If preemption is enabled, a backup router with a priority that is higher than the current master “preempts” the master and becomes the master itself. The backup router preempts the master by beginning to send its own VRRP advertisements. The master router examines these advertisements and discovers that the backup router has a higher priority than itself. The master then stops sending advertisements, while the backup continues to send, thus, making itself the new master.
Preemption is useful in situations in which a lower-performance backup router becomes the master when a higher-performance router fails. In this case, a new higher-performance router can be brought online, and it automatically preempts the lower-performance backup.