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

The SOA record contains a serial number that identifies the version of the zone. When a secondary name server queries the primary and finds the primary's SOA serial number is higher than its own, it indicates that changes have been made to the zone, prompting the secondary to request a zone transfer to synchronize the data.

If a secondary's SOA serial number is higher than the primary's, it implies the secondary might have newer data, which is an anomalous situation in a typical primary-secondary setup and would not trigger a transfer from the primary.

While a primary name server restart might be followed by zone changes, the direct condition that triggers a secondary to request a transfer is the serial number difference in the SOA record, not the restart itself.

When a secondary name server restarts, it will check the primary's SOA record to determine if a zone transfer is needed, but the actual condition for the request is the primary having a higher SOA serial number, not just the restart.

The TTL (Time To Live) value applies to individual resource records and dictates how long clients should cache a record, it does not directly trigger zone transfers between DNS servers.