With standard VRRP , the backup router takes over only if the router goes
down. With VRRP interface tracking, if a tracked interface goes down on the
VRRP master, the priority decrement value is subtracted from the router
priority. If the master router priority becomes less than the priority on the
backup router, the backup router takes over. If the tracked interface becomes
up, the value of the priority decrement is added to the current router priority.
If the resulting priority is more than the backup router priority, the original
VRRP master resumes control.
VRRP route tracking monitors the reachability of an IP route. A tracked route
is considered up when a routing table entry exists for the route and the route
is accessible. When the tracked route is removed from the routing table, the
priority of the VRRP router will be reduced by the priority decrement value.
When the tracked route is added to the routing table, the priority will be
incremented by the same.