Suppose a domain [dom.net] has the following:
- one primary NS [PNS]
- one secondary NS [SNS1]
- one seconday NS [CNS2] loads from SNS1
- one caching-only NS [CNS]
The SOA records for:
PNS:
21600 ; refresh - 6 hours
3600 ; retry - 1 hour
2592000 ; expire - 1 month
345600 ) ; TTL - 4 days