Controlling Updates via DNS SOA records

[Dave Rajnauth]

My SOA record on my primary server indicates that updates should done
once per day (24 hours) and retries should be done every minute.  I have 
noticed that the retries don't ocure every minute.  Is there a minimum
retry time? Must the retry time be greater that one minute?

How are the updates governed on secondary servers which load from my primary
server?  I guess it will use the SOA record for the domain their in.  From
what point will a secodary server start performing updates.  For example,
when will my secondary servers start performing the updates.  Will they
start from the time they are rebooted or reloaded and then 24 hours after 
that point or will they use some kind of a "master time" from relative to the
primary server. 

What is the SOA record in the local host defination (named.local) used for?
How does this record affect updates from the primary server?

Thanks for you help,
Dave Rajnauth