Switches can operate in three different VTP modes, which affect the way in which they share or interact with VTP advertisements. Once configured, VTP information is stored in their VTP database. The three modes include:
Server. By default, every Catalyst switch configured to use VTP will be configured as a server. In any VTP domain, at least one server must exist. When in server mode, a switch can be used to add, modify or delete VLAN related information, which will be passed to all other switches in the VTP management domain.
Client. In client mode, a switch receives VTP advertisements and makes changes according to their contents. Note that a VTP client cannot change VLAN information.
Transparent. In transparent mode, a VTP switch will forward VTP messages, but will not actually use the configuration information it receives. VLANs that are added, modified or deleted on a VTP switch in transparent mode will only apply to that particular switch.
When a switch receives a VTP update, it checks the VTP domain name and revision number information stored in the advertisement. If the information is for a different VTP domain, it is ignored. If the revision number of the advertisement is below the number currently stored in its database, it is similarly ignored.