Spanning tree catalyst 4507

Unanswered Question
Jan 26th, 2010

Recently, i fiber port on our 4507 switch has been shutting down periodically, and i dont know why...

to a 4510R  on the other end...

see attached message....  (it states a spanning tree issue)


rob davis

I have this problem too.
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sachinraja Tue, 01/26/2010 - 07:04

Hi Rob

Can you post your error message ? I think you missed it with your previous post ? Is the port going to err-disabled ?


sachinraja Tue, 01/26/2010 - 07:52

Hello Rob

This means that you havent configured trunk on Gig 3/1 which is used to forward traffic ? can you check if you have the "switchport mode trunk" command on that interface ? can you copy+paste the configs on gig 3/1 ? is Gig 3/1 connecting to another switch ? If so, you need to configure trunk and the same type of encapsulation (dot1q or isl) on both the sides. Once you configure trunk, spanning tree automatically unblocks the port.

Hope this helps...All the best


davisr651 Tue, 01/26/2010 - 08:16

When the problem did appear, one of our techs did do a show trunks, and found that the fiber port was no longer a trunk port, so he manally assigned it as a trunk, then everything started working..... my question to you is, what would cause the port to lose its configuration as a trunk port?... also does the switch automatically detect that it is connected to another switch and set the port as a trunk port?..... or is this a manual operation


rob davis

sachinraja Tue, 01/26/2010 - 08:24

Hi Rob

You can either set a port to manual trunk "by forcing switchport mode " command, or configured for dynamic trunking. With DTP, the switch negotiates parameters and automaticaly brings the port for trunking. Normally you would have it configured for auto/desirable etc... you can set the port to manual trunk if you dont want DTP.. Refer to the following document for best practices of DTP..

Hope this helps.. all the best


Mohamed Sobair Tue, 01/26/2010 - 08:29

Hi Rob,

DTP or Dynamic trunking protocol negtiate trunking between connected switch ports if itset to negotiation mode, however, if switches failed to negotiate trunking on the ports, the Switch places the port into Access mode.

The point here is that, DTP is ONLY performed if the switches are part of the same VTP domain, if the switches are in different domains, then DTP is not performed. The best approach from also a securtiy prepective is not having DTP on ports enabled and have the ports to negotiate trunking rather than setting it manually.




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