Named slave zone not updating
The NS records don't tell us which server is the primary master for a zone and which servers are slaves -- they only say who the servers are.(Globally, DNS doesn't care; as far as the actual name resolution goes, slave servers are as good as primary master servers.) What's the difference between a primary master name server and a slave name server?
With a backup copy, the slave has zone data, although it might be slightly out of date.
Two name servers are the minimum -- if you have only one name server and it goes down, no one can look up domain names.
A second name server splits the load with the first server or handles the whole load if the first server is down.
All directives and resource records should be entered on individual lines.
Comments can be placed after semicolon characters ( Sets the default Time to Live (TTL) value for the zone.