q1) VTP pruning is a bit more complicated than that example suggests.
So it talks about active ports but trunks are also active ports not just ports connected to end devices. It's important to understand this because the example you linked to is quite a straightforward one but take this example -
h1 (vlan 10) -> sw1 -> trunk -> sw2 -> trunk -> sw3 -> h2 (vlan 10)
all trunk links are allowing vlan 10 but there are no access ports on sw2 in vlan 10.
For obvious reasons sw2 cannot simply prune vlan 10 off it's trunk links even though it has no active access ports in vlan 10 otherwise h1 cannot send traffic to h2.
The key thing to understand here is that sw2 cannot make a decision as to which vlans to prune based on the state of it's own ports which is the logic of what you are suggesting. It can only make a decision based on information received from neighboring switches.
Going back to the example you linked to if switch 2 pruned vlan 10 off the trunk using the logic you are suggesting it has done that regardless of what ports in vlan 10 are active on switch 1.
Following on with the same logic if a port in vlan 10 now becomes active on switch 2 it will begin to forward traffic for vlan 10 to switch 1 again regardless of whether switch 1 needs it or not.
What if in the meantime the active port on switch 1 in vlan has gone down ?
Because switch 2's decision as to whether to prune or not is based only on it's own port status it has no way of knowing this.
On the other hand if switch 2 does not prune vlan 10 off the trunk link because it knows there are active ports on switch 1 in vlan 10 then if a port becomes active in vlan 10 on switch 2 it can begin forwarding on that trunk link.
And if the port in vlan 10 on switch 1 went down switch 2 could prune that vlan regardless of whether it had active ports in vlan 10 or not.
It should also be noted you gain nothing from pruning on both sides in your example ie. switch 1 has pruned the vlan so it won't be sending any traffic and by definition switch 2 won't either otherwise the vlan would not have been pruned in the first place.
Jon
