Replication is key to the health and stability of an Active Directory environment. Without proper and timely replication, a domain will be unable to function effectively. There are three main elements or components that are replicated between domain controllers: the domain partition replica, the global catalog and the schema.
The domain partition replica is the Active Directory database of a domain. Each domain controller maintains a duplicate copy of its local domain partition replica. Domain controllers do not maintain copies of replicas from other domains. When an administrator makes a change to the domain, that change is replicated to all domain controllers immediately.
Each forest contains only a single global catalog. By default, the first domain controller installed into a forest is the global catalog server. The global catalog contains a partial replica of every object within each domain of the forest. The global catalog serves as a master index for the forest, which allows for easy and efficient searching for users, computers, resources and other objects. Any domain controller can be configured to act as a peer global catalog server. You should have at least two global catalog servers per domain and at least one per site. As changes are made to objects within the forest, the global catalog is updated. Once the global catalog is changed on one domain controller, it is replicated to all other domain controllers in the forest.
Tip: Keep the usage of universal groups to a minimum. If universal groups are needed, place users in global groups and global groups into universal groups. Otherwise, placing users directly into universal groups will greatly increase the size of the global catalog and lengthen the time required by each user to initially log into the network.
Every domain controller in a forest has a copy of the schema. Just as with changes to the Active Directory database (i.e., domain partition replica), any changes to the schema are replicated to all other domain controllers in the forest. Fortunately, the schema is usually static so there is little replication traffic caused by schema changes.
James Michael Stewart is a partner and researcher for ITinfopros, a technology-focused writing and training organization.