Important SOA Parameters (2/2)

Serial number
specifies whether zone transfer is necessary
a popular cause of DNS trouble
do not forget updating it when modifying a zone file

"Minimum" TTL
previously used for indicating the "default" TTL
tend to be large (e.g., 1-2 days)
RFC 2308 update: TTL for negative caching
recommended value: 10-30 minutes
revisit your configuration
stick to the RFC 2308 definition
use "$TTL" for the default TTL