Each VTP device is configured in Transparent, Client or Server mode (except VTP v3 which has an extra mode – ).
The purpose of VTP is copy the contents of the VLAN Database to neighbouring switches so that VLAN configuration in a given operational domain are synchronised – that way, all VLANs are consistently configured, VLAN names are the same and much more.
Note that VTP was especially useful when first introduced since configuring VLAN over Token Ring and FDDI interfaces was complex, and it took many years for network administrators to understand VLAN configuration.
Importantly, VTP was useful to help stabilise Spanning Tree in the early days by ensuring consistentcy, especially in large networks (well, they were large in 1999 anyway….).
A Switch in VTP Server mode will always actively participate in sending and receiving VTP and synchronising the VTP data file. A Switch in VTP Client mode will always actively participate in VTP data file synchronisation. A switch in VTP v1 Transparent Mode will not send, receive VTP data or participate in file synchronisation.
A switch in VTP v2 Transparent Mode will send, receive VTP data, but doesn’t participate in VTP file synchronisation.
The only configuration that DOES NOT pass VTP packets is a switch configured in VTPv1 Transparent Mode and VTPv3 in off mode. VTPv3 has four modes: server, client, transparent and off.
The off mode was formerly only available with CAT OS.
The configuration of off on an interface will apply to all VTP instances.