1.8 Global catalog

The GC is a special type of DC. As a DC it contains all the objects and attributes of its own domain. It is a full replica of the other DCs in the same domain. However, in addition to the objects of its own domain, a GC contains a subset of the attributes of all the other domain NC objects in the forest as well as the schema NC and the configuration NC. The GC contains all the objects in the AD. The role of the GC is to provide the AD with a search engine. By containing an indexed partial replica of all the objects in the forest, GCs become extremely fast search engines. The subset of the attributes is defined and modifiable in the schema and contains by default the attributes most commonly searched, such as name, e-mail address, and phone number for a user.

Any DC can be configured to become a GC. A GC incurs more overhead than a DC because it performs more replication and requires more disk space for its DB; however, because users and applications connect to the GC to search for objects in the AD, it is recommended to deploy enough GCs to ensure that clients can make a high-quality connection. Normally, this implies a Local Area Network (LAN) quality connection established by locating a GC close to each major user community.

In order to understand how a search works against a GC, let us imagine that we are looking...