An efficient Active Directory topology is a must if you're planning to deploy Exchange 2007. Unlike its predecessors, the latest version of Exchange doesn't use Routing Groups to handle messaging traffic; rather, it works directly off of the existing AD infrastructure. Fortunately, achieving the level of AD efficiency you need has become far easier, thanks to the much-improved Knowledge Consistency Checker (KCC).
KCC first saw the light of day in February 2000, when Microsoft released Windows 2000 and offered admins the ability to connect remote sites together under one structure: Active Directory. To assist in that task, Microsoft added a behind-the-scenes algorithm to the structure: the Knowledge Consistency Checker.
KCC was promoted as an automatic site topology builder for monitoring physical connections between sites and subnets. Moreover, it was celebrated for forming logical connection links that included a replication schedule and site link costs -- that is, the costs to connect two sites based on connection speeds and bandwidth utilization.
This automatic manager of site replication for AD seemed too good to be true -- and it was, in fact. KCC was criticized for being a poor algorithm, and hence administrators ended up creating their own manual links and costs.
Some time later, according to administrative legend, a mathematical wiz took the algorithm apart, enhanced its capabilities, and gave it back to Microsoft. In fact, in 2004 InfoWorld's previous Enterprise Windows author Oliver Rist wrote, "For enterprises with an increasing number of branch offices or other remote locations, check out 2003's KCC (Knowledge Consistency Checker). Under Windows 2000, you were asking for trouble by attempting to connect more than 100 sites to a central server farm. KCC, however, has been reworked to handle well above 200 sites with no complaints whatsoever."