In the past article we've talked about the role and functionality of VLANs and network trunks. In today's post we will talk about a feature that was invented in order to make VLAN administration much easier, VTP (VLAN Trunking Protocol). If you are a network Administrator in a mid to large network, you will probably encounter a lot of VLAN problems. Usually, big companies have a lot of configured VLANs and they tend to change them frequently. Imagine what would happen if you'd have to configure 100 switches with 30 VLANs each, the process would take a very long time. VTP is a a protocol that allows switches to propagate VLAN information throughout the network. The mechanism of VTP is quite simple, a VTP server switch is configured by Administrators and any VLAN information configured here is automatically sent to all VTP-enabled switches in the network. Other switches except the VTP server are configured as clients and they are the ones that receive VLAN information from the server. Every VLAN change (creation, modification, deletion) is notified by the VTP server. The benefits of using VTP are visible from the start: you configure VLANs from a single point of administration, VLAN information is automatically propagated without the Administrator's intervention, makes VLAN monitoring easier and all switches share the same VLAN information.
Before showing you how to configure VTP on a Cisco switch, I want to explain the main elements of the VLAN Trunking Protocol:
There are three VTP Modes:
VTP Domain: this element describes the network devices that are part of the same VLAN configuration. All switches configured with the same VTP domain will share the same VLAN information.
VTP Client: a switch that is configured to receive the VTP server's VLAN information. A VTP client will not be able to modify any VLAN configuration.
VTP Transparent: a switch that will receive and forward VLAN information but it will not participate in the VTP operation. Any VLAN configuration made on a VTP Transparent switch will not be propagated to other network devices.
VTP Messages: information exchanged between switches.
VTP Pruning: mechanism for controlling VTP traffic.
There are some steps that you need to take in order to configure VTP in a network. When you first configure a Cisco switch, it will be automatically selected as a server switch if no other server is configured as so. To verify the VTP mode of a switch , type show vtp status from the privilege exec mode: