VTP Advertisements

VTP Advertisements

Every Cisco switch advertises (as multicast frames) VLAN information (VLANs 1-1005), revision number and VLAN parameters out its trunk ports to notify other switches in the domain using VTP (VLAN Trunking Protocol). A VTP frame received over a trunk port by a switch is forwarded out trunk ports to other switches.

A VTP switch in Server mode is configured with VLAN information and all other VTP switches in Client mode in the domain receive this VLAN information using VTP.

By default, VTP domain is non-secure but a same VTP password can be configured on all VTP switches so that advertisements are sent to only those VTP switches.

VTP uses configuration revision number to keep track of the most recent information. The VTP Server sends the first VTP advertisement with configuration revision number set to 0. When changes are made on a VTP server, the revision number is incremented before a VTP Advertisement is sent. The listening switches receive VTP Advertisement and if the revision number is higher than the one locally stored, the advertisement overwrites any stored VLAN information.

Note

Even though the switch in Client mode only listens to VTP Advertisement from the switch in Server mode, the Client sends its own VTP Advertisement. When the Client powers up, it sends a summary advertisement and if it receives an inferior advertisement, it sends a subset advertisement with its higher revision number and VLAN information and Server will accept it due to higher revision number.

Tip

To cause a switch to reset the configuration revision number to 0, there are 2 options-

1. Change the mode to Transparent then change back to Server.
2. Change the VTP domain name to something irrelevant and then change back to the original name

VTP Advertisement Types

There are 3 types of VTP Advertisements-

1. Summary Advertisement

VTP Servers send summary advertisements every 300 seconds and every time a VLAN database change occurs.This advertisement carries information like VTP version, domain name, revision number, number of subset advertisements to follow, time-stamp, etc. If VLAN configuration changes, one or more subset advertisements follow the summary advertisement.

2. Subset Advertisement

VTP Servers send subset advertisements after a VLAN configuration change occurs. These advertisements contain the specific changes that have been performed, like add or delete of a VLAN, change of name of a VLAN, status of a VLAN, etc.

3. Advertisement Requests from Clients

A VTP Client may request any VLAN information. This could be due to the Client switch might be reset and has lost VLAN information or it might hear a higher revision number than its locally stored. The Server switch responds to these Advertisement Requests with a summary advertisement followed by a subset advertisement to bring the Client up-to-date.