Tip: Debugging with 'debug ip packet'

The 'debug ip packet' command is a brilliant way to help diagnose problems with traffic traversing the router - although there are a few drawbacks - one being that only packets that are switched using process switching (i.e. switched with the help of the CPU) will be visible in the 'debug ip packet' output - other switching mechanisms like Fast Switching and CEF will not.

Although we can use the 'no ip route-cache' within interface mode to force packets to be switched with process switching - although note that this can have an adverse affect on the CPU is busy environments and should only be used if absolutely necessary.

int gi0/0
no ip route-cache

In larger scale environments you might be better of using tcpdump or Wireshark to inspect traffic.