To use VTP, you must configure a VTP domain. You'll also need to know how to set the mode on your switches. Further configuration options include setting VTP passwords and configuring VTP pruning.
The default VTP domain is null. Bear this in mind when implementing VTP because trunks negotiated using the null VTP domain will break if you assign a different domain to one side.
This behavior differs from switch to switch. For example, the Catalyst 5000 will not negotiate trunks unless a VTP domain has been set for each switch.
On some switches, such as the Cisco 6500, the null domain will be overwritten if a VTP advertisement is received over a trunk link, and the switch will inherit the VTP domain from the advertisement. (If a VTP domain has been previously configured, this will not occur.)
Note also that once you've changed a switch's VTP domain to something other than null, there is no way to change it back to null short of erasing the configuration and rebooting.
Setting or changing the VTP domain in IOS is done with the
vtp domain command:
vtp domain GAD-LabChanging VTP domain name from NULL to GAD-Lab 3550-IOS(config)# 1w4d: %DTP-5-DOMAINMISMATCH: Unable to perform trunk negotiation on port Fa0/20 because of VTP domain mismatch.
In this case, changing the domain has resulted in a VTP domain mismatch that will prevent trunk negotiation from occurring on port Fa0/20.
You can set or change the VTP domain on CatOS with the
set vtp domain ...