Error disable due to loopback

[tiwariharish44]

Hi,

We have a campus network. A student's hostel room port that is connected to a 2960 switch get error-disabled time and again and it shows the reason being "loopback".

From switch patch panel to user's LAN port in room , we have tested the connectivity through cable tester and it is found to be proper.

He has changed his LAN cable also, what can be the exact reasons causing this problem

[Madhukrishnan Gopinathan Nair]

Hi,

Looking at the symptoms, I feel that there could be a possible loop causing this.

The port gets err-disabled since it is getting its own keep-alive packet looped back to it. 

Could you please ensure that student is not introducing any physical loop in the room? For example connecting a hub unamanged device etc?

Hope this helps you.

Thanks,

Madhu.

[tiwariharish44]

Hi Madhu,

The student is not using any kind of non-manageable device that could cause this problem, In fact the problem is very frequent in some other buildings LAN ports also. Can this problem be related to some switch software mis-behaviour? What other troubleshooting steps should I take

Thanks

[johnlloyd_13]

hi,

madhu is right. it's caused by a hardware/cable loop.

this is seldom due to switch's software.

i wouldn't trust the student/user and would instead put security measure/harden the switch ports to avoid this kind of problem from re-occuring.

[tiwariharish44]

Hi all,

Please go through the attachement and let me know what can be done further.

[Leo Laohoo]

A student's hostel room port that is connected to a 2960 switch get error-disabled time and again and it shows the reason being "loopback".

Post the output to the command "sh interface status err" when this happens.  I want to see this for myself. 

Next, re-enable the port and post the output to the command "sh interface <PORT>".  Do not reboot the switch or attempt to clear the counters.  

[tiwariharish44]

I have not enabled the port as of now. Please see the attachement for your reference

[Leo Laohoo]

Thanks for the output.  

Your output doesn't make sense.  The output tells me the port has been in "error disable" state TEN (10) DAYS after the "sh interface Fa0/23".  Is this correct?  

Can you post the complete output to the command "sh version"?  I want to see the exact model of the switch and the IOS version. 

In fact the problem is very frequent in some other buildings LAN ports also. 

Can you please elaborate this line?  I want to be clear I understand what you're saying.

[tiwariharish44]

Yes, the port got error-disabled 10 days back, since then I have not re-enabled the port.  This port had earlier also got error-disabled many times and every time I re-enabled it, but this time I did not re-enable because I am yet to receive the complaint from that user.

And the other building line means that-

The same loopback error disabled problem is coming on many other different switches ports in different locations of my campus network.

Show version is attached herewith.

[Leo Laohoo]

Thanks for the output to the command "sh version".  

Look at the uptime of the switch.  Because of this the output to the "loopback_error_2960" is totally useless.  Why?  If there was any line errors which can determine if there was a cabling issue, a NIC card issue or something more sinister, then there's only 4-days worth of data.  Not much to run with.  

The IOS is very, very old.  

Currently, the only thing I can think of is remove the configuration of setting the speed to the port to 10 Mbps.  (I don't see the benefit of punishing students by slowing down their network speed.)  Once the port is running auto speed/auto duplex, wait for approximately 30 minutes and run a TDR on the port.   Even better if you can move the cable to the GigabitEthernet port and run the TDR there so you'll get a better picture of all the pairs.

Another thing, post the output to the command "sh post".