Investigating Cisco's built-in TDR
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 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# sh 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.
About the Author
Jeremy Stretch is a freelance networking engineer, instructor, and the maintainer of PacketLife.net. He currently lives in Fairfax, Virginia, on the edge of the Washington, DC metro area. Although primarily an R&S guy, he likes to get into everything, and runs a free network training lab out of his basement for fun. You can contact him by email or follow him on Twitter.
Comments
On a 7600 with WS-X6748-GE-TX, there is disruption on the traffic :(. On a 3750 i didn't see any.
Definately saw a drop on my 3560, also got the following result:
CNPAS226#sh cable-diagnostics tdr int g0/1
TDR test last run on: November 14 18:06:36
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- -----------
Gi0/1 100M Pair A N/A Pair A Normal
Pair B N/A Pair B Normal
Pair C 180 +/- 2 meters Pair C Short
Pair D 164 +/- 2 meters Pair D Short
That's a really cool feature. Seems to work only with gig interfaces, though - bummer :)
This works on one of my 2960's with 100Mb too. Although I did have something plugged into it far side - no packets dropped on testing. Impressed!
Cisco IOS Software, C2960 Software (C2960-LANBASE-M), Version 12.2(25)SEE3, RELE ASE SOFTWARE (fc2)
DARWIN_SW2#sh cable-diagnostics tdr int fa0/3
TDR test last run on: June 29 00:44:25
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/3 100M Pair A 0 +/- 15 meters Pair A Normal
Pair B 0 +/- 15 meters Pair B Normal
Pair C N/A Pair C Not Supported
Pair D N/A Pair D Not Supported
On some of my 2960 switches, I get length "N/A" when I have lost connectivity to the end-node. Has anyone found what this means - other than the switch couldn't determine the length?
There was a drop on 6500 with 6748s as well. Should have scrolled down to the comments before trying it....
Can this get the full length of a cable as it is connect from the switch ---> to the patch panel ---> to the user office ?