VTP Pruning is enabled and working, but traffic is still allowed on the trunk.

[Cauli Medeiros]

Hi guys, I'm a little confused about how the VTP Pruning acts, monitoring traffic on an interface realized that traffic from one pruned vlan still able to cross a trunk link. 

In attached image 1 you can see that the pruning is active. The only allowed vlan on the link is vlan 1

In attached image 2 you can see that traffic is only interrupted at the interface when the pruning is done manually. (with the command "switchport trunk allowed  vlan")

The traffic in question is in a video vlan. They are multicast frames. Anyone know if this is a normal behavior? is there any reason for this traffic be going through the link?

[paul driver]

Hello

VTP pruning is a switches non deterministic way of dynamically pruning vlan membership off a given trunk.

If the switch doesn’t receive/or send a vlan membership announcement (VMA) for a given vlan the neighboring switch will not send traffic for that vlan, so that traffic will be pruned.

Personally I am in favor of manually pruning ( switchport trunk allowed) as you can be sure what is being pruned and what isnt.

sh int x/x pruning

res
Paul

[Cauli Medeiros]

Hi paul , thanks for the feedback!

I agree that manual pruning would be the best option,  unfortunately I have a layer 2 topology involving more than 100 switches and some VLANs (such as vlan of cameras) spread across all of them.

The pruning seems to be working normally, ( follow the output attached) the correct VLANs are marked as "pruned for lack .. " ( attached image ), but traffic from vlan 5 still being sent through the trunk (I'm sure the traffic is being sent , based on the chart that I sent and on the fact that the command "switchport allowed vlan except 5" takes the traffic to nearly 0).