Introduction to Administering Intersite Replication

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

An Active Directory Site object represents a collection of Internet Protocol (IP) subnets, usually constituting a physical local area network (LAN). Multiple sites are connected for replication by Site Link objects.

Sites are used in Active Directory to:

• Enable clients to discover network resources (published shares, domain controllers) that are close to the physical location of the client, reducing network traffic over wide area network (WAN) links.

• Optimize replication between domain controllers.

Managing sites in Active Directory involves adding new subnet, site, and site link objects when the network grows, as well as configuring a schedule and cost for site links. You can modify the site link schedule, cost, or both, to optimize intersite replication. When conditions no longer require replication to a site, or clients no longer require the sites to discover network resources, you can remove the site and associated objects from Active Directory.

Managing sites:

• Enables clients to discover network resources (printers, published shares, domain controllers) that are close to the physical location of the client, reducing network traffic over wide area network (WAN) links.

• Optimizes replication between domain controllers.

The KCC and Replication Topology

The Knowledge Consistency Checker (KCC) uses site link configuration information to enable and optimize replication traffic by generating a least-cost replication topology. Within a site, for each directory partition, the KCC builds a ring topology that tries to set a maximum number of hops (3) between any two domain controllers. Between sites, the KCC creates a spanning tree of all intersite connections. Therefore, adding sites and domains increases the processing that is required by the KCC. Before adding to the site topology, be sure to consider the guidelines discussed in “Adding a new site” later in this document.

Significant changes to site topology can affect domain controller hardware requirements. For more information about domain controller hardware requirements, see Planning Domain Controller Capacity on the Microsoft Web site (https://go.microsoft.com/fwlink/?LinkId=42682).

Bridgehead Server Selection

By default, bridgehead servers are automatically selected by the intersite topology generator (ISTG) in each site. Alternatively, you can use Active Directory Sites and Services to select preferred bridgehead servers. However, it is recommended for Windows 2000 deployments that you do not select preferred bridgehead servers.

Selecting preferred bridgehead servers limits the bridgehead servers that the KCC can use to those that you have selected. If you use Active Directory Sites and Services to select any preferred bridgehead servers at all in a site, you must select as many as possible and you must select them for all domains that must be replicated to a different site. If you select preferred bridgehead servers for a domain and all preferred bridgehead servers for that domain become unavailable, replication of that domain to and from that site does not occur.

If you have selected one or more bridgehead servers, removing them all from the bridgehead servers list restores the automatic selection functionality to the ISTG.