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Does a switch still send BPDUs in VTP transparent mode?

Posted on 2008-06-19
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Last Modified: 2008-06-21
Hi experts,

I'm hooking two cisco switches together via a crossover cable. For example, on Switch A port Gi3/15 I have Spanning-tree BPDU enable.
It will be connected to Gi4/18 on Switch B. I have switch B in VTP Transparent mode. Why is it that switch A keeps putting it's port Gi3/15 in ERR-Disabled everytime I connect the two switches?
It works when I remove the Spanning-tree BPDU enable on Switch A so that's how I know BPDU's are causing it.
I thought since switch B is in VTP transparent mode, it won't send out BPDUs.
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Question by:typertec
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Expert Comment

by:from_exp
ID: 21828642
It's not due to vtp mode, but because your switch A receives bpdu's from switch B
so if you will configure port gi3/15 with spanning-tree bpdu filter everithing should be fine.
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Expert Comment

by:Don Johnston
ID: 21829826
I've never heard of the command "spanning-tree BPDU enable" before. But it sounds like you have BPDU guard enabled. With BPDU guard enabled, if any BPDU's are received on the port, it will go error disabled. This feature is used in conjunction with portfast.

Neither of these features should be used with switch-to-switch links. Enabling BPDU filter will stop the port from sending BPDU's out the port. This is very risky as you are effectively disabling spanning-tree on the port. If a loop ever occurs (intentionally or un-intentionally), spanning-tree will not be able to detect it and a broadcast storm could occur.

I would suggest disabling BPDU guard and portfast on the switch-to-switch links.

Then again, this could be caused if Root guard is enabled. In that case, if a superior BPDU is received (from a switch that is attempting to be the root), the port will go into error-disable state. In that case, either lower the bridge priority of Switch A or raise the bridge priority of Switch B.
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Author Comment

by:typertec
ID: 21830658
Well, why is switch B sending BPDUs when it's in VTP transparent mode? Am I correct to say that a switch in Transparent mode does not send BPDUs?
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Accepted Solution

by:
from_exp earned 350 total points
ID: 21830713
no, incorrect, vtp and spanning tree are different things.
as it was mentioned by donjohnston, more likely you have switched on spanning tree bpdu guard on the port towards switch B.
remove that setting. and if you don't want switch A receive BPDUs from switch B, configure switch A with spanning tree bpdu filter
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Assisted Solution

by:Don Johnston
Don Johnston earned 150 total points
ID: 21830879
Concur. VTP and STP are close (by one letter), But have ABSOLUTELY nothing to do with each other.
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