9.2 VTP  
  9.2.4 VTP implementation  
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 configured in the transmitting device. A new VLAN must be created and configured on one device only in the management domain. All the other devices in the same management domain automatically learn the information.

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. As changes are made the configuration revision number is increased incrementally by one, (n + 1). The revision number continues to increment until it reaches 2,147,483,648. When it reaches that point, the counter will reset back to zero.

There are two types of VTP advertisements:

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

There are three types of VTP messages:

  • Advertisement requests
  • Summary advertisements
  • Subset advertisements

With advertisement requests, clients request VLAN information and the server responds with summary and subset advertisements.

By default, server and client Catalyst switches issue summary advertisements every five minutes. Servers 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.

Subset advertisements contain detailed information about VLANs such as VTP version type, domain name and related fields, and the configuration revision number. The following can trigger these advertisements:

  • Creating or deleting a VLAN
  • Suspending or activating a VLAN
  • Changing the name of a VLAN
  • Changing the maximum transmission unit (MTU) of a VLAN

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.

 

Web Links

Understanding and Configuring VLAN Trunk Protocol (VTP)

http://www.cisco.com/en/US/tech/
tk389/tk689/technologies_tech_note
09186a0080094c52.shtml