There may be instances where you will need to customize the global catalog to include additional attributes. However, you will want to carefully consider your options as changes to attributes can impact network traffic. By default, the global catalog contains an object’s most common attributes for every object in the entire forest, which applications and users can query. For example, you can find a user by first name, last name, e-mail address, or other common properties of a user account.
To add additional searchable attributes to the global catalog, administrators can use the Active Directory Schema snap-in. For more information about adding additional attributes to the global catalog, see To add an attribute to the global catalog.
When determining whether or not to add an attribute to the global catalog, consider only adding additional attributes that are frequently queried and referenced by users or applications across the enterprise. Also consider how frequently an attribute gets updated during replication. Attributes that are stored in the global catalog are replicated to every global catalog in the forest. The smaller the attribute, the lower the impact of that replication. If the attribute is large, but very seldom changes, it will have a smaller replication impact than a small attribute that changes often.
For more information about attribute definitions, see the Active Directory Programmer's Guide at the Microsoft Web site(http://www.microsoft.com/).
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