Active Directory Back to Basics – Replication

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Active Directory is a very scalable, very reliable directory service. Its multi-master approach means makes it suitable for both large and small scale organisations and provides reliability whilst also removing single points of failure. This is vitally important when you consider that Active Directory becomes the foundations for many other services, including Microsoft Exchange Server. 

To achieve this highly available robust platform we need to ensure that our Active Directory design contains at leaf two Domain Controllers. To ensure these domain controllers can work together, and provide all the AD services they need to be synchronised so they contain the same sets of information. This is achieved using Active Directory Replication. 

This article will help you understand how Active Directory Replication works. 

The foundations

Each naming context (configuration, schema, domain) in Active Directory has its own replication metadata which contains various bits of information used to perform replication with another domain controller. Each domain controller users update sequence numbers, USN, this is a 64bit number which is only applicable to that domain controller. When ever a change is made to an object that domain controller will update its USN by 1. Each DC also keeps track of the highest committed USN value and can be viewed through the LDP tool connected to the RootDSE.