Intrasite replication occurs between domain controllers within a site. For intrasite replication, be aware of the following:

  • By default, replication occurs once every hour.
  • To modify the replication frequency, edit the NTDS Settings for the site. For each hour, you can configure the following options for the replication frequency:
    • None (replication does not take place)
    • Once per hour
    • Twice per hour
    • Four times per hour
  • Bridgehead servers, site links, or site link bridging are not used. Connections are created automatically as necessary.

Intersite replication occurs between bridgehead servers between sites. The following table describes configuration steps you can take when managing intersite replication.

Configuration Description
Preferred bridgehead server A preferred bridgehead server is a domain controller in a site that has been designated as a potential bridgehead server.
  • To designate preferred bridgehead servers, edit the server properties and add the transport protocol to the preferred bridgehead server list.
  • The preferred bridgehead server should be a global catalog server.
  • You can designate more than one server as a preferred bridgehead server. If multiple servers in a site are preferred bridgehead servers, the replication process automatically selects one of the servers during replication.
  • When at least one preferred bridgehead server exists in a site, replication will only use preferred servers for intersite replication; non-preferred servers will never be used. This means that:
    • To prevent a specific server from being used for intersite replication, configure one or more preferred bridgehead servers.
    • If all bridgehead servers in a site are unavailable, intersite replication will not occur. For this reason, you should assign more than one preferred bridgehead server.
  • If no preferred bridgehead servers are designated, the system chooses which server to use for the bridgehead server from the list of servers in the site that are enabled for the transport protocol.

Note: In Windows Server 2008 R2, load-balancing was introduced to distribute the workload evenly among bridgehead servers. The Windows Server 2008 R2 forest or domain functional level is not required for the load balancing feature, only Windows Server 2008 R2 domain controllers.

Replication schedule The replication schedule identifies the hours of the day when replication is possible.
  • To edit the replication schedule for intersite replication, edit the properties of the site link and click the Change Schedule... button.
  • The schedule identifies which days and hours of the day that replication is allowed.
  • By blocking replication, you give priority to other traffic, but you also increase replication latency.
  • Domain controllers store time in Coordinated Universal Time (UTC). Time settings in site link object schedules conform to the local time of the site and computer on which the schedule is set. When a domain controller contacts a computer that is in a different site and time zone, the schedule on the domain controller displays the time setting according to the local time for the site of the computer.  
  • It is best to synchronize your SMTP site link replication schedule with the times your network's SMTP connections are available. Do not configure site link replication availability on SMTP site links unless the following is true:
    • Scheduled connections are used by the site links.
    • The SMTP queue is not on a schedule.
    • Information is being exchanged directly from one server to another. This does not include exchanges that use intermediaries such as a network Ethernet backbone.
Replication frequency The replication frequency identifies how often replication occurs (if it is allowed). The replication frequency works together with the replication schedule to control when replication occurs.
  • To modify the replication frequency for intersite replication, edit the properties of the site link.
  • The replication frequency is scheduled in 15 minutes intervals. The default replication interval is 180 minutes (3 hours).
  • A small interval decreases latency but increases the amount of wide area network (WAN) traffic. To keep domain directory partitions up to date, low latency is preferred.
  • The replication frequency is dependent upon the times when replication over this site link is scheduled to be available. For example, if the schedule allows replication between 02:00 am and 04:00 am:
    • If the replication interval is set for 30 minutes, replication can occur up to four times during the scheduled time.
    • If the replication interval is set for 180 minutes, replication might occur once, or not at all. To ensure that replication takes place, configure the replication frequency to be shorter than the scheduled time interval.
Site link cost The site link cost is a number assigned to a site link that identifies the overall relative cost of using that site link. The cost is used to select the optimal path between sites when more than one path exists.
  • Cost is usually based not only on the total bandwidth of the link but also on the availability, latency, and monetary cost of the link.
  • The cost value is a relative value. The number has meaning only in relation to other site link costs.
  • The default link cost is 100. If you do not modify the site link cost, all site links will have an equal cost value.
  • To force traffic over one link, set a lower cost. For example, set a lower cost for high-speed links to force traffic over the high speed link. Configure a higher cost for dial-up links that are used as backup links.
  • To modify the cost, edit the properties of the site link object.

Active directory also uses the site link cost to determine which site will provide coverage in the event a domain controller cannot be located at the clients' own site, and in the event of multiple failures will try other sites according to the cost until it locates a viable domain controller. Automatic site coverage factors in the cost associated with the site links of a site without a domain controller. Use the autositecoverage setting in the HKLM\SYSTEM\CurrentControlSet\Services\Netlogon\Parameters key in the registry to adjust the site coverage:

  • When set to 0, the system cannot add sites to the coverage area of the domain controller. This setting must be set to 0 on the domain controller that should not provide coverage to sites that do not have a domain controller.
  • When set to 1, the system can add sites to the coverage area of the domain controller.
Bridged site replication Site link bridging enables transitivity between site links, so that replication between sites that are not directly connected together with a site link can still take place. Be aware of the following when planning for bridged sites:
  • By default, all site links are bridged. To prevent automatic bridging, edit the properties of the transport protocol (such as IP) and deselect the Bridge all site links option. By disabling this option, you must manually create site link bridges.
  • To create a site link bridge, right-click the transport protocol and select New Site Link Bridge....
  • For replication between bridged sites to be successful, the following conditions must be met:
    • Schedules set on the site links between the two bridged sites must overlap. The intersection of the replication schedules on all the relevant links determines the connection schedule between the two sites.
    • The replication frequency must be sufficient for replication to occur. The replication interval is the maximum interval along the minimum-cost path of site link objects from one end of the connection to the other.

    An easy way to ensure replication occurs between bridged sites is to set the same replication schedule and frequency on all site links connecting the two bridged sites.

  • Link costs are cumulative when multiple links are required between sites. If more than two paths are available (including bridged or non-bridged paths), the one with the lowest cost is used (even if this path crosses more sites and site links). Click Bridged Site Costs for an example.
Forced replication You can force replication to take place using one of the following methods:
  • To force replication between two sites, right-click the connection object and choose Replicate Now.
  • To force replication to or from a domain controller, right-click the NTDS Settings object below the server and choose one of the following:
    • Replicate configuration from the selected domain controller
    • Replicate configuration to the selected domain controller
  • Run repadmin.exe /replicate from a command prompt to force replication between a source and a destination domain controller. List the target system first, followed by the source system. Use /syncall to force replication between all domain controllers in a site. Use /rodcpwdrepl to force replication of the passwords to read-only domain controllers (RODC).

Note: To configure sites, subnets, and replication, you must be a member of the Domain Admins group or the Enterprise Admins group.