Active Directory 2008: Replication Configuration Facts…

Replication Configuration Facts

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).
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.


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