Vlan Trunking Protocol Explained

VTP Versions
There are three versions of the VTP protocol: 1,2, and 3. Version 1 and 2 is the most commonly used. Even after Version 3 was released, it was not originally supported on core/distribution class switches(chassis based) running in Native (IOS Only) mode. Version 3 was first supported on this class of switches running CatOS 8.1(1) or later.

Transport Mechanism
VTP packets are sent in either Inter-Switch Link (ISL) frames or in IEEE 802.1Q (dot1q) frames. This means in order to use VTP, there must be an active trunk between switches. A standard access port (switchport mode access) will not pass VTP packets.

Revision Number
The configuration revision number is a 32-bit number that indicates the level of revision for a VTP packet. This information is used in order to determine whether the received information is more recent than the current version. Each time that you make a VLAN change in a VTP device, the configuration revision is incremented by one.

To Reset VTP Configuration Revision Number:
In order to reset the configuration revision of a switch, change the VTP domain name, and then change the name back to the original name.

Advertisement Summaries
By default, Catalyst switches issue summary advertisements in five-minute increments.

Handling of VTP packets received
When the switch receives a summary advertisement packet, the switch compares the VTP domain name to its own. 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. If its own configuration revision is higher or equal, the packet is ignored. If it is lower, an advertisement request is sent. Upon receipt of an advertisement request, a VTP device sends a summary advertisement.

Modes
Server – In VTP server mode, you can create, modify, and delete VLANs and specify other configuration parameters, such as VTP version and VTP pruning, for the entire VTP domain. VTP servers advertise their VLAN configuration to other switches in the same VTP domain and synchronize their VLAN configuration with other switches based on advertisements received over trunk links. VTP server is the default mode.

Client – VTP clients behave the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client.

Transparent – VTP transparent switches do not participate in VTP. A VTP transparent switch does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements, but transparent switches do forward VTP advertisements that they receive out their trunk ports in VTP Version 2.