How does a VTP client know that the VTP database has been updated?

A. The CDP protocol checks for VTP updates.
B. The VTP protocol checks the MD5 checksum for changes.
C. The VTP protocol checks the database revision number.
D. The VTP protocol pulls a full copy of the database every 60 seconds.

C. The VTP clients track changes via a 32-bit number inside of the VTP frame called the revision number. By default, every 5 minutes a summary advertisement is sent from the server. However, when an update occurs, a new VTP frame called a subset advert message is sent.