Before you start
Objectives: Learn what is Global Catalog and why it is used.
Prerequisites: you have to know what is Active Directory.
Key terms: Global Catalog, Domain Controller, DC, GC, replication.
What is Global Catalog
Global Catalog (GC) is a database which contains replicated parts (attributes) of objects from the whole forest. So, it includes all objects from all domains within a forest. Server which has a copy of that database is called a Global Catalog server.
Global Catalog essentially enables faster searching of objects. Let’s say that we have two domain controllers (DCs) which are both classified as GC servers. Inside of our domain we have several organizational units with many user objects. Every user object has several attributes (user name, first name, last name, etc). The Global Catalog will contain a subset of those attributes (for example, only user name) of all users. Because Active Directory uses Multi-master replication model, replication of all attributes would take a very long time. So, since GCs can replicate a subset of important attributes, replication is fast. This enables faster searching for objects because GCs will know where that object is based on the subset of attributes they contain. In that way, when searching for objects, multiple DCs don’t have to be referenced, since the global catalog will know where that object is located.