Active Directory Sites
Active Directory uses sites to map a organizations physical network.A site is a collection of well-connected subnets.A simple method of transfering updates and other changes to Active Directory is to let all servers communicate directly with each other,this is not ideal because it places high requirements on network bandwidth and is ineffective in network environments that uses slower WAN links.
By using sites Active Directory can automatically determine the best method for performing replicaton.Sites take into account the organizations network infrastructure and Active Directory uses sites to determine the most efficent method for synchronizing information between domain controllers.
Whenever a change is made it is given an update sequence number,the domain controller can then propagate the changes to the other domain controllers based on the replication settings. Windows Server 2016 uses a feature called linked value replication that is only active when the domain is in Windows Server 2003,2008,2008 R2,2012,2012 R2 or 2016 domain fuctional level.