Loop monitor

Unanswered Question
Oct 8th, 2007


we have a huge LAN consists of Cisco and HP switches and some unmanaged switches. I doubt that there are some problems about loops in LAN. But i don't know how to discover it. Does anybody know any tool,etc to discover loops? It is not important if it is free or not.

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elfrancesco Tue, 10/09/2007 - 06:38

On CISCO switch u can use this command to automatically disable a port which cause a loop in your network :

errdisable detect cause loopback

You have to analyse Syslog messages to see if a port get disabled.

If a port get errdisabled, you will see its status using the "sh interfaces status err-disabled" command.

Concerning HP switches, procurve generally generate logs concerning loops or "broadcast storms", again, you will have to check the logs.

In all cases, activating the errdisable function on your cisco device should let you find from where the problem is comming.

For instance if you have a switch A (cisco) and a switch B (hp), after activating err disabling function on switch A, if you create a loop on switch B, the trunk port between A and B will come into err-disable state quickly so you will be able to locate where the problem is coming from.

One last thing, remember if you are in a production environment that you could isolate some parts of your network by enabling this function. So be careful about using it.

P.S : you could also use thoses commands so, when a port goes err-disabled, it will enable again automatically after a 30 seconds period :

errdisable recovery cause loopback

errdisable recovery interval 30

yatisspor Tue, 10/09/2007 - 22:10

I don't know if there is any loop or not. But i want to know if someone does a mistake etc, so i want to know if any loop occurs.

elfrancesco Wed, 10/10/2007 - 02:17

As explained before, you need to use the errdisable command.

But you will have to remember that it will disable ports which receive traffic which was previously transmitted (=loop).

In what kind of environment are you ? do you have times during night for instance where it would be acceptable to unlink some network segments ?


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