Propagating VLAN Information with VTP
Describing VTP operation

Switches within a VTP management domain synchronize their VLAN databases by sending and receiving VTP advertisements over trunk links. VTP advertisements are flooded throughout a management domain by switches running in specific modes of operation. Advertisements are sent every five minutes or whenever there is a change in VLAN configuration. VTP advertisements are transmitted over VLAN1 using a Layer 2 multicast frame. VLAN advertisements are not propagated from a switch until a management domain name is specified or learned.

The process of VLAN synchronization over VTP follows this general order:

Step 1 Configure the VTP domain, VTP mode, and VTP password (optional) on each switch. This proactively determines which switches will send updates.
Step 2 Switches running VTP server mode then send VTP updates across trunk links.
Step 3 A device that receives a VTP advertisement will check that the VTP management domain name and password in the advertisement match those configured in the local switch.
Step 4 If a match is found, a switch further inspects the VTP update to see the configuration revision number.
Step 5 If the configuration revision number of the message is greater than the number currently in use and the switch is running in VTP server or client mode, the switch overwrites its current VLAN information with that in the received update.
Step 6 The switch may also request more information.

Configuration Revision Number

One of the most critical components of VTP is the configuration revision number. When initially configured, the VTP configuration revision number is set to 0. Each time a VTP server modifies its VLAN information, it increments the VTP configuration revision number by one. It then sends out a VTP advertisement referencing the new configuration revision number. If the configuration revision number being advertised is higher than the number stored on other switches in the VTP domain, they will overwrite their VLAN configurations with the new information.

CAUTION:

Given this overwrite process, if the VTP server sending the advertisement had deleted all VLANs and sent an advertisement with a higher revision number, the receiving devices in the VTP domain will delete those VLANs as well.

VTP Advertisement Types
There are three types of VTP advertisements exchanged between switches:

  • Summary Advertisements – an update sent by VTP servers every 300 seconds or when a VLAN database change occurs. Among other things, this advertisement lists the management domain, VTP version, domain name, configuration revision number, timestamp, and number of subset advertisements. When the advertisement results from a VLAN database change, one or more subset advertisements will follow.
  • Subset Advertisements – an update that follows a summary advertisement that results from a change in the VLAN database. A subset advertisement cites the specific change that was made to a specific VLAN entry in the VLAN database. One subset advertisement will be sent for each VLAN ID that encountered a change.
  • Advertisement Requests from Clients – an update sent by a switch requesting information in order to update its VLAN database. If a client hears a VTP summary advertisement with a configuration revision number higher than its own, the switch may send an Advertisement Request. A switch operating in VTP server mode, then responds with summary and subset advertisements.
NOTE:

VTP advertisements are associated with VLAN database information only, not VLAN information configured on specific switch ports. Likewise, on a receiving switch, the receipt of new VLAN information does not change the VLAN associations of trunk or access ports on that switch.

VTP Versions
Currently Catalyst switches run VTP versions 1, 2, or 3. Version 2 is the most prevalent, and provides these features:

  • Forwarding of VTP updates from transparent mode switches without checking the version number
  • Consistency checks on new VTP and VLAN configuration parameters
  • Support for Token Ring Switches
  • Propagation of VTP updates that have an unrecognized type, length or value

VTP version 3 is available on some switches now using the Cat OS operating system version.


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