Windows Server 2008 : Understanding Active Directory Sites (part 1)

1/31/2011 4:07:11 PM
The basic unit of AD DS replication is known as the site. Not to be confused with actual physical sites, the AD site is simply a group of highly connected computers and domain controllers. Each site is established to more effectively replicate directory information across the network. In a nutshell, domain controllers within a single site will, by default, replicate more often than those that exist in other sites. The concept of the site constitutes the centerpiece of replication design in AD DS.


Intrasite replication is approximately 15 seconds when the forest functional level is set to Windows Server 2003 or higher. The intrasite replication is set to 5 minutes for Windows 2000 Server forest functional level.

Outlining Windows Server 2008 R2 Site Improvements

Specific functionality that affects sites has evolved since the early days of Active Directory. Windows Server 2003 introduced numerous replication enhancements that directly affect the functionality of sites and allow for greater design flexibility in regard to site design. These changes continue to exist in Windows Server 2008 R2 and have been further improved. These enhancements include the following:

  • Read-Only Domain Controllers (RODCs) and Read-Only Global Catalogs (ROGCs)

  • AD DS optionally installed on Server Core

  • GC universal group membership caching

  • Media-based domain controller creation

  • Linked-value replication

  • ISTG algorithm improvements

  • No global catalog full synchronization with schema changes

  • Ability to disable replication packet compression

  • Lingering object detection

Associating Subnets with Sites

In most cases, a specific site in AD DS physically resides in a specific subnet. This idea stems from the fact that the site topology most often mimics, or should mimic, the physical network infrastructure of an environment.

In AD DS, sites are associated with their respective subnets to allow for the intelligent assignment of hosts to their respective domain controllers. For example, consider the design shown in Figure 1.

Figure 1. Sample client site assignment.

Server1 and Server2, both members of Site1, are both physically members of the 10.1.1.x subnet. Server3 and Server4 are both members of the 10.1.2.x subnet. Client1, which has a physical IP address of, will be automatically assigned Server3 and Server4 as its default domain controllers by AD DS because the subnets have been assigned to the sites in advance. Making this type of assignment is fairly straightforward. The following procedure details how to associate a subnet with a site:

Open Active Directory Sites and Services.

Drill down to Sites\Subnets.

Right-click Subnets and choose New Subnet.

Enter the network portion of the IP range that the site will encompass. In our example, we use the (subnet mask of, as shown in Figure 2. Select a site for the subnet, and click OK.

Figure 2. Associating a subnet with a site.

Using Site Links

By default, the creation of two sites in AD DS does not automatically create a connection linking the two sites. This type of functionality must be manually created, in the form of a site link.

A site link is essentially a type of connection that joins together two sites and allows for replication traffic to flow from one site to another. Multiple site links can be set up and should normally follow the WAN lines that your organization uses. Multiple site links also ensure redundancy so that if one link goes down, replication traffic follows the second link.

Creation of site links is another straightforward process, although you should establish in advance which type of traffic will be utilized by your site link: SMTP or IP.

Site link replication schedules can be modified to fit the existing requirements of your organization. If, for example, the WAN link is saturated during the day, a schedule can be established to replicate information at night. This functionality enables you to easily adjust site links to the needs of any WAN link.

With the assumption that a default IP site link is required, the following steps will create a simple site link to connect Site1 to Site2. In addition, the replication schedule will be modified to allow replication traffic to occur only from 6:00 p.m. to 6:00 a.m. at one-hour intervals:

Open Active Directory Sites and Services.

Drill down to Sites\Inter-Site Transports\IP.

Right-click IP and choose New Site Link to open a properties page similar to the one shown in Figure 3.

Figure 3. Site link creation properties page.

Give a name to the site link that will easily identify what it is. In our example, we named it Site1-Site2.

Ensure that the sites you want to connect are located in the Sites in This Site Link box.

Click OK to create the site link.

Right-click the newly created site link, and choose Properties.

Click Change Schedule.

Select the appropriate time for replication to occur.

Click OK twice to save all settings to the site link.

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