[27202] in resnet
Ethernet ports "burning out"
daemon@ATHENA.MIT.EDU (Brad Coburn)
Wed Jan 25 21:08:09 2012
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Date: Wed, 25 Jan 2012 21:07:49 -0500
Reply-To: Resnet Forum <RESNET-L@listserv.nd.edu>
From: Brad Coburn <coburn@tcnj.edu>
To: RESNET-L@listserv.nd.edu
Hi all.
I have a curious situation that perhaps the community might be able to
offer some insight on. I apologize for the length of my post; I hope the
detail will serve to spark some insight.
Some background:
Two, 10-story residence halls. One Ethernet port per student (so
2-person rooms have two Ethernet ports). Three equipment rooms per hall,
serving approximately 1/3 of each building. 15, 48-port 10/100 switches
total in each building.
The situation:
Suddenly this Fall I have a recurring issue with a single student room.
The initial report is that they cannot establish connection with their
laptops. One is a PC, one is a MAC; neither looks terribly old (I'm on
the network side, not the desktop side).
First time, troubleshooting turns up "bad" Ethernet ports on that switch
for these two connections. Not an uncommon occurrence across residence
halls. If it's important, the two patch cords are adjacent (jacks were
patched in, in-order).
Moved patch cords to two spare ports (happen to be adjacent) on that
switch; service restored (confirmed with my tester and customer machines).
Two weeks later (second time), call comes back. No link. Troubleshooting
shows two "bad" Ethernet ports again. There are only three of us who
work on switches in the field, and nothing's been touched (so the patch
cables are where they were). Moved both patch cords to another switch,
adjacent ports, same model.
Some days later (third time), call comes back. It's "bad" ports again.
So I and my tech go out there. Standard tests come back fine.
Re-terminated the jack "just in case". I brought my cable pair tester to
try advanced functions like TDR, as well as resistance and voltage
measurements pair-to-pair and pair-to-ground, with computers connected
and without computers connected. Nothing comes up.
By now I'm out of sufficient spare ports, so I place these two
connections on a little desktop Netgear switch and patch that into the
stack. This is just before Thanksgiving break. Student connections again
confirmed working with tester and customer computers.
Shortly thereafter we replace the two switches with "bad" ports (there
had been other port failures over the years) with two new switches from
spare stock. One switch is the same model. One switch is the newest
model of the 10/100 class - a model I'm placing in other buildings that
won't support or can't afford GigE.
The Netgear setup worked the whole time, but it's a non-standard setup
and will only come back to haunt us later. So we pulled the Netgear out
and placed the room patch cords back in their usual spot, which is now
also on the new replacement switch.
Everything worked fine from approximately post-Thanksgiving Break until
last Thursday, January 19. Most students returned from Winter Break on
Tuesday January 24.
Call is back (fourth time). Ports are "bad" again, on this brand-new
switch. This model supports extended diagnostics, which sometimes report
bad ports. Nothing comes up this time (this is not atypical).
Tech and I followed the cable in the ceiling all the way to the IDF
(only one floor down). This room is roughly in the middle of the cable
runs, and the room next door shares a common route from the hallway.
Inspection turned up nothing in the way of obvious damage. Moved the
customer patch cords to two spare ports on the new switch.
So tomorrow I have Facilities scheduled to go up and run new cable for
this room. The parents are irate at this point.
I'm more than sure that this is a customer equipment issue, but I can't
figure out what could be happening. We've not seen anything like this
before. A single-port occurrence would be more likely rather than a
simultaneous 2-port event.
It may be important to know that when the campus was originally cabled
someone took advantage of the "economy" offered by 10Mbps Ethernet's
operation over 2-pair. So we split the cable at the faceplate and then
again back in the IDF. That's how it was done back then, and we're still
operating successfully at 100Mbps now (but we stopped the practice for
retro-fits some time ago).
Given our "split" architecture in this case, I suppose there's a remote
possibility that a damaging event on one half of the cable could
transfer to the entire cable. It's also just as likely that if customer
equipment damages the first connection, the customer tries the second
connection immediately and blows that too.
I certainly understand what this looks like for the customer, but I
don't know what else to say. Of course the customers aren't doing
anything out of the ordinary, and neither is anyone else they might
invite into the room.
I have not tried:
- Swapping horizontal cables with the neighbor. If the problem follows,
then it's ours. It's too late now though; Facilities will replace this
cable and we will pull the old one down to more thoroughly inspect for
physical damage.
- Separating the two patch cords for the room between switches. It just
hasn't happened.
- Installing a surge suppressor on the line (I have pricing).
I really would like to know what I can do to damage an Ethernet port. By
the way, my tester "sees" the bad ports as a switch connected, but
powered off. Which should mean that it sees the transformer at the front
of the interface. In no case has the tester reported any "error"
condition. Oh, and it's a Fluke LinkRunnerPRO.
Any thoughts?
Thanks.
-Brad Coburn
Associate Director, Communications Technologies
Information Technology
The College of New Jersey
15 switches
48 ports each
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