VTP Pruning

mattrgee

Hi all,

I've added a vlan to the vtp server with pruning enabled and I didn't expect it to appear on my second switch as no ports were part of that vlan, however it did appear on the second switch. Is that correct?

Thanks matt.

cisco_trooper

The vlan will be on the switch. The vlan won't be allowed on the trunk to the switch.

do a show int trunk...

bighornsheep

There's a different between active vlan in the VTP domain, and actually used in forwarding state.

example:

Show int <x/y> trunk

Port Mode Encapsulation Status Native vlan
Gi2/10 on 802.1q trunking 1

Port Vlans allowed on trunk
Gi2/10 1-4094

Port Vlans allowed and active in management domain
Gi2/10 1,95-99,208-209

Port Vlans in spanning tree forwarding state and not pruned
Gi2/10 1,99

First section shows the vlans that are allowed, in this case all are allowed.
Second section shows the vlans that are defined on VTP server and existing in VTP domain.
Third section shows the vlans that have ports defined on the downstream switch.

The key here is DOWNSTREAM, meaning in the simple case of just two switches one being vtp server, the other being vtp client, the above output will be on the server side of the trunk port. If you do the above on the vtp client side trunk port, it will show all the vlans in forwarding state & not pruned even though there are no port active ports for those vlans, this is because the server is pruning it off the downstream trunk.

mattrgee

Brilliant, just played with the config and added a port to one of the previously pruned vlans and it now appears on the downstream side of the trunk list under the 'not pruned' section.

I love this ****.

Thanks Matt.

bighornsheep

Now try and remove vlan 1 from the allowed list on the trunk. Then see if any of the switch features which rely on vlan 1 will continue to function. ie. cdp, if so, how come? if not, why not?

mattrgee

Interesting, it looks like by removing vlan 1 it stops VTP traffic yet CDP still functions. I this because VTP requires vlan 1 as a management vlan, whereas CDP does not?

Matt

kryolla

it just stop data traffic, management/control traffic still traverses vlan 1.

To see what is going on with the pruning messages type sh int fa0/xx pruning