VTP pruning

[c.cornett]

I have a question on VTP pruning. If I have VLANs 1-15 in my VTP domain. Then I setup a trunk between two 6500 switches with the following commands.

clear trunk 1/1 2-4094

set trunk 1/1 des dot1q 1,2,3

Now, on both 6500 switches I have ports active in VLAN 1,2,3. At this time I have manually pruned all VLANs except 1,2,3 from this trunk.

Next, VTP pruning is enabled.

Then someone sets a user port to VLAN 4 on both 6500 switches and plugs clients into these ports.

Will VLAN 4 be added to the trunk on port 1/1 automatically ?

[milan.kulik]

Hi,

there are two different terms:

VLAN disabling and VLAN prunning.

If you

clear trunk x/y 4

you have disabled VLAN4 on the trunk. There is no VLAN4 traffic allowed on the trunk, STP tree for VLAN4 has been disrupted. This is the way how to control spreading of particular VLANs in your network for security or just saving CPU load purposes.

If you

set vtp pruneeligible 4

set vtp pruning enable

you have enabled prunning for VLAN4. This means that VLAN4 user traffic is not sent through the trunk (even VLAN4 broadcasts are not sent) if there is no port assigned to VLAN4 on the downstream switch. The only traffic sent in VLAN4 on the trunk are STP BPDUs, CMP, and similar "Cisco service" protocols.

I think all VLANs are pruneligible but pruning is disabled by default (I'm not 100% sure).

Use sh vtp dom command to see which VLANs are pruneligible and if pruning is enabled.

To your question:

If you configured only

clear trunk 1/1 2-4094

set trunk 1/1 des dot1q 1,2,3

VLAN4 will NOT be added to the trunk automatically.

You need to add it manually to the trunk by

set trunk 1/1 4

Regards,

Milan