Active Directory replication is like pull rather than push; it means that replicates pull changes from the server where the changes are affected. The Knowledge Consistency Checker (KCC) creates a replication topology of site links using the defined sites to manage traffic. Intrasite replication is frequent and automatic as a result of change notification, which triggers peers to begin a pull replication cycle. Intersite replication intervals are typically less frequent and does not use change notification by default, although this is configurable and can be made identical to intrasite replication.
Each link can have a 'cost' and the site link topology will be altered accordingly by the KCC. Replication may occur transitively through several site links on same-protocol site link bridges, if the cost is low, although KCC automatically costs a direct site-to-site link lower than transitive connections. Site-to-site replication can be configured to occur between bridgehead servers in each site, which then replicates the changes to other DCs within the site.
Replication of Active Directory uses Remote Procedure Calls (RPC) over IP (RPC/IP). Between Sites you can use SMTP for replication, but only for changes in the Schema, Configuration, or Partial Attribute Set (Global Catalog) NCs. SMTP cannot be used for replicating the default Domain partition..