Chapter 10. The Physical Topology: Sites and Replication

It is easy to focus exclusively on the logical Active Directory design. After all, the logical design is the new and challenging aspect of Active Directory, and so we tend to spend a lot of time getting it right. However, the physical design of Active Directory is also important, and it requires close consideration to ensure that Active Directory is custom fit to the network that supports it.

Active Directory services are provided to clients and applications through Active Directory Domain Controllers (DCs). Each Active Directory DC hosts at least three different database partitions (also known as naming contexts as mentioned in Chapter 7, "Designing the Windows 2000 Domain Structure" ) that make up these services. These partitions include the domain partition, the configuration partition, and the schema partition. Each DC in an Active Directory Forest has a complete copy of each partition with the domain partition belonging to the DCs domain. Each DC can read and write to the domain partition, making Active Directory a multi-master directory. This means that each DC can write changes to its domain database. These changes then have to be replicated to all other DCs in the domain so that they too reflect that change and can provide consistent information to the client computers and users.

Replication is a tricky business. The more changes made to the directory, the more it is necessary to replicate. The more DCs replicate, the more network bandwidth is consumed with replication. Replication intervals can be configured. For example, if 100 changes are made to a DC in an hour, you can replicate those 100 changes incrementally throughout that hour or all at the end of the hour. Either way, all 100 changes to the directory are going to replicate, and they will consume the same amount of network bandwidth. To help mitigate replication traffic, it is important to consider domain and site design. The physical design is built with sites and site link connectors. Sites, and how they are connected, help to determine the performance, and possibly the reliability of your Windows 2000 Active Directory. This chapter looks closely at Active Directory sites and the Active Directory replication that occurs within them.