Any discussion of Microsoft Exchange 2000 Server performance necessitates mention of Active Directory (AD). Exchange 2000 depends on AD as the receptacle for organizational configuration information, the source of the Global Address List (GAL), and the basis for all routing decisions. A Global Catalog (GC) server must be in close network proximity to every Exchange 2000 server to permit Exchange 2000 to query AD as quickly as possible.
Performing a GC lookup for each and every address that Exchange 2000 needs to check wouldn't make sense, so each server maintains a cache of recently accessed directory information. Exchange 2000 always checks this cache first, then proceeds with a lookup against the GC only if it can't find the data. Although the cache can hold many thousands of items, fast access to a GC is still a prerequisite for good performance.
In small implementations, one server might run Exchange 2000 and also act as a GC. (If your Windows 2000 deployment spans only one domain, all domain controllers—DCs—are GCs.) GC placement probably won't be a concern in small environments but will be an important decision for all enterprise or distributed deployments. (For more information about how Exchange 2000 interacts with AD, see "Exchange 2000 Server over Active Directory," September 2000.)