Trunk misconfiguration troubleshooting question

Hi everyone,

I was going through some trunking labs and began to dig a little deeper
into DTPs role in dynamically creating trunks. I found that DTP does not
perform a negotiation in the true sense of the word. Each switch simply
changes the port into a trunk if it receives a DTP frame with the right DTP
status and type codes. Also I have found that the switches can give you a
false trunk indication if incompatible encapsulation is used on either
side.

An example this is when one end is configured as a static trunk with
802.1q and the other end is configured as a static trunk with ISL, they
will both say they are in a trunking state even if no data is passing
between the two switches due to the encapsulation mismatch. Also, I
receive no error messages indicating the encapsulation mismatch.

I am trying to find out from a troubleshooting perspective, if you only
have access to view the configuration on one end, how can you tell there is
an encapsulaion mismatch? I believe the frames between the two switches
are being dropped because of the encapsulation mismatch but don't know if a
way to tell this is happening. Does anyone know of a debug or show command
that can indicate the presence of an encapsulation mismatch on a statically
trunked link?

I posted my findings on my wordpress site (
http://tweqouu.wordpress.com/2012/09/11/dynamic-trunking-protocol-dtp/) if
anyone is interested.

Thanks for your time.

Regards,
Andrew

Blogs and organic groups at http://www.ccie.net
Received on Tue Sep 11 2012 - 09:49:16 ART