VTP Facts

VTP Facts

The VLAN Trunking Protocol (VTP) simplifies VLAN configuration on a multi-switch network by propagating configuration changes to other switches. With the VTP, switches are placed in one of the following three configuration modes.

Mode	Characteristics
Server	A switch in server mode is used to modify the VLAN configuration. On a server: Changes can be made to the VLAN configuration on the switch. The switch advertises VTP information to other switches in the domain. The switch updates its VLAN configuration from other switches in the domain. The switch saves the VLAN configuration in NVRAM.
Client	A switch in client mode receives changes from a VTP server and passes VTP information to other switches. On a client: Changes cannot be made to the VLAN configuration. The switch advertises VTP information to other switches in the domain. The switch updates its VLAN configuration from other switches in the domain. The switch does not save the VLAN configuration in NVRAM.
Transparent	A switch in transparent mode allows for local configuration of VLANs, but does not update its configuration based on the configuration of other switches. On a transparent switch: Changes can be made to the VLAN configuration on the switch. Local VLAN information is not advertised to other switches. VTP information received from other switches is passed through the switch. Note: The transparent switch only relays VTP information if it is in the same VTP domain or if it has a null (blank) VTP domain. The switch does not update its VLAN configuration from other switches in the domain. The switch saves its VLAN configuration in NVRAM.

Keep in mind the following facts about VTP:

• By default, switches are preconfigured in server mode. If you do not intend to use VTP, configure each switch to use transparent mode.
• You can have multiple VTP servers in the same domain on the network. Changes made to any server are propagated to other client and server switches.
• To make VLAN changes on a switch, the switch must be in either server or transparent mode. You cannot modify the VLAN configuration if:
  • The switch is in client mode
  • The switch is in server mode and without a configured domain name.
• VTP uses the following process for communicating updates:
  • VTP summary advertisement packets contain the domain name, MD5 version of the password, and the revision number.
  • When a switch receives a summary packet, it compares the domain name and password in the packet with its own values. If the domain name and password do not match, the packet is dropped.
  • If the domain name and password match, the switch compares the revision number in the packet.
  • If the revision number in the packet is lower or equal, the packet is ignored. If it is higher, the switch sends an advertisement request for the latest updates.
  • When the updates are received, the VLAN configuration and the revision number is updated.
• If you lose your only VTP server, the easiest way to recover is to change one of the VTP clients to server mode. VLAN information and revision numbers remain the same.
• Switches must meet the following conditions before VTP information can be exchanged:
  • The switches must be connected by a trunk link. VTP is not used on access ports.
  • Switches must be in the same domain. Switches in different domains do not share or forward VTP information. Transparent switches must be in the same domain or have a null domain name to pass VTP information to other switches.
  • Passwords on each device must match. The password is included in each VTP advertisement. The receiving switch compares the password in the advertisement with its configured password. It will only accept information in the packet if the passwords match. The password provides a method of authenticating the packet contents that they came from a trusted source.
• Connecting two switches with different VTP domains works only if you manually turn trunking on. VTP information is carried in DTP packets, so only switches in the same domain can use DTP for automatic trunking configuration. However, when two switches with different domains are connected, VTP information will not be passed between the switches.
• When you change the VLAN configuration on a server, the revision number is incremented. The revision number on a transparent switch remains at 0, even when changes are made to the VLAN configuration.
• All devices in the domain must use the same VTP version. By default, VTP version 2 is disabled. Only enable VTP version 2 if all devices support version 2.