VTP Modes and VTP Protocol Explained

VTP is a Cisco proprietary VLAN management protocol. It allows us to manage VLANs from a centralized switch. It works in three modes: Server, Transparent, and client.

In a small network, you can manually manage VLANs. For example, if your network has only two switches and you want to add a new VLAN, you can easily add that VLAN on both switches. But if your network has 50 switches, this process becomes complex and tedious. You might make a mistake in configuring the new VLAN or forget to add the new VLAN to one of the 30 switches.

VTP solves this issue. It allows you to manage VLAN information from a single switch. With it, you need to add or remove VLAN information only on the VTP Server. VTP Server automatically propagates the new VLAN information, via VTP messages, to all of the other switches in the network. VTP shares VLAN configuration information between Cisco switches on trunk connections. It allows switches to share and synchronize their VLAN information, ensuring the network has a consistent VLAN configuration.

VTP domains

A VTP domain is a group of switches that share VTP information. For example, you have 50 switches in a network. you want only 30 of them to share VTP information. In this case, you can keep them in a VTP domain. If a switch is not a member of a VTP domain, it ignores VTP messages shared in that domain.

A switch can belong only to a single domain. When switches generate VTP messages, they include the domain name in their messages. A switch processes incoming VTP messages only when the domain name in the message and the domain name configured on it are the same.

VTP modes

There are three VTP modes: Server, Transparent, and client. Based on the mode you configure on a switch, the switch interacts with VLANs.

VTP Server mode

This mode allows you to add, modify, and delete VLANs. It automatically propagates VLAN information to other switches. You can configure multiple VTP Server switches in a VTP domain.

If a VTP server receives VLAN information from another VTP server in the same VTP domain, it compares the received information with the locally saved information and keeps the updated information. To compare information, it uses the configuration revision number. VTP Server adds a configuration revision number to each VLAN information it advertises through VTP messages.

Initially, this number is set to zero (0). When you make a change on a server switch, it increments its revision number and advertises this to the other switches. When a client or other server switch receives this information, it compares the revision number in the message to the last message it received. If the newly arrived message has a higher number, it assumes that the VLAN information on the server has changed and updates the locally stored information from the received message.

If the message has the same revision number, it ignores the messages.

VTP Client mode

In this mode, a switch depends on VTP Server for VLAN information. It can not make any change in VLAN information itself. It incorporates VTP messages received from the VTP Server and implements VLANs the VTP messages have.

VTP Transparent mode

In this mode, a switch works as a bridge between VTP Server and VTP clients. It connects VTP Servers with VTP clients. It forwards the messages it receives from VTP Server to VTP clients. It does not change or incorporate VTP messages.

You can add, modify and delete VLAN on a transparent switch. But these changes are locally significant. The server does not forward them to other switches.

Differences between VTP modes

The following table lists the differences between VTP Server, transparent, and client mode.