I was troubleshooting an AP that was in another building. I thought it would come up with PoE but there seemed to be an issue. Further investigation lead to an interface status of up/down. A little weird for an AP.
Checking the configuration looked okay. After spending some time looking over the same information, I researched to see if there was any way to test an individual port. In the past, I’ve had to replace this specific line card because ports were going bad.
What is TDR
Cisco has a built in cable diagnostic feature called TDR, Time-Domain Reflectometer. TDR is used to test cables to find faults. There’s a good resource on the Cisco Support Forums.
How To Use TDR
The syntax is simple.
switch#test cable-diagnostics tdr interface g0/1
Diagnostics runs quickly so you can follow that up with the results.
switch#show cable-diagnostics tdr interface g0/1
Going back to my issue, this is what I found:
switch#show cable-diagnostics tdr int g9/1
TDR test last run on: February 22 08:55:49
Interface Speed Local pair Pair length Remote pair Pair status
---------- -------- ----------- ------------------- ------------ ------------
Gi9/1 1000 Pair A N/A Invalid Terminated
Pair B N/A Invalid Terminated
Pair C 4 +/- 2 meters Invalid Open
Pair D N/A Invalid Terminated
The table above is very easy to read.
Interface – The interface tested
Speed – Speed the interface is running.
Local pair – Pairs detected on the cable connected to the switch.
Pair length – Approximate length of the cable.
Remote pair – Pairs identified on the far end of the cable.
Pair status – Whether cable is terminated to the far end device.
Right away, the pair length wasn’t determined and the remote pair was invalid. Not what I want to see. By the way, the switch does a surprisingly great job and determining the cable length. I assumed I had a bad cable so I swapped that out. Unfortunately, I had the same results.
The stupid fix to this was powering the switch with a power adapter. Turns out the switch wasn’t giving PoE to the AP (another problem). After providing power to the AP, I re-ran the cable diagnostics and received the following output.
switch#show cable-diagnostics tdr int g9/1
TDR test last run on: February 22 09:13:04
Interface Speed Local pair Pair length Remote pair Pair status
---------- -------- ----------- ------------------- ------------ ------------
Gi9/1 1000 Pair A 1 +/- 20 meters Pair A Terminated
Pair B 1 +/- 20 meters Pair B Terminated
Pair C 2 +/- 20 meters Pair C Terminated
Pair D 1 +/- 20 meters Pair D Terminated
Everything looks good! The test even got the correct cable length.
My Thoughts
TDR is a useful feature especially when troubleshooting remotely. I didn’t have a cable tester on-hand but the built-in Cisco TDR proved to be beneficial. My problem was not a bad cable although it seemed like it in the cable diagnostics results.