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.
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: 22.214.171.124, Source count: 0, Packets forwarded: 0, Packets received:
0 Group: 126.96.36.199, 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.