A failure of the VTP protocol will prevent switches from agreeing about the
VLAN configuration within a network. This may cause communications between
switches to fail or may result in entire VLANs being deleted.
The first
indication of a VTP problem often occurs when a VLAN is created on a switch but
the VLAN fails to propagate to all other switches within the domain. When this
occurs the first thing to check is the VTP mode. Each switch can be placed into
one of the following VTP modes:
- Server
- Client
- Transparent
Changes to the VLAN configuration within a VTP domain can only be made
on a switch that is in the VTP server mode. The VTP mode and domain can be
verified using the show vtp status command.

Assume
that the switch has been previously used, and already has an appropriate VTP
domain name entered. The switch is configured as a VTP client, and then
connected to the rest of the network. The instant the trunk link is brought up
to the rest of the network, the whole network goes down. What could have
happened?
The most likely explanation is that the configuration revision number of the
inserted switch was higher than the configuration revision of the VTP domain.
Therefore, the recently introduced switch with minimal VLAN information has
erased all VLANs through the VTP domain.
This situation will happen
whether the switch is a VTP client or a VTP server. A VTP client can erase VLAN
information on a VTP server. This is evident when many of the ports in the
network go into inactive state and are assigned to a non-existing VLAN.
Quickly reconfigure all of the VLANs on one of the VTP servers. Always make
sure that the configuration revision of all switches inserted into the VTP
domain is lower than the configuration revision of the switches already in the
VTP domain.