link
Click on the red underlined text to get to the source
... A given zone will be available from several name servers to insure its
availability in spite of host or communication link failure. By
administrative fiat, we require every zone to be available on at least
two servers, and many zones have more redundancy ...
... to resolve a particular request. This condition can be caused by a
resolver which becomes separated from the rest of the network due to a
link failure or gateway problem, or less often by coincident failure or
unavailability of all servers for a particular domain ...