A DTP question.

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Aug 7th, 2010
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Suppose you have two 2950 Cisco Switch.

They default to dynamic desirable switchport mode.

I link fa0/1 on a switch to fa0/1 on the other one.

I do not understand why, with Boson simulator, the link does not become a trunk.

Also if I put the switches in the same VTP domain, as stated by the configuration guide.

Is it possible that the simulator is so greedy?

What would happen in the real world?

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Sebastian Helmer Sat, 08/07/2010 - 06:11
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By default, interfaces on most Cisco switches are set to dynamic  desirable mode. (Catalyst 2950 and 3550 switches.)
Desirable mode will create a trunk link if the neighboring interface is  set to desirable, trunk, or auto mode.
Because both interfaces by default are in desirable mode, this means a  link between two Cisco switches will automatically become a trunk link  unless configured otherwise. It is reccomended that all switch ports be  set to access mode for security.

You can check the configuration with

show interface fa 0/1  switchport

how the simulator work I can't say sorry..



glen.grant Sat, 08/07/2010 - 08:12
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  In a real world scenario 2950's are dynamic desirable by default as th

e other poster indicated and the link should come up and trunk .

Make sure the speed and duplex are the same on both sides and the vtp domain name

. Maybe Boson made the program so it thinks it should be just "switchport mode trunk "  instead of switchport mode dynamic desirable " which is the default on the 2950 .

speculor_cisco Sat, 08/07/2010 - 10:37
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I have found this note in the book "Cisco field manual: Catalyst switch configuration" by Hucaby and McQuerry:

"Cisco 2950 and 3500XL switches do not support DTP and are always in a mode similar to nonegotiate. If you turn trunking on for one of these devices, it will not negotiate with the other end of the link and requires that the other link be configured to on or nonegotiate."

I imagine that this note come from a real world experience, also because the 2950 official configuration guide says that the default should be dynamic desirable.

That is incredible! Is it possible that also the official document is not clear about this argument?


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