Troubleshooting Toolbox - Last Input/Output
The show interface command contains a lot of great information. One of the often overlooked pieces of output from this command is the last input/output field. You can use this field to determine if packets are flowing in both directions on an interface, and - if they are not - when the last packets were registered ingress and egress on the interface. The only caveat is that only process-switched packets will be registered by these counters.
Last input - Number of hours, minutes, and seconds since the last packet was successfully received by an interface and processed locally on the router.
Last output - Number of hours, minutes, and seconds since the last packet was successfully transmitted by an interface.
output hang - Number of hours, minutes, and seconds (or never) since the interface was last reset because of a transmission that took too long.
Note: When the number of hours in any of the "last" fields exceeds 24 hours, the number of days and hours is printed. If that field overflows, asterisks are printed.