Cisco Cable Test 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 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.

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