Cable test for 100 full duplex cable comes up with pair 3 "short"??

[ronnieshih]

I'm still trying to grasp the inaccuracy or "bug" in the cable test result of a connection that's connected at 100 full duplex on Meraki MS switches.  The result always shows for example:

port  Link         Length  Status  Pair 1  Pair 2  Pair 3  Pair 4

46    100fdx   1 m       OK       ok        ok       short    -

Coming from doing physical cabling as part of my job for years.  I've used physical cable tester to come up with result of all 4 pairs being good, but Meraki would always show pair 3 being short when it clearly isn't shorted.  I understand pair 4 is not tested in 100 full duplex mode.  My coworker is literally arguing with me that there is a problem with this cable even though I just broke it out of a fresh bag and it's brand new.

please advise.

[kYutobi]

I've sometimes had it happen where it was either the connection to the port wasn't seated all the way. This one time I had a cable that always worked start to fail when I noticed that at one point the walls were painted (which I didn't know) to then look at the cable having dried paint on the gold pins. 😂

All I mean is try another cable even though you know it's functional or flip the ends over and try again to make sure it clicks in port. I've had so many of the weird cable tests.  

[MerakiDave]

@ronnieshih I've also seen some occasional oddities with the cable test tool and TDR results.  Definitely try this with a different cable if possible just to determine if the same results are reported.  If so, it could certainly be an issue inside the switch port, from a bent or broken pin to something in the electronics.  Perhaps also open a case with Meraki Support once you have the data points on reproducing the issue with multiple cables (along with your own physical cable tester you have), and/or try different devices connected at the far end.  For example, if you just use a patch cable to a laptop in that same switch port, same result?  

[PhilipDAth]

Was the cable plugged into a device at the time - perhaps the device has the issue.

[PhilipDAth]

Also, the Meraki cable test will "throw up" when the impedance of the cable is outside of spec.  Is your cable test tool able to check the impedance?

[ronnieshih]

It IS a brand new cable out of a NEW bag, connected directly to the switch in the same rack, no patch panel involved in between.  In fact, I tried 3 different new cables, with the same result every time.  Every single connection in this company that negotiates that 100 full duplex comes up with pair 3 being "short".  I'd make a fool out myself if I call a certified cabling guy out to test with a $4000 Fluke meter.  I went to one of our branches and tested almost 10 connections showing this state and each connection shows pair 3 having fully terminated properly.  These are either postage machines or APC PDU's with no need to run 1000/full.  But why does the status show "OK" if pair 3 is shown as in short status?  This baffles me completely.  I've done cabling for years and am no stranger to layer 1 punchdowns and terminations, used to do analog phone lines + T1 circuits also.  I don't ever believe Meraki's cable test at this point.

[cmr]

@ronnieshih only pairs 1 and 2 are used for Ethernet connections, hence if they are okay the cable passes.  PoE power uses the other pairs.  This is why you can use cable splitters to run two Ethernet connections down one 4pr cable, unless you want to use PoE.  Actual wires are 1,2,3,6 for the data and 4,5,7,8 for power.

[ronnieshih]

I understand that have done splits using the same technique.  I am not questioning what pairs are used for what and the basics of ethernet or pairs used for POE.

My question is why does Meraki cable test shows pair 3 as "short" when there is clearly NO short??  If indeed pair 3 isn't used then don't test it but testing it and showing me it's a short is misleading.  This caused me to visit a site and tested over 10 cables for no reason. 

[ronnieshih]

To ALL and EVERYONE using Meraki cable testing feature on switch ports, I got confirmation from Meraki support today that this is a KNOWN ISSUE/BUG and development knows about it.  I have been correct all along using physical cable tester showing all pairs are sound, but the Meraki cable test is incorrect showing pair 3 as "short" or "open" when tested against devices connected at 100Mbps or 10Mbps. 

Case closed, until Meraki fixes this.

[KRobert]

Does anyone know if this was fixed in a newer version yet?

[cmr]

Just cable tested a non PoE device running at 100Mbps on an MS210 with 14.24, pair 3 shows open (which technically is correct):

Tested a 100Mbps connection on an MS225 with 14.22, pair 3 still shows short there:

[KRobert]

Thanks @cmr . Anyone have a response from support? I can also confirm that all of the 100Mbps devices on MS210 with 12.28.1 have pair 3 showing a "short"

While all of the 1Gbps links show all pairs as "ok"