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Replication is a process through which the changes made to a replica on one domain controller are synchronized to replicas on all other domain controllers in the network. Each domain controller stores three types of replicas:
•Schema partition: This partition stores definitions and attributes of objects that can be created in the forest. Changes made in this partition are replicated to all the domain controllers in all the domains in the forest.

•Configuration partition: This partition stores the logical structure of the forest deployment. It includes the domain structure and replication topology. Changes made in this partition are replicated to all the domain controllers in all the domains in the forest.

•Domain partition: This partition stores all the objects in a domain. Changes made in this partition are replicated to all the domain controllers within the domain.
Note: Windows supports a new type of directory partition named Application directory partition. This partition is available only to the Windows 2003 (or above) domain controllers. The applications and services use this partition to store application-specific data.

Creating, modifying, moving, or deleting an object triggers a replication between domain controllers. Replications are of two types:
•Intrasite: In the intrasite (within a site) replication, the data is not compressed, as the replication mostly uses LAN connections. This saves the computer’s CPU time of processing data. In the intrasite replication, the replication partners poll each other periodically and notify each other when changes need to be replicated, and then pull the information for processing. Active Directory uses the remote procedure call (RPC) transport protocol for intrasite replication.

•Intersite: As intersite (between sites) replication uses WAN connections, a large amount of data is compressed to save bandwidth. For the same reason, the replication partners do not notify each other when changes need to be replicated. Instead, administrators configure the replication schedule to update the information. Active Directory uses the IP or SMTP protocol for intersite replication.
For intrasite replication to take place, the connection object is required. Active Directory automatically creates and deletes connection objects as and when required. Connection objects can be created manually to force replication.