TDR testing – testing cables

A time-domain reflectometer (TDR) is an electronic instrument used to determine the characteristics of electrical lines by observing reflected waveforms. It can be used to characterize and locate faults in metallic cables (for example, twisted pair wire or coaxial cable).

With Cisco, you can run TDR tests without any hardware.

1. Command to start the TDR: “test cable tdr interface <interface of your choice>”;
2. Wait for about 5 to 7 seconds for the test to run; and.
3. Command to show the result of the TDR test: “show cable tdr interface <interface of your choice>”

Here is a TDR from a defective cable:

TDR test last run on: September 01 14:59:13

Interface   Speed Local pair Pair length        Remote pair Pair status
———   —– ———- —————— ———– ——————–
Gi4/0/39    100M  Pair A     57   +/- 10 meters N/A         Normal
Pair B     62   +/- 10 meters N/A         Normal
Pair C     14   +/- 5  meters N/A         Short
Pair D     0    +/- 5  meters N/A         Open

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