In any network troubleshooting scenario it is always wise to investigate the health of Layer 1 as a first step in the process. It is useful to check for recent interface changes, the current link state, and errors or drops on the suspect interface.

1. If the problem is not currently ongoing then document the time that it occurred.
2. Check the current interface state: Use the show interfaces command.
3. Check the interface counters and statistics: Use the show interfaces extensive and look for increasing errors or drops.
4. Examine the interface details in depth, as appropriate to the deployment scenario. For example: show interface dataplane dp0s4; show interface switch sw0a.