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Replication between sites

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Replication between sites

Active Directory handles replication between sites, or intersite replication, differently than replication within sites because bandwidth between sites is usually limited. The Active Directory Knowledge Consistency Checker (KCC) builds the intersite replication topology using a least-cost spanning tree design. Intersite replication is optimized for bandwidth efficiency, and directory updates between sites occur automatically based on a configurable schedule. Directory updates replicated between sites are compressed to preserve bandwidth.

For information about intrasite replication, see Replication within a site.

For information about site design, see "Designing the Site Topology" at the Microsoft Windows Resource Kits Web site.

Building the intersite replication topology

Active Directory automatically builds the most efficient intersite replication topology using information you provide (through Active Directory Sites and Services) about your site connections. The directory stores this information as site link objects. One domain controller per site, called the intersite topology generator, is assigned to build the topology. A least-cost spanning tree algorithm is used to eliminate 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 you provide when you create your site links. For more information, see Managing replication.

Determining when intersite replication occurs

Active Directory 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 (3 hours). You can adjust this frequency to match your specific needs. Be aware that increasing this frequency increases the amount of bandwidth 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 site link replication frequency and Configure site link replication availability.


  • With certain restrictions, you can use the Simple Mail Transfer Protocol (SMTP) for replicating to sites that do not have a direct or reliable Internet Protocol (IP) connection. For more information, see "Active Directory Replication" at the Microsoft Windows Resource Kits Web site.

  • Intersite replication through a firewall or virtual private network requires some special considerations. For more information, see Active Directory at the Microsoft Web site.