VTP Pruning

No, it's not necessary, but it saves lots of wasted traffic.

The condition where you should enable it, it where you want to stop wasted traffic.

:-)

Configuring VTP Pruning:

a more basic Q is should you use VTP at all.

personally i think it a p[rooblem, not a solution, and i believe the need vanished once high performance L3 switching arrived.

VTP mode transparent effectively turns it off.

VTP and L3 switching have nothing to do with each other. VTP is a method to create layer 2 vlans across across a set of switches. We leave it on because I can't count how many times I've been called because "traffic isn't flowing" or "DHCP isn't working" and the problem is that a VLAN was created on Switch A, but not on Switch B.

agreed - what i was trying to point out is "why design sprawling VLANs in the 1st place?".

L3 switching is the tool that makes it easy to confine VLANs to small areas of a network.

We leave it on

OK - personally i would try to keep any 1 VLAN to a single switch and avoid the need for VTP is the 1st place (but that is an ideal solution - the real world tends to be more messy).

VTP isnt a generic solution anyway - many of the instances where i have had problems with VLANs have not allowed VTP to be used - other manufacturers kit for example.