The Global Catalog (GC) contains an entry for every object in an enterprise forest but only a few properties for each object. An entire forest shares a GC, with multiple servers holding copies. You can perform an enterprisewide forest search only on the properties in the GC, whereas you can search for any property in a userís domain tree. Only Directory Services (DSs) or domain controllers (DCs) can hold a copy of the GC.
Configuring an excessive number of GCs in a domain wastes network bandwidth during replication. One GC server per domain in each physical location is sufficient. Windows NT sets servers as GCs as necessary, so you donít need to configure additional GCs unless you notice slow query response times.
Because full searches involve querying the whole domain tree rather than the GC, grouping the enterprise into one tree will improve your searches. Thus, you can search for items not in the GC.