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| 2.7 |  | VLAN Trunking Protocol (VTP) |
| 2.7.2 |  | VTP operation |

A VTP domain is made up of one or more interconnected devices that share the same VTP domain name. A switch can be configured to be in one VTP domain only. Global VLAN information is propagated across the network by way of connected switch trunk ports.

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. 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

It is important to note that switches can be configured not to accept VTP information. These switches will forward VTP information on trunk ports in order to ensure that other switches receive the update, but the switches will not modify their database, nor will the switches send out an update indicating a change in VLAN status. This is referred to as transparent mode.

By default, management domains are set to a nonsecure mode, meaning that the switches interact without using a password. Adding a password automatically sets the management domain to secure mode. A password must be configured on every switch in the management domain to use secure mode.

Detecting the addition of VLANs within the advertisements serves as a notification to the switches that they should be prepared to receive traffic on their trunk ports with the newly defined VLAN IDs, emulated LAN names, or 802.10 SAIDs.

For example, C5000-3 transmits a VTP database entry with additions or deletions to C5000-1 and C5000-2. The configuration database has a revision number that is notification +1. 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.

In order to use VTP, VTP domain name must be assigned to each switch. VTP domain names are case sensitive. VTP information remains in the VLAN management domain. The following are conditions for a VTP domain:

- Each Catalyst switch in a domain must have the same VTP domain name, whether learned or configured.
- The Catalyst switches must be adjacent, meaning that all switches in the VTP domain form a contiguous tree in which every switch is connected to every other switch in the domain via the tree.
- Trunking must be enabled between all Catalyst switches.

If any one of these conditions is not met, the VTP domain becomes disconnected, and information will not travel between the separate parts of the network.