Replication Working in AD
Solution 1:
Domain controllers pull changes from partner DC's by requesting the highest USN, and replicates changes for the USN it previously had for that DC and the current highest USN. Within a site, the originating DC notifies its replication partners of the changes when they occur. Between sites the replication is performed on a schedule, unless the site link has been configured to use change notifications.
https://social.technet.microsoft.com/wiki/contents/articles/4592.how-active-directory-replication-works.aspx