Cisco CLI Cable Testing Command

Cisco has a command that can run TDR tests on an interface. A TDR (time-domain reflectometer) is an electronic instrument used to characterize and locate faults in metallic cables. It will help determine if there’s a cabling problem.

The command is “test cable-diagnostics tdr interface”. To see the results “show cable-diagnostics tdr interface”. This is NON DISRUPTIVE because we are above IOS 12.2(46)SE. So it will NOT affect the end user or link. The output will show you the Interface, Speed, Local Pair, Pair Length, Remote Pair, and Pair Status.

Look at the results below for NYC’s port Gig1/0/7. The AP they are having issues with is plugged into this port. You can see the cabling is bad. The Pair status should read “Normal”.

NYC-SW01#show cable-diagnostics tdr interface gigabitEthernet 1/0/7
TDR test last run on: October 10 10:13:59

Interface    Speed     Local pair      Pair length                Remote pair    	Pair status
---------    -----     ----------      ------------------         -----------           --------------------
Gi1/0/7      auto      Pair A          28   +/- 1  meters         N/A                    Open
                       Pair B          27   +/- 1  meters         N/A                    Short/Crosstalk
                       Pair C          27   +/- 1  meters         N/A                    Short/Crosstalk
                       Pair D          28   +/- 1  meters         N/A                    Open

Pairs A & B = typical receive/transmit
Pair C = POE
Pair D = GigabitEthernet

On 100Mb cables, you will probably see the status “Short” on Pairs C & D. This is normal.