When transmitting VTP messages to other switches in the network, the VTP message is encapsulated in a trunking protocol frame such as ISL or IEEE 802.1Q. Figure shows the generic encapsulation for VTP within an ISL frame. The VTP header varies, depending upon the type of VTP message, but generally, four items are found in all VTP messages:

• VTP protocol version: Either Version 1 or 2
• VTP message type: Indicates one of four types
• Management domain name length: Indicates size of the name that follows
• Management domain name: The name configured for the management domain

VTP switches operate in one of three modes:

• Server
• Client
• Transparent

VTP servers can create, modify, and delete VLAN and VLAN configuration parameters for the entire domain. VTP servers save VLAN configuration information in the switch NVRAM. VTP servers send VTP messages out to all trunk ports.

VTP clients cannot create, modify, or delete VLAN information. This mode is useful for switches lacking memory to store large tables of VLAN information. The only role of VTP clients is to process VLAN changes and send VTP messages out all trunk ports.

Switches in VTP transparent mode forward VTP advertisements but ignore information contained in the message. A transparent switch will not modify its database when updates are received, nor will the switch send out an update indicating a change in its VLAN status. Except for forwarding VTP advertisements, VTP is disabled on a transparent switch.

VLANs detected within the advertisements serve as notification to the switch that traffic with the newly defined VLAN IDs may be expected.

In Figure , Switch C transmits a VTP database entry with additions or deletions to Switch A and Switch B. The configuration database has a revision number that is incremented by one. A higher configuration revision number indicates that the VLAN information that is being sent is more current then the stored copy. Any time a switch receives an update that has a higher configuration revision number the switch will overwrite the stored information with the new information being sent in the VTP update. Switch F will not process the update because it is in a different domain. This overwrite process means that if the VLAN does not exist in the new database, it is deleted from the switch. In addition, VTP maintains its own NVRAM. An erase startup-configuration clears the NVRAM of configuration commands, but not the VTP database revision number. To set the configuration revision number back to zero, the switch must be rebooted.