I work as a junior network admin at an ISP and I would like your help in resolving an issue whose cause is not readily apparent. Rn I am trying to troubleshoot a case where a switch interface randomly goes down for about 3-5 and then right back up. It is a poe switch and the connected device is a wireless point to point antenna getting power from the switch itself.


  1. switch interface goes down
  2. switch interface goes back up and connectivity resumes as normal
  3. autonegotiation sometimes results in 100mbit full duplex but is mostly 1gbit full duplex

Now the problem can be one of many things: nic of the point to point antenna, cable, switch interface, the switch itself (correct if I'm wrong and please add other possible causes). Is there a way I can tell which of the above is to blame without having to physically go to the location and start changing stuff? What errors should I be looking for in each case? Obviously, an on-site inspection might be of help but it is not certain and requires people

For the record, I have looked at the statistics provided by the switch but the only errors I get in this case are fifo errors which increment when the interface autonegotiates to 100mbit and cannot support all traffic passing through it. But that is expected. I have not gone into much detail about the specific equipment used because I would like a more general approach.

1 Answer 1


There is no common cause for that problem. Possible problems include cabling, especially with PoE, a defective device or switch port.

autonegotiation sometimes results in 100mbit full duplex but is mostly 1gbit full duplex

That's a near giveaway for a cable problem. Check grade, termination, length, gauge for PoE, and indicated error rates (on both sides and especially in 1000BASE-T mode).

  • Thanks for your reply Zac! Well I actually forgot to check the cable using the cable checker built into the switch so I'll go ahead and try that. Started looking into the linux directories for errors and forgot to do the basics :P. To be honest, cabling would be the last thing I would suspect because we're using a pretty solid cat5e cable (ubiquiti toughcable) for such installations and never had any issues. Commented Aug 14, 2023 at 18:12
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    Agreed that the likely cause is hardware related. Most likely it is a fault in the cable OR that the cable is too long. I have seen POE issues with a cable that is simply too long to deliver the current required by the device at the end of the cable. It boots and runs fine for some time and eventually draws too much power and either reboots itself when it can't get enough current or the switch reboots it because it draws more than the port is rated to provide. Other issues could be insufficient power capacity at the switch for all devices using POE. Commented Aug 15, 2023 at 4:02
  • Hi Frame, thanks for chipping in! When you mention "for some time" how long do you mean? This specific setup I'm referring to has been running issue-free for at least 3 years (that's when I started working here) so I don't think it is length-related. Could it be that the cable has degraded over the years and is now causing problems? After all, it's not indoors (although the cable itself is meant for outdoor usage). Also, the issue only occurs about 3-4 times a day if frequency is relevant. Thank you both for your contributions!!! Commented Aug 15, 2023 at 12:45
  • You were right guys, it was indeed the cable. We replaced it yesterday and the problem hasn't occurred since. Thanks again for your help! Commented Aug 19, 2023 at 12:03

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