Got bit of situation here. Our tech's noticed that  AD1/AD2 are not replicating with AD3 (AD3 has most recent updates on every user / pwd etc). So my question is, I know how to fix the replication (DNS fix) , but I am afraid that if AD1/AD2 will replicate old stuff to AD3 and AD3 won't replicate new users/pwd changes to AD1/AD2... 

Can you give me an overview as to what is going to happen once replication is fixed? 

changes are timestamped - new data is replicated. Old data is replaced.

Hi

 Agree with Joseph,plus if the dc does not replicate long time (depends on tombstone life time),there will be possible garbage collections occurs.

Hi PushMyCar,

Thanks for your post.

Also add to Joseph, changes are timestamped when it keeps the clocks on all directory servers synchronized.

Active Directory replication does not depend on time to determine what changes need to be propagated. It relies instead on the use of update sequence numbers (USNs) that are assigned by a counter that is local to each domain controller. Because these USN counters are local, it is easy to ensure that they are reliable and never "run backward" . The second element is a timestamp.

You could refer to the articles for more details.

https://technet.microsoft.com/en-us/library/Cc961798.aspx

Best Regards,

Mary Dong