroute
  2. Enable PIM on the interface

RPF Failure with Tunnel Interface

As described above, this could happen if PIM is not enabled on the tunnel interface or if the unicast routing table does not match the multicast distribution tree.


Troubleshooting commands

show ip mroute

show ip rpf [ip]

show ip route [ip]

show ip pim neighbor

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