I was recently faced with a remote troubleshooting situation caused by a newly installed IP phone that could receive PoE power, but was unable to acquire a DHCP lease. I then remembered a useful feature of the 3750 that allows for a cable test to be executed on any copper interface. To perform this test simply run the following command:
Catalyst-3750# test cable-diagnostics tdr interface gig 2/0/2
TDR test started on interface Gi1/0/2
A TDR test can take a few seconds to run on an interface
Use ‘show cable-diagnostics tdr’ to read the TDR results.
Catalyst-3750#show cable-diagnostics tdr interface gig 2/0/2
TDR test last run on: December 30 08:31:09
Interface Speed Local pair Pair length Remote pair Pair status
——— —– ———- —————— ———– ——————–
Gi2/0/2 auto Pair A 40 +/- 4 meters N/A Open
Pair B 12 +/- 4 meters N/A Open
Pair C 40 +/- 4 meters N/A Open
Pair D 40 +/- 4 meters N/A Open
From this output it was easy to see that Pair B was not connected end-to-end and needed to be checked by our cabling vendor.
The technology in use here is TDR (time-domain reflectometry) and more information can be found at the following Wikipedia page here
More information from Cisco can be found here