There is a nice little trick or tool in Cisco that we can use when we troubleshoot network problems. It is very simple and useful:)
We create an Access list that filters only ICMP traffic and we turn on the debugging for that traffic.
Just follow this procedure below:
R1(config)#access-list 101 permit icmp any any
R1(config)#do debug ip packet 101 detail
After creating an ACL and turning the debugging on we get the message “IP packet debugging is on (detailed) for access list 101”.
All we have to do now is just PING the other side that we suspect is causing us trouble.
As you can see that we do not have Layer 2 reachability but we know that Layer 3 routing is in order (FIBipv4-packet-proc: packet routing succeeded).
After fixing the issue we try again:)