Handy TDR Capability In Cisco Switches- How Long Is That Attached Cable?

OK, so this isn’t exactly earth-shaking. Nor does it replace the accuracy of the calibrated Time Domain Reflectometer (TDR) functionality found in your garden variety higher-end UTP tester. But at the same time, there is a TDR utility built into certain Cisco Catalyst switches that provides some handy diagnostics of the physical layer. If this is old news to you (and yes, it has been written about in other places through the years), just click on off to more interesting terrain. If you’re not familiar with the notion, please continue reading.

A Real-World Example

This morning, I got an alert from one of my far-off sites that a wireless access point went off-line. I accessed the switch that the AP should have been connected to, and saw that indeed the interface for the AP was down. Looking in the switch’s log, I could see where the port went dead before I got the alert. So… either the AP itself failed hard enough to show no link, the switchport itself died (unlikely), or I was facing a physical layer problem.

So an AP mysteriously went out as verified by the switch log… Now let’s figure out where it got disconnected. First, login to the switch. My issue was on Interface Gig 0/46, so my whiz-bang TDR command looks like the following (with switch feeback shown):

Switch#test cable-diagnos tdr int gig0/46

TDR test started on interface Gi0/46
A TDR test can take a few seconds to run on an interface
Use ‘show cable-diagnostics tdr’ to read the TDR results.

 OK- so the test ran. Then to see the results:

Switch#sh cable-diagnostics tdr int gig0/46

TDR test last run on: March 10 10:18:06

Interface Speed Local pair Pair length        Remote pair Pair status
——— —– ———- —————— ———– ——————–
Gi0/46    Pair A     66   +/- 10 meters N/A         Open
               Pair B     70   +/- 10 meters N/A         Open
               Pair C     70   +/- 10 meters N/A         Open
               Pair D     67   +/- 10 meters N/A         Open

 What’s It Mean?

Given that we see all pairs open at around 70 meters from the switch, it’s a good bet that the AP got disconnected at the field end. Had the length been a meter or two, you’d guess that the problem is closer to the actual switch itself. If this was a new cable with no host device at the end, you’d get a general sense of the characteristics of the overall cable and individual pairs and whether any obvious problems could be afoot.

In this case, I was able to track someone down on the far end via telephone who was able to explain that construction was going on in the vicinity of the problem AP, and that it was in fact disconnected as a result.

Again, this is hardly revolutionary, but certainly is as handy as the likes of “show cdp/lldp neighbors”, “show power inline” etc when trying to figure out why an AP on a Cisco switch may be misbehaving.

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