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ReplicationActive Directory replication between domain controllers is managed by the system administrator on a site-by-site basis. As domain controllers are added, a replication path must be established. This is done by the Knowledge Consistency Checker (KCC), coupled with Active Directory replication components. The KCC is a dynamic process that runs on all domain controllers to create and modify the replication topology. If a domain controller fails, the KCC automatically creates new paths to the remaining domain controllers. Manual intervention with the KCC will also force a new path. The Active Directory replaces PDCs and BDCs with multimaster replication services. Each domain controller retains a copy of the entire directory for that particular domain. As changes are made in one domain controller, the originator communicates these changes to the peer domain controllers. The directory data itself is stored in the ntds.dit file. Active Directory replication uses the Remote Procedure Call (RPC) over IP to conduct replication within a site. Replication between sites can utilize either RPC or the Simple Mail Transfer Protocol (SMTP) for data transmission. The default intersite replication protocol is RPC. Intersite and Intrasite ReplicationThere are distinct differences in internal and intersite domain controller replication. In theory, the network bandwidth within a site is sufficient to handle all network traffic associated with replication and other Active Directory activities. By the definition of a site, the network must be reliable and fast. A change notification process is initiated when modifications occur on a domain controller. The domain controller waits for a configurable period (by default, five minutes) before it forwards a message to its replication partners. During this interval, it continues to accept changes. Upon receiving a message, the partner domain controllers copy the modification from the original domain controller. In the event that no changes were noted during a configurable period (six hours, by default), a replication sequence ensures that all possible modifications are communicated. Replication within a site involves the transmission of uncompressed data. NOTE Security-related modifications are replicated within a site immediately. These changes include account and individual user lockout policies, changes to password policies, changes to computer account passwords, and modifications to the Local Security Authority (LSA). Replication between sites assumes that there are network-connectivity problems, including insufficient bandwidth, reliability, and increased cost. Therefore, the Active Directory permits the system to make decisions on the type, frequency, and timing of intersite replication. All replication objects transmitted between sites are compressed, which may reduce traffic by 10 to 25 percent, but because this is not sufficient to guarantee proper replication, the system administrator has the responsibility of scheduling intersite replication. Replication Component ObjectsWhereas the KCC represents the process elements associated with replication, the following comprise the Active Directory object components:
When it is necessary to link multiple sites, two additional objects are created to manage the replication topology.
Preventing Data Replication CollisionThe Active Directory issues a unique identifier known as the Update Sequence Number (USN), which is given to every change made to an object. This number is incrementally changed whenever the object is modified. Each property of an object is also issued a USN. A source domain regularly communicates USN sequence changes to the peer domain controller. The latest USN is then registered in each domain controller to ensure the freshness of an object's current state. The Active Directory uses a timestamp only when changes are made at approximately the same time to the same object. At this point, in order to avoid data collisions, the change with the latest timestamp will be replicated by default. In all other cases, the Active Directory disregards the timestamping process. In the next article in this series, we examine the specialized roles played by domain controllers. What type of replication does Active Directory use?Directory replication is communicated by using the RPC protocol. RPC is a communication protocol that allows developers to execute code on a local or remote system without having to develop specific code for remote execution.
What type of Active Directory replication takes place between domain controllers in the same site?Intrasite replication takes place between domain controllers within an Active Directory site. Domain controllers in a site are well connected with less than 10 milliseconds of latency. The Knowledge Consistency Checker creates a bidirectional ring of domain controllers in a site.
How replication happens in Active Directory?By default, for domain controllers that are in the same site (intra-site replication), replication occurs every 15 seconds. As soon as you change an attribute of an AD object, for instance the job title of your newly promoted systems engineer, the DC will send out the update to its replication partner.
Is AD replication push or pull?Creating Replication Topology
Active Directory replication is a one-way pull replication whereby the DC that needs updates (the target DC) gets in touch with the replication partner (the source DC).
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