You can use remote procedure call (RPC) over IP or SMTP to link sites. After you define the site links, replication schedules, cost factors, and site link bridges (if appropriate), the Knowledge Consistency Checker (KCC) can create the connection objects as long as the site links are transitive.
Using SMTP has some limitations. You can use SMTP to replicate the Global Catalog (GC) information, as well as schema and configuration data. However, SMTP can’t replicate full domain-name context data such as the data that domain controllers (DCs) in a domain exchange, because some domain operations require the File Replication Service (FRS) (e.g., the global policy), which SMTP doesn’t support.
Intersite replication uses a spanning tree topology. As long as the KCC can establish a replication route between all the sites in the enterprise forest, the replication tree is complete. The administrator manually creates the links between sites. Creating the links involves defining costs for each link (the cost relates the network’s speed and reliability) and establishing a replication schedule.
You use the Microsoft Management Console (MMC) Active Directory Sites and Services snap-in to create and maintain site links. By default, your original site is part of the site link DEFAULTIPSITELINK. You can add sites to this site link when you create them. (When you create a site, you must specify a site link.)
Replication data that travels between sites is 10 to 15 percent of its original size. This smaller size is important because intersite links are usually over WAN links, which tend to be slow.
You need to create only the necessary links between sites. The KCC creates the required connection objects.