VTP Pruning and trunked hosts

Answered Question
Oct 21st, 2009
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What would the impact be to servers that have a trunk running to them when/if I enable VTP Pruning?

I have a number of ESX Boxes with VMs on various VLANs... what would happen? Thanks.

Correct Answer by Peter Paluch about 7 years 9 months ago

Hello,


If you enable VTP Pruning while having trunk lines to end hosts, the VLANs will be pruned from these trunks. The VTP Pruning feature requires the VTP Join messages to be received on a trunk port to decide whether a VLAN should be pruned or not. As the end hosts do not generate VTP Join messages, all VLANs will appear as unused to a switch so it will prune them off the trunk.


You can, however, configure a list of pruning-ineligible VLANs on a trunk, i.e., VLANs that should never be pruned. This is done using the command


switchport trunk pruning vlan none


This way, you can have the VTP Pruning turned on and at the same time prevent the VLANs from being pruned on a selected trunk interface.


Best regards,

Peter


Correct Answer by Jon Marshall about 7 years 9 months ago

Ryan


Not sure because the servers don't participate in VTP.


However rather than use VTP pruning you should actually just be specific on the trunk link with the "switchport trunk vlan allowed.." command and only allow the vlans that the servers need access to.


Jon

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Correct Answer
Jon Marshall Wed, 10/21/2009 - 05:30
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Ryan


Not sure because the servers don't participate in VTP.


However rather than use VTP pruning you should actually just be specific on the trunk link with the "switchport trunk vlan allowed.." command and only allow the vlans that the servers need access to.


Jon

rtjensen4 Wed, 10/21/2009 - 05:38
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I'm not really concerned about the trunks going to the ESX hosts as far as Pruning goes...It's OK to trunk everything to them... I'd just like to clean up the uplinks to switches that are directly connected to our core. I.E. The switch on Floor 2 only uses 2 VLANs, why trunk 20 VLANs to it and have that traffic take up bandwidth on the uplink. I Could use static assignments (which I'm leaning towards), but am just curious what the impact would be. If there won't be an impact on the trunked servers, then I'll turn on pruning. If I can't find a definite 100% answer, I'll avoid the headaches of an outage and just use static assignments for the trunked uplinks. Thanks!

Giuseppe Larosa Wed, 10/21/2009 - 07:55
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Hello Ryan,

be aware that VTP pruning can manage unnecessary flooding of multicast, broadcast, unknown unicast traffic but it does not limit the number of STP instances running on your switches.


switchport trunk allowed vlan list


has the added benefit to provide STP scalability by removing unneeded STP instances.


This has to be take in account if you use PVST+ or Rapid PVST.


20 vlans are not a problem but 130 vlans can be a problem for some low end switches.


also the limits on number of vlans can be different for VTP and for STP:

there are devices accepting up to 128 vlans in their VTP DB but only 64 STP instances.

The result that has been seen also in the forums are vlans with STP disabled without any human intervention!


Hope to help

Giuseppe


Correct Answer
Peter Paluch Wed, 10/21/2009 - 05:41
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Hello,


If you enable VTP Pruning while having trunk lines to end hosts, the VLANs will be pruned from these trunks. The VTP Pruning feature requires the VTP Join messages to be received on a trunk port to decide whether a VLAN should be pruned or not. As the end hosts do not generate VTP Join messages, all VLANs will appear as unused to a switch so it will prune them off the trunk.


You can, however, configure a list of pruning-ineligible VLANs on a trunk, i.e., VLANs that should never be pruned. This is done using the command


switchport trunk pruning vlan none


This way, you can have the VTP Pruning turned on and at the same time prevent the VLANs from being pruned on a selected trunk interface.


Best regards,

Peter


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