Introduction to Administering AD LDS Sites and Replication

Updated: August 8, 2008

Applies To: Windows Server 2008

In Active Directory Lightweight Directory Services (AD LDS), the replication topology is the set of physical connections that is used to replicate directory updates among AD LDS instances in a configuration set. AD LDS builds the most efficient replication topology for a configuration set based on the site or sites in which members of the configuration set reside. AD LDS treats replication within a site differently than replication between sites.

One AD LDS server in each site, called the intersite topology generator, is assigned to build the topology. Together, these intersite topology generators build a least-cost spanning tree algorithm that eliminates redundant replication paths between sites. The intersite replication topology is updated regularly to respond to any changes that occur in the network. You can control intersite replication through the information that you provide when you create your site links.

AD LDS preserves bandwidth between sites by minimizing the frequency of replication and by allowing you to schedule the availability of site links for replication. By default, intersite replication across each site link occurs every 180 minutes (three hours). You can adjust this frequency to match your specific needs. Be aware that increasing this frequency increases the amount of bandwidth that is used by replication. In addition, you can schedule the availability of site links for use by replication. By default, a site link is available to carry replication traffic 24 hours a day, 7 days a week. You can limit this schedule to specific days of the week and times of day. You can, for example, schedule intersite replication so that it only occurs after normal business hours. For more information, see Configure Replication Availability On a Site Link.

The Knowledge Consistency Checker (KCC) on each AD LDS instance automatically builds the most efficient replication topology for intrasite replication, by using a bidirectional ring design. This bidirectional ring topology attempts to create at least two connections to each AD LDS instance (for fault tolerance) and no more than three hops between any two AD LDS instances (to reduce replication latency). To help prevent connections of more than three hops, the topology can include shortcut connections across the ring. The KCC updates the replication topology regularly.

Intrasite replication is optimized for speed, rather than bandwidth, because bandwidth within a site is assumed to be high-speed. Replication within a site occurs automatically on the basis of change notification. Intrasite replication begins when a directory update occurs. By default, the source AD LDS instance waits 15 seconds and then sends an update notification to its closest replication partner. If the source AD LDS instance has more than one replication partner, subsequent notifications go out by default at three-second intervals to each partner. After receiving notification of a change, a partner AD LDS instance sends a directory update request to the source AD LDS instance. The source AD LDS instance responds to the request with a replication operation. The 3-second notification interval prevents the source AD LDS instance from being overwhelmed with simultaneous update requests from its replication partners.

If no directory updates have occurred within a given time period, intrasite replication still occurs, based on a scheduled interval. By default, this scheduled interval is once per hour. For information about modifying this time period, see Configure Replication Frequency Within a Site.

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