Replication Basics

To design an Active Directory topology that optimizes the replication of information in a business, you must understand a couple of replication basics first. This section discusses some of the concepts behind Active Directory replication, including the following:

• Sites

• Site links

• Replication schedules and compression

Sites

Chapter 2, "Overview of Active Directory Design Elements," briefly introduced you to the concept of Active Directory sites. If you recall, an Active Directory site is basically a collection of well-connected IP subnets. The links between the subnets in a site are generally fast enough to support replication, available (not saturated ), and very reliable.

Because the creation of sites is based on the physical topology, a site can contain IP subnets from multiple domains.

Active Directory sites are designed to optimize replication. However, replication in a site occurs differently than it does between two separate sites. Intrasite replication is designed to take advantage of the fact that the IP subnets in a site are connected by fast, reliable links, whereas intersite replication is designed to occur differently because it is assumed that the links connecting two sites are slow and unreliable.

Table 9.1 compares replication as it occurs in a site and between sites.

Table 9.1. Comparison of Intersite and Intrasite Replication

Intrasite Replication	Intersite Replication
Information replicated in a site is uncompressed.	Information replicated between sites is compressed to optimize bandwidth.
Domainontrollers in a site notify each other when a change occurs, which reduces the time for changes to appear throughout a site.	To optimize bandwidth, no notification process exists between sites, meaning information on domain controllers might not always be up-to-date.
Domain controllers in a site poll each other for changes on a regular basis.	Domain controllers between sites poll each other at a preconfigured interval during scheduled times.
Intrasite replication can occur between multiple domain controllers.	Replication between sites occurs only between specific domain controllers.

The following traffic-generating events are controlled and optimized by the creation of sites:

• User authentication ” When a user attempts to log on to a workstation, a domain controller in the same site is contacted to authenticate the user. Therefore, the logon process is more efficient (the user does not have to log on over a slow, unreliable link).

• Controlled replication ” Creating multiple sites enables replication across slow, unreliable links to be controlled by specifying a schedule, frequency, and cost (these options are covered later in the chapter).

• Site-aware applications ” Applications that are site aware, such as the distributed file system (DFS), can take advantage of the site topology by attempting to connect to a domain controller in the same site before attempting to connect to a domain controller in another site.

Two important points to note are that site creation is easy and that changing a site is as simple as adding or deleting a subnet. Therefore, if any erroneous assumptions were made in designing a site hierarchy, correcting the design is straightforward.

After the Active Directory sites have been defined and the IP subnets have been added to the appropriate site, site links must be established. Before doing so, however, you need to understand the function of site links.

Site Links

Site links are similar in concept to trust relationships. A trust is the link between two domains; a site link is the connection between two Active Directory sites. The site link established between two sites is used to control replication across the physical link.

Site links are transitive by default. So, if a link is defined between sites A and B and another is defined between sites B and C, it is automatically assumed that sites A and C can communicate. These transitive site links establish a replication path so information can be replicated throughout the organization.

Site links are transitive, unless the default settings have been modified. To change from transitive to nontransitive site links, the Bridge All Site Links check box must be deselected for a given site protocol.

For example, if the site structure created for the XYZ Corporation were based on geographical locations, it would look similar to Figure 9.1. If site links were established between LA and NJ and also between NJ and NY, a replication path would be created between LA and NY due to the transitive nature of the site links. Additionally, a site link created between Paris and NY would establish a replication path that would allow for the replication of information throughout the business.

Figure 9.1. Site links in Active Directory are transitive in nature, enabling a replication path to be created in an organization.

When you're defining a site link, certain options are used to control replication over the link. A site link is defined by the following characteristics, each of which is discussed in detail later in the chapter:

• Transport ” The method used to transfer data between two sites. You can use one of two methods : RPCs or SMTP.

• Schedule ” The schedule defined for a site link specifies the times when replication can occur over the link.

• Cost ” The value assigned to the site link. If multiple site links exist, the one with the lowest cost is tried first.

• Interval ” Specifies how often during the scheduled times the site link can be used to check for updates in another site (this process is referred to as polling ).

You can only select a transport protocol for intersite replication ”all intrasite replication occurs using RPC.

Replication Schedules and Compression

One of the main differences between intersite and intrasite replication is that, for intersite replication, the replication events can be scheduled by an administrator to occur only at certain times. Intrasite replication is based on a notification process: Anytime a change occurs, other domain controllers in the site are notified. As already mentioned, sites are designed to optimize bandwidth usage; therefore, there is no notification process between sites. Instead, a schedule is defined specifying when the link is available for intersite replication. This enables an administrator to schedule the site link to be available only during off-hours.

For example, if the site link shown previously in Figure 9.1 between Paris and NY is heavily used during the morning hours, an administrator can choose to schedule that site link to be available during the afternoon or evening hours when the link is not being used as much.

Another feature of intersite replication that optimizes throughput is data compression . Any replication cycle in which more than 50KB worth of updates has been generated is automatically compressed to save bandwidth. Replication compression is extremely efficient, reducing the data transferred by approximately 90%. Compression does require additional CPU and RAM resources, however.

Conversely, because it is assumed that the links in a site have plenty of bandwidth, replication in a site remains uncompressed. This also reduces the processor load on domain controllers in a site.

Now let's take a look at some considerations to keep in mind when determining how many sites to establish and which IP subnets to place into which sites.