From: Enrique Langmaack (langmaack@gmail.com)
Date: Wed Apr 16 2008 - 16:05:20 ART
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
Pass the CCIE in six weeks, Guaranteed!
http://www.certscience.com/CCIE
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