http://www.dummies.com/WileyCDA/Dummies … USING.html You can provide DNS redundancy in two ways: * Master/slave: In the traditional master/slave DNS relationship, (one or more) DNS slave servers load zone data from the master server on startup and at intervals specified in the start of authority (SOA) record for each zone. This method of redundancy has one huge advantage: When a zone file is changed, the changes are automatically propagated to the slave servers. This process normally happens as soon as the changes are made if the NOTIFY DNS feature is supported, and it happens after the time interval in the SOA record if NOTIFY is not supported. The master/slave DNS server relationship has a disadvantage also: If the master goes down, the slave is restarted, and the zone data cannot be transferred. Also, if the master goes down and isn’t restored by the time the DNS record becomes stale (because it cannot update from the master server), the zone is no longer accessible.]]>