Critical service. Defining a critical service greatly reduces the traffic
generated by the poller when an interface is DOWN. When an interface is
DOWN only the critical service is polled. If and when the critical service
comes back UP then the interface's other services are polled to determine
their status. When an interface is UP all services are polled as expected.
If the critical service goes DOWN, all services are considered to be DOWN
and therefore the interface is also considered DOWN.