Zone transfer (AXFR)

Synchronize primary and secondary
Copy the entire zone file from primary to secondary
Use TCP as transport
(the transfered data is large)

When does it take place?
primary server notifies secondary of the need for transfer by 'notify' message
periodically happens based on SOA parameters
SOA serial matters
AXFR happens only when the primary has a larger serial
do not forget update SOA serial at the primary