VLAN Trunking Protocol (VTP) is a Cisco‐proprietary protocol that runs over trunk links and synchronizes the VLAN databases of all switches in the VTP domain. A VTP domain is an administrative group; all switches within that group must have the same VTP domain name configured, or they do not synchronize databases.
VTP works by using Configuration Revision numbers and VTP advertisements:
■ All switches send out VTP advertisements every five minutes or when there is a change to the VLAN database (when a VLAN is created, deleted, or renamed).
■ VTP advertisements contain a Configuration Revision number. This number is increased by one for every VLAN change.
■ When a switch receives a VTP advertisement, it compares the Configuration Revision number against the one in its VLAN database.
■ If the new number is higher, the switch overwrites its database with the new VLAN information and forwards the information to its neighbor switches.
■ If the number is the same, the switch ignores the advertisement.
If the new number is lower, the switch replies with the more up‐to‐date information contained in its own database.