VTP Pruning Question

Unanswered Question
Sep 4th, 2008


I have a network spanning roughly 12 switches. 2 Core 6500's with 10 dual connected 3560's, all running MST, and one of the 6500's is a VTP server.

I have around 240 VLANs configured, and hanging off of some of the 3560s are 2950s that only support 250 VLAN's before the switch will change to transparent mode, because the VTP database on those does not go beyond 250.

What I am looking for is a way to use VTP, but not send all VLANs to some of the switches.

I thought VTP pruning would solve this, but it seems it only blocks traffic, mainly broadcast from being sent over the trunks that connect to the 2950s, and am looking for a way to be able to keep using VTP, and have client databases have only the VLAN's they need. Of course making them transparent would work also, just does not scale well, with creating the VLAN on all switches along the way etc.

Any suggestions are welcome, thanks.

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glen.grant Thu, 09/04/2008 - 16:36

All you have to do on the 2950 is to "manually" prune off the connecting links to the 2950 . Only allow the vlans across the trunk links that you need on the 2950's , "switchport trunk allowed vlan XX " . This prevents the switch from using spanning tree instances that are not actually needed on that switch. If you only need 3 vlans on the switch say vlan 1-3 then manually prune off 4-1025 on the connecting trunk links and this prevents those stp instances from being implemented on that switch.

Jasonch518_2 Thu, 09/04/2008 - 16:57


But does that actually remove the VLAN from the VTP database? That is where my problem is. I am running MST network wide, so spanning tree instances are not the problem, here is show vtp output, part of it anyway.

Maximum VLANs supported locally : 250

Number of existing VLANs : 237

The switchport trunk allowed vlan does not remove VLAN's from the VTP database, from what I have seen anway, correct me if I am wrong. So if I added 14 more VLAN's on my server, the switch above would auto enter transparent mode, nothing would break, just wanted to avoid manually adding VLAN's on downstream switches.

I am using the switchport trunk allowed vlan on the upstream switch, but to prevent broadcast traffic from flowing to the 2950s, and it does not remove VLAN's from VTP database, which I did not think it would.

Sorry if I was unclear in my first post on what I was trying to do, if my only option is transparent mode, then so be it, but wanted to see what else was available, if anything.

Edison Ortiz Thu, 09/04/2008 - 20:15

I'm afraid you will need to go transparent on switches that support less than 250 Vlans and leave the rest on the VTP Server/Client mode.

Then you can have a justification to upgrade the hardware :)



jpoplawski Fri, 09/05/2008 - 06:18

Can I ask why you have so many VLANs? 10 VLANs per switch seems kind of high. I would be interested to see how you're implementing them.



paul.matthews Fri, 09/05/2008 - 07:24

20 VLANs per switch seems a trifle excessive!

On your numbers, you have 240 VLANs, but the switches support 250, so it should work, you just have no room for expansion.

I would also be careful about pruning when using MST - you need to make sure that you prune in line with the group rather than simply where you want to prune.

I would either review the design to reduce the number of VLANS, or carefully plan the groups and manually prune to simplify the groups as much as possible - simpler topology within the instance leads to better stability, and if that means switching over to VTP Transparent everywhee, heck that

jpoplawski Fri, 09/05/2008 - 08:14

My math is wrong, I thought he had 24 switches. 20 VLANs is even worse then 10 per switch, yikes! :)


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