Dear Lawrence m,
Here are my answers:
How does pruning work?
VTP pruning enhances network bandwidth use by reducing unnecessary flooded traffic, such as broadcast, multicast, and unicast packets. VTP pruning increases available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to access the appropriate network devices. By default, VTP pruning is disabled.
For VTP pruning to be effective, all devices in the management domain must either support VTP pruning or, on devices that do not support VTP pruning, you must manually configure the VLANs allowed on trunks.
How is pruning controlled? What is the default pruning behavior?
Enabling VTP pruning on a VTP server enables pruning for the entire management domain. VTP pruning takes effect several seconds after you enable it. By default, VLANs 2 through 1000 are eligible for pruning. VTP pruning does not prune traffic from pruning-ineligible VLANs. VLAN 1 is always ineligible for pruning; traffic from VLAN 1 cannot be pruned.
When the last switchport in a vlan is removed from said vlan, how and when is VTP triggered to prune said vlan?
Normally VTP does everything triggered and pruning will be done on the switch right away and then it triggers a vtp update. If you want to know the exact timer, then we will have to dig into Wireshark and do the experimental.
Now in case the cable is unplugged from the port, then it may take up to 30 sec to prune and that’s because of the Spanning-tree instance that will be created.
I hope I could answer your questions.