Expire: How long a secondary will still treat its copy of the zone
data as valid if it can't contact the primary. This value
should be greater than how long a major outage would typically
last, and must be greater than the minimum and retry
intervals, to avoid having a secondary expire the data before
it gets a chance to get a new copy. After a zone is expired a
secondary will still continue to try to contact the primary,
but it will no longer provide nameservice for the zone. 2-4
weeks are suggested values.