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I have inherited a network. I have 12 separate sites, all of which have at least 10 vlans. Each site is it's own VTP domain. I have an MPLS ethernet handoff into a switch at each site. From each switch, I have trunks distributing vlans to the other access switches.I have some switches where all vlans are distributed but I have no member ports. If I understand the concept correctly, if I have vlans on switch A and B, but no member ports on C, VTP pruning removes the unneeded vlans from C thus reducing bandwidth usage on the trunks.

The actual question is: do I have any reason NOT to use VTP Pruning?

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  • @pedro/avery--good information, for sure. I should clarify this is not a purchase of equipment, or anything like that. I have finally been given a chance to move up/forward and I am now in charge of our network. Everything at this point is up and functioning, albeit not very well.
    – Rex Sheffield
    Jul 24, 2014 at 20:57
  • Rex, the biggest downside of vtp pruning is VTP. If you like VTP and you accept the risks of using it, then there's no problem. I personally don't use it and disable it in all our switches.
    – Mike Pennington
    Jul 25, 2014 at 12:42

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The short answer is 'no' there is no downside to VTP pruning that I'm aware of.

@Pedro Brito gives some good background on the operation and gotcha's of VTP. Since you are using site specific VTP domains the risk of revision number issues is lessened.

I would just add to be very specific which switches are your VTP servers at each site and set the rest to client. I usually make the spanning-tree primary and secondary root bridges also the VTP servers.

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Not exactly a reason not to use VTP pruning but just a few attention points:

If running version 1, transparent mode will disable forwarding of VTP packets, hence making all downstream switches outside of your VTP domain and unaware of VLANs.

If you're running version 2, a switch in transparent mode will forward every VTP packet if the switch is set to NULL domain. If the domain name on the switch is set, it will forward packets only for this domain.

Most importantly: be really cautious about used equipment! When you use the "write erase" command to wipe your new-used equipment, it doesn't erase VLAN information, then if you plug it in your network and start to configure VTP, bad things might happen. If it had a bigger revision number than you have on your network, its VLAN information will now be flooded to all your switches. This will happen even if set to null domain, because when in server or client mode, the first non-null domain packet that arrives determine the new VTP domain and then it will flood its VLANs and disrupt your network.

But I would say it's a really good protocol which will save you some time if used wisely.

Edit: sorry. I misread this question as regarding VTP and not only VTP pruning and I'm not able to delete my answer :(

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  • To add to this, to clear the existing VLAN information off of a switch (good practice for buying second-hand switches or for selling yours) is to delete the vlan.dat file and reboot the switch.
    – Avery Abbott
    Jul 24, 2014 at 16:26

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