Measuring Cable Lengths on a Catalyst Switch

A while back, I was playing on a 3750 switch in a customers lab and came across something I’ve never seen before. It seems that some Cisco switches have a built in Time-domain reflectometer, or TDR. While this is pretty standard on something like my Fluke gear, its nothing that I would expect to find on a Catalyst switch!

According to what I could find, this should be present on the 3560, 3750, and some modules for the 4500, 6500 and 7600 series. Quite an interesting feature if you ask me! This could be quite helpful for discovering cables that have been damaged. You can find the Cisco command reference for this feature here.

Switch#test cable-diagnostics tdr interface g1/0/12
TDR test started on interface Gi1/0/12
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
Switch#show cable-diagnostics tdr int g1/0/12
TDR test last run on: May 21 21:31:09

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/14  auto  Pair A     25   +/- 4  meters N/A         Open
                Pair B     25    +/- 4  meters N/A         Open
                Pair C     24   +/- 4  meters N/A         Open
                Pair D     25   +/- 4  meters N/A         Open