3.2.a Troubleshoot reverse path forwarding

3.2.a [i] RPF failure

The Unicast RPF feature helps to mitigate problems that are caused by the introduction of malformed or spoofed IP source addresses into a network by discarding IP packets that lack a verifiable IP source address. These drops are accounted as packets that have failed RPF check.

http://blog.ine.com/2008/01/02/troubleshooting-multicast-rpf-failure/

http://en.wikipedia.org/wiki/Reverse_path_forwarding

3.2.a [ii] RPF failure with tunnel interface

You can use show ip mroute count command to see RPF failures. A sample output of this command and its increasing counters for RPF failure are shown in the output below:

Router# show ip mroute count

IP Multicast Statistics

3 routes using 1642 bytes of memory

2 groups, 0.50 average sources per group

Forwarding Counts: Pkt Count/ Pkts per second/ Avg Pkt Size/ Kilobits per second

Other counts: Total/ RPF failed/ Other drops( OIF-null, rate-limit etc)

Group: 224.0.1.40, Source count: 0, Packets forwarded: 0, Packets received:

0 Group: 239.1.1.20, Source count: 1, Packets forwarded : 11, Packets received: 50 Source: 10.1.1.1/ 32, Forwarding: 11/ 0/ 100/ 0, Other: 30/ 19/ 0

You can also use show ip rpf source command to verify if RPF information is correct.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2449-2450).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/ip/ip-multicast/43584-mcast-over-gre.html