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markysharkey
Premium
join:2012-12-20
united kingd

[Info] Cable testing built in to IOS

A colleague showed me this yesterday:

Switch#test cable-diagnostics tdr interface type number
 

Read the on-screen output, wait a few seconds then run

sho cable-diagnostics tdr int type number
 

Here's me having a crack at it.
Switch#sho cable-diagnostics tdr int gig 1/0/48
TDR test last run on: January 24 21:55:55
 
Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/48  1000M Pair A     28   +/- 0  meters    Pair B      Normal
                Pair B     26   +/- 0  meters    Pair A      Normal
                Pair C     29   +/- 0  meters    Pair D      Normal
                Pair D     28   +/- 0  meters    Pair C      Normal
 
 

Not quire sure why there is such a discrepancy in the length, and this example is a crossover cable worked out by MDIX and is part of an etherchannel group.
This was on a 2960S with 12.55 IOS. I believe the command was introduced somewhere around 12.2.
Anyway, just thought I'd share. It could be handy if you're caught short without a cable tester, or you need to test cables remotely ;)

There are lots of other things to test here:
»www.cisco.com/en/US/docs/ios-xml···dem.html

--
Binary is as easy as 01 10 11

HELLFIRE
Premium
join:2009-11-25
kudos:18

And here I was thinking it could only be done on 6500 switches.

Tried the command on a 3750G for chuckles.... NIIIIIICE!

3750G#sh cable-diagnostics tdr int gi 1/0/23
TDR test last run on: January 25 20:41:18
 
Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/23  100M  Pair A     N/A                Pair B      Normal
                Pair B     N/A                Pair A      Normal
                Pair C     2    +/- 2  meters Pair D      Short
                Pair D     2    +/- 2  meters Pair C      Short
 

Regards

cramer
Premium
join:2007-04-10
Raleigh, NC
kudos:9
reply to markysharkey

The readings will not be 100% because an accurate (true) TDR is expensive, if it's even doing a real TDR. Most cheap length tests are just capacitance readings.

But, yes, the pair lengths are actually different, due to different amounts of twist on each pair.


HELLFIRE
Premium
join:2009-11-25
kudos:18
reply to markysharkey

Suffice to say, "good enough for twits that say 'there must be a network / cable issue!,'" but if you want to be sure
pack a good 10K dollar Fluke meter.

Been saving this command for the former situation....

Regards


markysharkey
Premium
join:2012-12-20
united kingd
reply to markysharkey

I pack a Fluke Microscanner when I go to site. But when telnetted in the accuracy of the length measurement is less important than confirming one of the sparky's has drilled through a cable... again...
--
Binary is as easy as 01 10 11