show cable-diagnostics tdr

[asaroth22]

Hi everyone, i have this result of an tdr test: 

Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa2/1 100M Pair A 22 +/- 15 meters Pair B Normal
                    Pair B 22 +/- 15 meters Pair A Normal
                    Pair C N/A Pair D Not Supported
                    Pair D N/A Pair C Not Supported

I know this is a fast ethernet port so pair C and D is not supported and its not suport PoE either. My question is about the length.  What does mean (in pair A) the number 22 and de number 15? what does mean the symbols + and -?

Thanks a lot!

[luis_cordova]

Hi @asaroth22 

Check this:

https://community.cisco.com/t5/networking-documents/how-to-use-time-domain-reflectometer-tdr/ta-p/3119327

Regards

[asaroth22]

Thanks Luis, the cable measures is about 22 meters and have an lengthwise error about of 15 meters. So the cable is defective  from what I see.

Fa2/1 100M Pair A 22 +/- 15 meters Pair B Normal
                    Pair B 22 +/- 15 meters Pair A Normal

Thanks again

[Martin L]

cable is defective? not necessary; why do u think is bad?  because of length?

Normal/Normal usually means cable is OK. Normal/Open or Normal/Short would be bad.

length is not always accurate; that is why there is a margin of error ( + /- thing).  I would re-run test 2 more times and compare results.

disconnect device from end of cable. Normal should change to Open and verify the length of cable.

what switch is that?

Regards, ML
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[Martin L · Accepted solution]

I worked with 3750s and 3850s and 3850s are more accurate. of course, this also depends on Cable Guy ;)

[asaroth22]

Hello Martin

The switch is a IE-3000-8TC. i thought the cable is defective because the margin of error is too long. I did the test several times and the result was the same

Thanks a lot.

[Leo Laohoo]

I call the "+/- 15" as the "error of probability" and I must admit "+/- 15" is a huge error.
This can be attributed to the following:
1. Firmware bug;
2. State & "quality" cable used.
If it is possible, try changing the patch cables (both ends) and see if it actually makes any difference.
Another thing is if the switch has a very high uptime, try rebooting the switch and see if makes any difference.