From: Mike Harrison (michael.h4@blueyonder.co.uk)
Date: Wed Apr 16 2008 - 16:34:56 ART
Enrique
Looks like you have something else going on - I have found loopback errors
usually occur when you telco loops a circuit to test it on a metro ethernet
connection or the like. The switch sees a physical loop between tx and rx of
the port - not to be confused with stp loops.
You can set the switch to auto recover from the err disable - something
like;
errdisable recovery cause loopback
errdisable recovery interval 30
Not sure why this would error in a lab situation though?
Mike
----- Original Message -----
From: "Enrique Langmaack" <langmaack@gmail.com>
To: <ccielab@groupstudy.com>
Sent: Wednesday, April 16, 2008 8:05 PM
Subject: loopback err-disable on switches
> Hi,
>
> While I have been doing some labs with pvst and mst using four switches
> and
> been connected forming physical loops, I was not able to have all of the
> trunks between the ports of these switches either in a fwd or blk state. I
> always got at least a couple of ports somewhere in the topology in the
> err-disable state.
>
> Everytime this happened, switches showed something like this:
>
> %PM-4-ERR_DISABLE: loopback error detected on Fa0/20, putting Fa0/20 in
> err-disable state
>
> So by searching this on cisco documentation I found the following:
>
>
> "A loopback error occurs when the keepalive packet is looped back to the
> port that sent the keepalive. The switch sends keepalives out all the
> interfaces by default. A device can loop the packets back to the source
> interface, which usually occurs because there is a logical loop in the
> network that the spanning tree has not blocked. The source interface
> receives the keepalive packet that it sent out, and the switch disables
> the
> interface (errdisable). This message occurs because the keepalive packet
> is
> looped back to the port that sent the keepalive:
>
> %PM-4-ERR_DISABLE: loopback error detected on Gi4/1, putting Gi4/1 in
> err-disable state"
>
> (
> http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00806cd87b.shtml
> )
>
> So basically what I did is disabling keepalives on trunk interfaces on my
> switches so now I don't fall into this issue.
>
> However I was wondering if this workaround would impact on anything else
> or
> would be allowed during the real ccie lab.
>
> Thanks for your help
>
>
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