Vlan Trunking Protocol

Vlan Trunking Protocols - Message Types

Once the VLAN Trunking Protocol (VTP) is configured on the switches, the switches start advertising VTP information between them on their trunk ports. The main information which the switches advertise are management domain name, configuration revision number and the configured VLANs. VTP advertisements are sent as multicast frames and all neighbor devices receive the frames.

Three types of VLAN Trunking Protocol (VTP) advertisement messages are

1 Client advertisement request: A client advertisement request message is a VTP message which a client generates for VLAN information to aserver. Servers respond with both summary and subset advertisements.

A switch needs a VTP advertisement request in these situations:

• The switch has been reset.
• The VTP domain name has been changed.
• The switch has received a VTP summary advertisement with a higher configuration revision than its own.

Upon receipt of an advertisement request, a VTP device sends a summary advertisement. One or more subset advertisements follow the summary advertisement.

2 Summary advertisement: Summary advertisements are sent out every 300 seconds (5 minutes) by default or when a configuration change occurs, which is the summarized VLAN information.

By default, Catalyst switches issue summary advertisements in five-minute increments. Summary advertisements inform adjacent Catalysts of the current VTP domain name and the configuration revision number.

When the switch receives a summary advertisement packet, the switch compares the VTP domain name to its own VTP domain name. If the name is different, the switch simply ignores the packet. If the name is the same, the switch then compares the configuration revision to its own revision. If its own configuration revision is higher or equal, the packet is ignored. If it is lower, an advertisement request is sent.

3 Subset advertisement: Subset advertisements are sent when a configuration change takes place on the server switch. Subset advertisements are VLAN specific and contain details about each VLAN

When you add, delete, or change a VLAN in a Catalyst, the server Catalyst where the changes are made increments the configuration revision and issues a summary advertisement. One or several subset advertisements follow the summary advertisement. A subset advertisement contains a list of VLAN information. If there are several VLANs, more than one subset advertisement can be required in order to advertise all the VLANs.