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 use this command “show cable-diagnostics tdr interface”. This is NON DISTRUPTIVE 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.
Below are a couple of exmples. The first two have issues. The last one is good. The Pair status should read “Normal”.
TPA-SW01#test cable-diagnostics tdr interface gigabitEthernet 1/0/5 TDR test started on interface Gi1/0/5 A TDR test can take a few seconds to run on an interface Use 'show cable-diagnostics tdr' to read the TDR results.
TPA-SW01#show cable-diagnostics tdr interface gigabitEthernet 1/0/5
TDR test last run on: May 11 14:34:44
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/5 100M Pair A 46 +/- 0 meters N/A Normal
Pair B 46 +/- 0 meters N/A Normal
Pair C 47 +/- 1 meters N/A Short
Pair D 46 +/- 1 meters N/A Short
TPA-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
TPA-SW01#show cable-diagnostics tdr interface gigabitEthernet 1/0/20
TDR test last run on: May 11 14:36:54
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/20 1000M Pair A 25 +/- 0 meters Pair A Normal
Pair B 25 +/- 0 meters Pair B Normal
Pair C 26 +/- 0 meters Pair C Normal
Pair D 26 +/- 0 meters Pair D Normal
Pairs A & B = typical receive/transmit
Pair C = POE
Pair D = GigabitEthernet
This is a pretty cool tool to help troubleshoot cabling issues.