Cisco 3750 port err-disabled. Any help please?

Hi. I've got a 3750 switch with the following problem. Middle of last week discovered one of the ports (Gigabit Ethernet) had become error-disabled due to loopback. Shut it down and restarted it and it came up OK and stayed up the rest of the week, but this morning it had disabled again over the weekend. Started it again and it has run all day so far. It is a link to a remote site through a BT circuit and just a basic unmanaged switch at the other end, a dozen hosts and a file server. Can anyone offer any assistance on how I might troubleshhoot further and any idea what might cause this problem? Thanks.

I'm not sure what a BT circuit is, so I don't know if that could be playing a part in the errdisable. First thing, check that interface's cable. Inspect it for kinks, poor terminations, etc. If it is obviously bad, then try replacing it or re-terminating. Second, try some debugs for the interface that is causing problems, hopefully you can catch the problem as it happens. Third, if this is a fiber trunk, be careful that the terminating end is compatible, whether it's a transceiver or a GBIC. Hope this gets you started.

First off, I'm making two assumptions:

a) BT = British Telephone (telecom)
b) This is a fiber link

If it is fiber, one suspect cause would be the carrier technicians performing maintenance on the line. They may be looping portions of the circuit path to test for dB loss, and if the circuit is looped in your direction, the GBIC will see itself talking and will error disable. If this is a leased circuit, I would suggest calling your carrier and asking if there was any scheduled maintenance on the line.

Hey Gruffalo, how's it going up there?
Firstly, yes guys BT is British Telecom our much beloved national telco here in the UK. Is this a fibre port Gruffalo, because if it is and you're using something like a BT LES1000 fibre service then I'm with the previous advice 110%; I think BT fiddled with it a couple of times and that caused your problem. Just keep an eye on it and if it proves to be an isolated incident then that's almost certainly what it was.
The other advice is good too; if this is a new installation check everything and make sure you're using the proper GBICs and patch cables to suit the distance the fibre covers

Make sure logging is enabled and try to recreate the issue, then check the logs. They will tell you why it disabled the port.

I've seen this occur when you plug a trunk link into an access port and when spanning-tree detects a non-root switch trying to become the root bridge when root guard is enabled.

Hi All. Sorry for not getting back on this and thanks for all your advice. I called BT, our Telco to check this out. Yes, it's a LES 10 circuit on a fibre. They came out and did some testing, swapped all of their equipment and said they couldn't find anything wrong, but as often happens it magically seemed to get better and ran fine for a week or so. I went on holiday and was convinced all was OK, but towards the end of last week it started again.

This one has really got me. It always happens overnight, every night and once re-enabled first thing in the morning will run fine until I leave in the evening. BT say they aren't up to anything so I'll enable logging on it and let you know what I find.