Updated: January 21, 2005
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
Active Directory relies on site configuration information to manage and optimize the process of replication. Active Directory provides automatic configuration of these settings in some cases. In addition, you can configure site-related information for your network using Active Directory Sites and Services. Configurable information includes settings for site links, site link bridges, and bridgehead servers.
Configuring site links
You can use site link settings to control replication between sites. Configurable settings include the relative cost of each site link, the frequency of replication on each site link, and the schedule availability of each site link for replication. For information about site links, see Replication between sites.
Site link cost
The cost of a site link determines the relative preference of the Active Directory Knowledge Consistency Checker (KCC) for using a site link in the replication topology. The higher the cost of the site link, the lower will be the KCC's preference for using the site link. For example, if you have two site links, site link A and site link B, and you set the cost of site link A to 150 and the cost of site link B to 200, the KCC will prefer to use site link A in the replication topology. By default, the cost of a newly created site link is 100. For information about setting site link cost, see Configure site link cost. For information about the KCC, see Replication overview.
The replication frequency of a site link determines how often replication occurs over that site link. By default, the replication frequency for a site link is 180 minutes, meaning that replication occurs over that site link every 180 minutes, or three hours. Using Active Directory Sites and Services, you can set the replication frequency from 15 minutes to 10,080 minutes (one week). A site link must be available for any replication to occur. If a site link is not available when the number of minutes between replication updates has passed, no replication will occur. For more information, see Configure site link replication frequency.
Site link availability
The availability schedule for a site link determines during which hours or days of the week a site link can be used for replication. By default, a site link is always available for replication, 24 hours a day and 7 days a week. You can change this schedule, for example, to exclude business hours during which your network is busy handling other types of traffic. Or, you can exclude particular days on which you do not want replication to occur. Scheduling information is ignored by site links that use the Simple Mail Transfer Protocol (SMTP) for replication. For more information, see Configure site link replication availability.
Configuring site link bridges
By default, all site links are bridged, or transitive. This allows any two sites that are not connected by an explicit site link to communicate directly, through a chain of intermediary site links and sites. One advantage to bridging all site links is that your network is easier to maintain because you do not need to create a site link to describe every possible path between pairs of sites.
Generally, you can leave automatic site link bridging enabled. However, you might want to disable automatic site link bridging and create site link bridges manually just for specific site links, in the following cases:
Your network is not fully routed (not every domain controller can directly communicate with every other domain controller).
You have a network routing or security policy in place that prevents every domain controller from being able to directly communicate with every other domain controller.
Your Active Directory design includes a large number of sites. For more information, see the Active Directory Branch Office Planning Guide at the Microsoft Web site.
For more information about site link bridges and their affects on replication, see "Active Directory Replication" at the Microsoft Windows Resource Kits Web site.
For information about disabling automatic site link bridging, see Enable or disable site link bridges.
For information about creating a site link bridge manually, see Create a site link bridge.
Configuring preferred bridgehead servers
When the KCC constructs the intersite replication topology, it automatically assigns one or more bridgehead servers for each site to ensure that directory changes only need to be replicated across a site link one time. It is recommended that you allow the KCC to make the bridgehead server assignments. You can make the bridgehead server assignments manually through Active Directory Sites and Services. However, doing so can potentially disrupt replication if one of your manually assigned bridgehead servers becomes unavailable. For more information, see "Active Directory Replication" at the Microsoft Windows Resource Kits Web site.
For information about manually configuring a bridgehead server, see Designate a preferred bridgehead server.