2nd DC w/DNS didn't authorize users

My domain servers are Windows 2003 R2 & both are a DC with DNS/WINS

There was a problem recently where my main DNS server was inaccessible to process logins. The "secondary" DNS server did not recognize this and begin offering authentication for logon requests to the domain. Later, the main DNS server was able to begin responding after some troubleshooting.

Is it my imagination that it should have automatically recognized this? What would be something to check/test to determine what might be missing?

If this should have worked, then does that mean my main DNS is being overloaded by handling the requests? Both can communicate with each other & there are forwarders present? This is a bit new to me, but I need to try and understand how to avoid this in the future.

Thanks!!

 

Could have been an authentication issue between the two DNS servers? Or perhaps a Replication issue?