In a previous article titled Active Directory Replication In Depth, Brian Desmond discussed the specifics of Active Directory (AD) replication technology with regard to how objects and attributes are actually kept in sync and how changes to them are tracked. This article discusses how AD decides which domain controllers (DCs) should replicate with one another. 

AD includes a very efficient background process known as the Knowledge Consistency Checker. The KCC is responsible for consuming information that administrators provide to AD in the form of subnets, sites, site links, and site link bridges to determine the best overall topology of connections between DCs. These connections are represented by connection objects, which the KCC automatically adds and removes as necessary. Your sites, site links, and site link bridges typically map closely to your network topology to form what’s called a site topology.