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| 2.7 |  | VLAN Trunking Protocol (VTP) |
| 2.7.3 |  | VTP modes |

Switches can operate in any one of the following three VTP modes: server, client, or transparent.

Server
By default, a Catalyst switch is in the VTP server mode and in the “no management domain” state until the switch receives an advertisement for a domain over a trunk link or a VLAN management domain is configured. A switch that has been put in VTP server mode and had a domain name specified can create, modify, and delete VLANs. VTP servers can also specify other configuration parameters such as VTP version and VTP pruning for the entire VTP domain. VTP information is stored in NVRAM.

VTP servers advertise their VLAN configuration to other switches in the same VTP domain, and synchronize the VLAN configuration with other switches based on advertisements received over trunk links. When a change is made to the VLAN configuration on a VTP server, the change is propagated to all switches in the VTP domain. VTP advertisements are transmitted out all trunk connections, including ISL, IEEE 802.1Q, IEEE 802.10, and ATM LANE trunks.

A critical parameter governing VTP function is the VTP configuration revision number. This 32-bit number indicates the particular revision of a VTP configuration. A configuration revision number starts at 0 and increments by 1 with each modification until it reaches 4294927295, at which point it recycles back to 0 and starts incrementing again. Each VTP device tracks its own VTP configuration revision number; VTP packets contain the sender’s VTP configuration number. This information determines whether the received information is more recent than the current version. To reset a configuration revision number to 0, simply disable trunking, change the VTP domain name, change it back to the original name, then reenable trunking.

If the switch receives a VTP advertisement over a trunk link, it inherits the VTP domain name and configuration revision number. The switch ignores advertisements that have a different VTP domain name or an earlier configuration revision number.

Client
The VTP client maintains a full list of all VLANs within the VTP domain, but it does not store the information in NVRAM. VTP clients behave the same way as VTP servers, but it is not possible to create, change, or delete VLANs on a VTP client. Any changes made must be received from a VTP server advertisement.

Transparent
VTP transparent switches do not participate in VTP. A VTP transparent switch does not advertise its VLAN configuration, and does not synchronize its VLAN configuration based on received advertisements. However, in VTP Version 2, transparent switches do forward VTP advertisements that the switches receive out their trunk ports. VLANs can be configured on a switch in the VTP transparent mode, but the information is local to the switch (VLAN information is not propagated to other switches) and is stored in NVRAM.

There is also an “off” VTP mode in which switches behave the same as in the VTP transparent mode, except VTP advertisements are not forwarded.