Impact of Schema Changes

A schema extension impacts the Active Directory forest in several ways:

• Schema changes are global. There is a single schema for an entire forest. The schema is globally replicated: a copy of the schema exists on every DC in the forest. When you extend the schema, it is extended for the entire forest.
• Schema object additions cannot be reversed. When a new class or attribute object is added to the schema, it cannot be removed. An existing attribute or class can be disabled, but not removed. For more information, see Disabling Existing Classes and Attributes. Disabling a class or attribute does not affect existing instances of the class or attribute, but prevents the creation of new instances. You cannot disable an attribute if it is included in any class that is not disabled.
• OIDs must be unique. When an attribute or class is added to the schema, no attribute or class with the same OID can be added. This is true even if a class or attribute has been disabled. For this reason it is you must use valid OIDs. Do not synthesize an OID, and do not reuse an existing OID. For more information about obtaining a valid OID, see Obtaining a Root Object Identifier.
• Some changes can be made after creation:
  • For a category 1 or category 2 class, you can add or remove values in the possSuperiors attribute. The possSuperiors values specify the object classes that can contain the class.
  • For a category 1 or category 2 class, you can add or remove values in the mayContain attribute. The mayContain values specify the optional attributes, but may be present in an instance of the class.
• The lDAPDisplayName for a Category 2 class or attribute can be changed after it has been created.Typically, you should not change the lDAPDisplayName. However, there is one legitimate reason for changing the LDAP name and that is if you made a mistake in defining the attribute or class and must create a new one to replace the old one. You do not have to rename the relative distinguished name (RDN) of the attribute or class schema to do this. When the LDAP name is changed this enables you to work around a mistake rather than rebuilding your whole Microsoft® Windows® 2000 infrastructure. For more information, see Disabling Existing Classes and Attributes.

  The lDAPDisplayName for predefined (category 1) classes or attributes cannot be changed. For more information about category 1 and 2 schema objects, see Restrictions on Schema Extension.