Under what conditions does a secondary name server request a zone transfer from a primary name server?

When a secondary server's refresh timer expires, it queries the primary for its SOA record and compares the serial numbers. If the primary's serial is higher, the secondary initiates an AXFR or IXFR request to synchronize its zone data. This serial-number-based comparison is the authoritative mechanism defined in RFC 1034 and RFC 1035 for detecting and replicating zone changes.

A secondary SOA serial higher than the primary's would indicate an abnormal state where the replica is somehow ahead, which does not trigger a transfer from the primary.

Restarting the primary name server service does not directly notify secondary servers to initiate a transfer - secondaries only detect changes through the serial number comparison at the next refresh interval.

A secondary server restart may prompt it to check the primary, but the zone transfer only actually occurs if the SOA serial comparison reveals the primary holds newer data.

TTL values govern how long caching resolvers retain individual resource records and are entirely separate from the zone transfer replication mechanism between authoritative servers.