VTP is VLAN Trunking Protocol which propagates VLAN numbers throughout a network. When a switch acting as VTP server with a higher revision number of the VTP database is inserted into the network it can “bomb” the network.
The higher VTP database number will cause VLAN information to be overwritten in all switches. Because the version of the latest database is usually from a switch in the lab which is nothing like the live network, your entire network effectively “dies”.
This is known as a “VTP Bomb”, especially is performed maliciously.

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As I’ve said once: VTP = plug-and-play wannabe gone in the wrong direction
http://blog.ioshints.info/2008/12/should-vtp-be-disabled-by-default.html
But it’s fixed in VTP v3 – http://etherealmind.com/vtp-3-making-comeback-review/
Unfortunately, v3 is widely unsupported on smaller switches. GVRP would be the right thing, but for some reason, Cisco has been ignoring this for years…
I’ve seen it mentioned in some 6500 documentation recently, though.
Yeah, it will start there and progressively move into the smaller switches over time (I should think and experience suggests). At a guess, 18 months.