VTP Pruning

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Sep 26th, 2007
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We have a VTP domain with one master switch connected to several downstream switches. The domain has vtp pruning enabled.

One switch wrongly prunes back vlans from the trunk which are active on the switch.

I am having trouble finding an in-depth explaination of how pruning works, but from my understanding the choice of which VLANs to prune off is made based on the join requests from the neighboring switch. Meaning the wrong information is actually being generated by the master switch and not the switch which is wrongly pruning these vlans? The master switch's pruned vlans IS correct.

All other switches are pruning the correct VLANs. All obvious steps have been taken ie; reload, reconfigure, reset vtp domain yet the problem still remains.

Does anyone have an in-depth document explaining the process vtp pruning uses to generate and process join messages so I can troubleshoot this further?

For now, I have configured vtp on the trunk interface to inform it that the active VLANs are not eligable for pruning, this fixes the symptoms, but does not resolve the underlying issue.

Many thanks in advance.


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glen.grant Wed, 09/26/2007 - 11:38
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Just something to check make sure the problem switch is in client mode and not accidently set as transparent . Other things to check is to make sure the vtp version numbers match on both the server and your client , if not then vtp is not working correctly . Make sure the vlans are allowed on the trunk on both sides of the link . Maybe you can post the configs from both sides .

gryphon55 Wed, 09/26/2007 - 14:05
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Hello, thanks for your reply.

All switches are in VTP server mode. The revision number is correct and VTP must be working because I cleared the VLAN database, then changed the VTP domain and the VLAN database was learned from the other switches. All VLANs are allowed by the trunk.

I don't have the exact configurations with me as they are at work, but it is very minimal; 'switchport trunk encapsulation dot1q', 'vtp mode server', 'vtp domain DOMAIN', 'vtp pruning', 'switchport access vlan 123'.

Thanks again,


arififtikhar Thu, 09/27/2007 - 21:03
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You should have all switches in client mode except the master switch(es). After you assign VTP domain assign a VTP password too.

Should be OK then.

You can also check "VTP status" copy and paste here for more clear picture.


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