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RE: can etherchannel and STP coexist? [7:131836] posted 07/11/2008
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Actually, what you should get at that point is a port-channel mismatch which
will cause link(s) to be err-disabled.

Kinda messes with your head, but is fun to watch.  At least when it occurs
on someone else's devices!  :)


Scott Morris, CCIE4 #4713, JNCIE-M #153, JNCIS-ER, CISSP, et al.
CCSI/JNCI-M/JNCI-ER
Senior CCIE Instructor

smorris@xxxxxxxxxxxxxxxxxxxxxx

 

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-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] 
Sent: Friday, July 11, 2008 6:21 PM
To: cisco@xxxxxxxxxxxxxx
Subject: Re: can etherchannel and STP coexist? [7:131836]

Hi Sachendra,

It's only a guess ... but I think that this was a result of the
Etherchannel.

The way that spanning tree works with Etherchannel is that each end selects
one of the connections to act as the "master" and forwards the BPDUs across
it. There's no signalling about this, and obviously it doesn't really matter
as each of the links originate and terminate within the same broadcast
domain.

If the links were to be accidentally terminated on different equipment, or
within different VLANs, then you potentially have no spanning tree ... and a
loop. I'm guessing that this is what happened in your case. LACP - if
available - would take care of preventing misconnection of the physical
layer, but there's nothing really that can prevent stupid mistakes in
configuration; other than a good change control mechanism of course!!!!!

You should also be aware that EXACTLY the same can happen if - for example -
you were to configure the Etherchannel with 4 ports on one side, but only 3
(the 4th got forgotten) on the other. LACP really comes to the fore in
preventing this due to the constant exchange of LACP BPDUs across every
member link. If the "A" side port isn't configured to be the mirror of the
"B" side then the link will never be added to the "Etherchannel".




Message Posted at:
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