A type of DTP and VTP interaction.

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Aug 6th, 2010
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In the "Catalyst 2960 Switch Software Configuration Guide" I can read that, t

o autonegotiate trunking, the interfaces must be in the same VTP domain.

As I have not found this type of information elsewhere and the Boson simulator do not help to clarify, could someone confirm this? Thanks.

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joshiaroon Fri, 08/06/2010 - 10:48
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cisco switch port are in dynamic desirable mode. that means they are sending DTP frame and in trunk negotiable mode. Even if two switches are in different VTP mode, they can send DTP message.

speculor_cisco Fri, 08/06/2010 - 10:58
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The fact that they can send DTP messages does not mean that they must negotiate a trunk.

Is it possible that the software configuration guide is in error too?

I can not think so, may be there is a control on the domain of the two switches.

joshiaroon Fri, 08/06/2010 - 11:15
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there are three modes: trunk , desirable and auto. All the combination form trunk besides auto-auto.

Giuseppe Larosa Fri, 08/06/2010 - 11:55
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Hello Speculor,

DTP frames probably carry an information about VTP domain name of frame sender, so the two switches can compare this information during negotiation if VTP domain name is different the trunk is not dynamically setup as a form of safety (if the VTP domain names are different the two switches may belong and may be managed by different entities/companies)

However, you can still configure a manual trunk between two switches that are in different VTP domains

Hope to help



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