VLAN Trunk Protocol (VTP) is a Cisco-proprietary protocol used to replicate VLANs using Layer 2 trunk frames to other switches. It is a method to ease administration by selecting a switch to be configured as a VTP server.
VLANs are configured on the VTP server and then replicated to other connected switches, called VTP clients. This reduces the amount of configuration — having to log onto each switch to create a VLAN. All VLAN and VTP data is saved in the vlan.dat file – stored in the switch’s flash memory.
VTP is organized into management domains, which share common VLAN requirements. These management domains are called VTP Domains. A switch can only be joined to one VTP domain. Each VTP domain is isolated from one another. Information is not shared between VTP domains.
Switches can be configured in one of three VTP modes:
Server Mode – switch has full control over creating and modifying VLANs. This is the default mode for a switch.
Client Mode – switch cannot create, modify, or delete any VLANs. Switches listen to VTP Advertisements.
Transparent Mode – switch doesn’t participate in VTP. A switch in Transparent Mode will forward VTP version 2 VTP Advertisements out it’s trunk ports — acting as a relay — if it receives VTP Advertisements.