How Active Directory determins what DNS to update
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1) AD uses the DNS servers listed in their TCP/IP settings to send
a query to.
	
2) The query is to find out the authoritative server in the SOA for
the zone they need to update.

3) The DNS server that is queried will return the SOA information
for that zone.  In the SOA record, is the server that is listed as
being authoritative for that zone.  There can be many different
servers authoritative for a zone (primary and secondary), but the
DC is looking for the DNS server that is specifically listed in the
SOA record.

4) The DC will then send the dynamic updates to that DNS server in
the SOA.

* Now, the case of AD, they use the multi-master scenario.  So,
whatever server is updated with the SRV records, that information
will be replicated to the other servers via DNS zone transfers.