When is the next hop of a route considered ``reachable''?

When a static route is added, or during an important event (eg:
interface up/down transition), the next hop for a route is looked up
from the routing table (i.e. recursive routing).

As a consequence, if a route which is depended upon for evaluation
of the next hop of a static route goes away, a mechanism is required
to remove that (now-invalid) static route.

Scanning all static routes each time the routing table changes is
too expensive, so instead, a period timer is used. One a minute, static
routes are added and removed from the routing table based on the routes
they depend upon.

It should be noted that a particular static route will be reevaluated
when its interface transitions up or down.