4.6 VLAN Trunking Protocol (VTP)
4.6.5 VTP advertisements
With VTP, each switch advertises on its trunk ports its management domain, configuration revision number, the VLANs that it knows about, and certain parameters for each known VLAN. These advertisement frames are sent to a multicast address so that all neighboring devices can receive the frames; however, the frames are not forwarded by normal bridging procedures. All devices in the same management domain learn about any new VLANs now configured in the transmitting device. A new VLAN must be created and configured on one device only in the management domain. The information is automatically learned by all the other devices in the same management domain.

Advertisements on factory-default VLANs are based on media types. User ports should not be configured as VTP trunks.

Each advertisement starts as configuration revision number 0. When changes are made, the configuration revision number increments (n + 1). The revision number in the management domain continues to increment until it reaches 2,147,483,648, at which point the counter will reset back to zero.

There are two types of advertisements:

  • Requests from clients that want information at bootup
  • Response from servers

There are three types of messages:

  • Advertisement requests - Clients request VLAN information, and the server responds with summary and subset advertisements. See Figure for examples of these.
  • Summary advertisements - By default, server and client Catalyst switches issue summary advertisements every five minutes. They inform neighbor switches what they believe to be the current VTP revision number. Assuming the domain names match, the receiving server or client compares the configuration revision number. If the revision number in the advertisement is higher than the current revision number in the receiving switch, the receiving switch then issues an advertisement request for new VLAN information. See Figure for an example.
  • Subset advertisements - These contain detailed information about VLANs such as VTP version type, domain name and related fields, and the configuration revision number. Creating or deleting a VLAN, suspending or activating a VLAN, changing the name of a VLAN, and changing the maximum transmission unit (MTU) of a VLAN can trigger these advertisements. See Figure for an example.

Advertisements may contain some or all of the following information:

  • Management domain name - Advertisements with different names are ignored.
  • Configuration revision number - The higher number indicates a more recent configuration.
  • Message Digest 5 (MD5) - MD5 is the key that is sent with the VTP when a password has been assigned. If the key does not match, the update is ignored.
  • Updater identity - The updater identity is the identity of the switch that is sending the VTP summary advertisement.