VLAN Trunking Protocol

VLAN Trunking Protocol

In Figure 4-4, two VLANs extend over multiple switches using trunking. Because each switch sharing trunks must support common VLAN information for the trunks to function correctly, Cisco created the VLAN Trunking Protocol (VTP) for creating and managing that VLAN information. It should be noted that any VLAN created on a switch is in an inactive state until VTP is configured.

A collection of switches that are under the same administrative control and will support the same range of configured VLANs are said to be in the same VTP domain. A domain name is simply a unique identifier up to 32 characters long used to identify the switches that will share the same VTP information. The domain name is also case sensitive.

In server mode, the switch has a list of all the VLANs for that domain. It can add, delete, or rename any VLAN, and the configuration information is stored in nonvolatile random-access memory (NVRAM). In client mode, the switch obtains its information for the VLAN database from a VTP server, and it cannot make any modifications to it. The information learned by the client switch is not stored in NVRAM. If the client switch is rebooted, the switch must dynamically learn all the VLAN information again from a VTP server. In transparent mode, a switch does not participate in VTP; it merely passes the VTP advertisements to other switches. In transparent mode, the switch can be configured to add, delete, and modify, and the information is stored in NVRAM.

Certain requirements must be met before VTP can be used to manage a domain and distribute VLAN information. Each switch must have a configured trunk port, use the same domain name, and be directly connected. As noted earlier, the trunk port is used to send the VTP information to the adjacent switch. VTP can automatically distribute VLAN information to all other switches in the same domain through a trunk port, or allow manually for each switch to be configured. The dynamic process using server/client mode is administratively palatable because it is easy to implement; a server switch is configured with VLANs, and the rest of the switches in that domain receive that information. On the other hand, server/client mode can pose potential risks on the network, which will be discussed in this section shortly. Transparent mode requires manually configuring each switch.

The two types of VTP versions, version 1 and version 2, have some major differences. Version 2 has support for Token Ring. In version 2, switches running in transparent mode forward VTP advertisements they receive regardless of VTP version or domain name; switches configured for VTP version 1 ignore VTP advertisements with a different VTP domain name than the one configured. Cisco switches default to version 1.