What is Active Directory Replication?

Windows 2003 Active Directory has a distributed directory structure and stores objects (users, computers, printers etc). Objects which are stored in Active Directory are distributed different domain controllers in a forest. Active Directory replication is the process by which the changes that originate on one domain controller are automatically transferred to other domain controllers in the forest.

Replication is a necessary factor in Active Directory to ensure

Fault tolerance: If one domain controller fails, the Active Directory database is still available from other domain controllers, which store the same information.

Load balancing: When many workstations are accessing Active Directory, the information they are requesting is retrieved faster when there is more than one domain controller to provide it.

Proximity of information: Workstations get the information from a local domain controller instead of across a slow WAN link.

Replication process ensures that changes made to a replica on one domain controller are synchronized to replicas on all other domain controllers within the domain.

The following actions trigger replication between domain controllers:

• Creating an object (When adding a user or a computer)
• Modifying an object
• Moving an object
• Deleting an object

You can use several different methods to force replication.

• Using the Active Directory Sites and Services MMC snap-in (Dssite.msc)
• Using Repadmin
• Using Replmon
• Using a script

There are two types of Active Directory Replication.

1) Intrasite (Replication within a site)
2) Intersite (Replication between sites).

The following table compares Intrasite and Intersite replication.

Function	Intrasite Replication	Intersite Replication
Compression	To save CPU time, replication data is not compressed.	To save WAN bandwidth, replication data greater than 50 kilobytes (KB) is compressed.
Replication model	To reduce replication latency, replication partners notify each other when changes need to be replicated and then pull the information for processing.	To save WAN bandwidth, replication partners do not notify each other when changes need to be replicated.
Replication frequency	Replication partners poll each other periodically.	Replication partners poll each other at specified intervals, only during scheduled periods. If updates are necessary, operations are scheduled to pull the information for processing.
Transport protocols	Remote procedure call (RPC).	IP or Simple Mail Transport Protocol (SMTP).