This chapter introduces a major feature of Active Directory: multi-master replication. Active Directory was one of the first LDAP-based directories to offer multi-master replication. Most directories replicate data from a single master server to subordinate servers. This is how replication worked in Windows NT 4.0 as an example. Obviously, there are several problems with a single-master replication scheme, including single point of failure for updates, geographic distance from master to clients performing the updates, and less efficient replication due to single originating location of updates. Active Directory replication addresses these issues, but with a price. To get the benefit of a multi-master replication, you must first create a site topology that defines how domain controllers should replicate with each other. Especially in large environments, maintaining a site topology can be a significant amount of overhead.
This chapter looks at the basics of how sites and replication work in Active Directory. In Chapter 9, we'll describe the physical infrastructure of a network layout using sites. We'll also discuss in that chapter how the Knowledge Consistency Checker (KCC) sets up and manages the replication connections and details on how to effectively design and tailor sites, site links, and replication in Active Directory.