When I explained about the replication, I mentioned Active Directory automatically creates replication links, select bridgehead servers…, and so on. But how does it really do it? KCC is the one responsible for all these. The KCC is a built-in service in Active Directory domain controllers, and is responsible for generating and maintaining the replication topology for intra- and inter-site replications. Every 15 minutes, KCC will revalidate its existing replication topology and make the topology changes if required. It gives enough time for domain controllers to replicate the changes if the existing replication topology is valid.

When it comes to inter-site, KCC selects a single KCC holder in a remote site to act as intersite topology generator (ISTG), and its ISTG's responsibility is to select the bridgehead servers for replication. ISTG creates the view of replication topology for all the sites it is connected to. ISTG is responsible for deciding topology for the site; and individual domain controllers (KCC) are responsible for making topology decisions locally.

The best way to understand KCC is to compare it with a network routing protocol. A network routing protocol is responsible for maintaining a routing path for connected networks. If network A needs to communicate with network B, routing table will tell it what path to go to. In the same way, the topology created by KCC will tell how the domain controller A can replicate the changes in the domain controller B. When I work on AD projects, I have seen engineers create manual replication links between domain controllers. But I really doubt if someone can be smarter than KCC on deciding replication topology.