VTP communication between switches is done through three different advertisements:
- Summary Advertisements
- Subset Advertisements
- Advertisement Requests
In 5 minute intervals, a VTP server will send a Summary Advertisement to it’s adjacent switches. Included in this VTP Summary Advertisement is the VTP domain name and configuration revision number.
Another VTP Server or Client receives the Summary Advertisement and compares the VTP domain name and revision number to its own. If the domain name is different, it ignores the advertisement. If its own configuration revision number is higher than the request, it ignores the advertisement. If the configuration revision number is higher than its own, it sends an advertisement request. Following Summary Advertisements are Subset Advertisements.
Changes made to VLANs will increment the configuration revision number and issue a Summary Advertisement followed by one or more subset advertisements. The Subset Advertisement is what holds the VLAN information.
When a switch needs VTP information it sends an Advertisement Request. This request is sent if the switch has been reset, VTP domain name has changed, or it received a VTP Summary Advertisement with a higher configuration revision. Following the Summary Advertisement are Subset Advertisements.