Networking

Cisco tdr command

I recently discovered a really cool command on a Cisco 3560 switch.

It has a TDR (Time-domain reflectometer). It basically means you can test how long the cable is, if it has a short, pair of wires open and any other problem.

TDR works on 10/100/1000BaseTx. Fibre optic ports are not supported.

TDR has been available after Cisco IOS Release 12.2(25)SE or later.

This is a table of the different pair status results and their meanings.

Pair StatusWhat does this mean?
NormalBest result. No issues.
OpenOpen circuit.
ShortShort circuit.
Impedance MismatchedBad cable.

More info is available here from Cisco.

Here is an example:

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

Have a read through TSF’s other networking articles here.

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  • Barb Cazorla

    Hey I was just looking at your site in Chrome and the graphic at the top of the page cant show up right. Just thought I would let you know.

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    By the way have you seen the RAVPower FileHub? It's a travel Router with wifi AC750 300Mbps Wi-Fi on 2.4Ghz or 433 Mbps on 5Ghz. It includes an SD Card Reader, 6700mAh Battery and can connect an SSD hd. Make this a portable NAS to store your pics and videos or upload your SD card pics/videos to a SSD hd. This is a bargain for under $60. (Amazon USA Link)

    Did our article help you? Here are some ways you can help us.

    Have a look at our IT Tech Tool Kit List, need ideas on what to buy? I have listed my top 29 tools here.

    Buy me a coffeeBuy me a coffee

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