On a BIND 9.x server all authoritative name servers for a zone must have the same version of zone information.

It is important to maintain the integrity of a zone file. The serial number of the SOA record is used to indicate to secondary name server that a change to the zone has occurred and a zone transfer should be performed. The serial number used in the SOA record provides the DNS administrator a method to verify the integrity of the zone file based on the serial number of the last update and ensure that all slave servers are using the correct zone file.