TDR test in Cisco Devices - Cable Testing

I love finding stuff like this. In the course of my daily Internet browsing, today I came across this post from a while back on the CCNP Recertification blog. It turns out a number of Cisco switches (apparently the 3560/3750, and some 4500 and 6500 modules) have a TDR (Time Domain Reflectometer) embedded in each interface. How cool is that?

Cisco's documentation shows the simple but very handy two-part command syntax.

   Switch# test cable-diagnostics tdr interface g0/1

TDR test started on interface Gi0/1

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 g0/1


TDR test last run on: March 01 00:09:06

Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- -----
Gi0/1 auto Pair A 20 +/- 4 meters N/A Open

Pair B 20 +/- 4 meters N/A Open

Pair C 21 +/- 4 meters N/A Open

Pair D 20 +/- 4 meters N/A Open
This output shows a cable approximately 20 meters in length connected to interface G0/1, with nothing attached to the far end. If you're like me, your first thought is "very cool, but is it accurate?" I grabbed a couple spare cables of various lengths and set about an unscientific benchmark. I measured each cable in three manners: by hand, with my Fluke TDR, and with a Catalyst 3560G-24PS-S. My observed results are below.

Cable Length Fluke 3560 ----------------------------------------------

A 69 ft 69 ft 20 +/- 4m (~66 ft)

B 21 ft 21 ft 6 +/- 4m (~20 ft)

C 83 ft 83 ft 25 +/- 4m (~82 ft)

As you can see, the 3560 is impressively accurate, even without its allowed 4m variance. My next consideration was whether a TDR test would disrupt interface operation. Trying it on a live port while running a constant ping yielded no observable effect, not even a brief disconnection pop-up. This is clearly a very handy tool. I only wish Cisco would do more to publicize cool features like this.
Taken From: http://packetlife.net/blog/2008/mar/28/investigating-ciscos-built-in-tdr/