VLAN TRUNKING PROTOCOL is designed to ease administration of a large number of switches. It manages addistions, deletions and renaming. You can only apply one VTP domain to a switch.
There are 3 versions of VTP and only two of those are actively used (V3 is CAT/OS). VTP is a method of synchronising the vlan databases of switches. The term domain is used to identify a cluster/group of switches. If the databases are to be shared then the domain name and any passwords set must match (not totally true, read below for more details).
VTP advertisements are based upon the revision number and are sent when a change is made or every 5 minutes. The advertisments are multicast frames.
A summary advertisment is sent out every 300 seconds and if a change occurs.
A subset advertisment after a configuration change. VLAN name, SAID value, type and MTU.
A request from client switch used to obtain up to date information.
Each change made to a vlan will increase the revision number. A switch will compare revision numbers when it receives an advertisement. A switch will overwrite its VTP database if the update from one of it’s peers is higher (potentially making an automatic change to the assigned vlan’s). The advertisement is forwarded onto any neighbours. If the switch receives a VTP advertisement with a lower revision it will reply with it’s advertisement to update it’s neighbour.
The roles are:
SERVER: This is the default and will allow the switch to create, delete and rename vlan’s.
CLIENT: Apparently clients cannot make changes. However, I have seen events where client switches have been able to pass on updates to server peers.
TRANSPARENT: Allows the creation, deletion and renaming vlan’s but all information remains local. This mode will forward VTP information to its peers.
The use of show vtp status identifies the version in use, the revision number and the number of vlans that are being passed around via VTP.