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Understanding Active Directory, Part IV

Some domain controllers are assigned specific roles to facilitate better performance and to reduce conflict. Although the principle of multimaster replication of services across all domain controllers is the underpinning of the Active Directory, certain specialized functions are best performed by a single domain controller, as discussed by experts Robert Williams and Mark Walla.
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Some domain controllers are assigned specific roles to facilitate better performance and to reduce conflict. Although the principle of multi-master replication of services across all domain controllers is the underpinning of the Active Directory, certain specialized functions are best performed by a single domain controller. Therefore, Windows 2000 supports two forms of specialized functions: the Global Catalog and operations masters. Forests share a common Global Catalog and some common operations masters.

Global Catalog

The Global Catalog (GC) has two primary functions. First, it acts as a domain controller that stores object data and manages queries about objects and their most common attributes (called the Global Catalog Partial Attribute Set, or PAS). Second, it provides data that permits network logon. In single domain controller environments, the Active Directory and GC reside on the same server. Where multiple domain controllers exist, as we discuss later, it is often advisable to move the GC to its own dedicated domain controller. All domain trees have a GC, and must reside on a domain controller.


In the absence of a GC, a user can log on only to the local system. However, a member of the Domain Administrators group can log on to the network without a GC.

The Global Catalog server stores and replicates an assortment of information, including the domain forest schema data and configuration data. It can also be seen as a data repository and engine for rapid object searches. The GC lists all the objects within a domain tree or forest. However, it differs from the Active Directory in that it is comprised of a partial list of object attributes. A list of the most requested or common object attributes is contained in the GC in an abbreviated format that results from partial replication of domain data. By cataloging domain objects, locating objects can be faster without the need to search the entire source domain. Clearly, the reason for a dedicated GC is to separate the inquiry process from the updating and management processes within a directory service.

An object's distinguished name typically provides sufficient data to identify the partition that holds it. The GC contains a partial copy of every distinguished name namespace on the Active Directory.

The Global Catalog supports a set of default object attributes that are considered the most common or the most frequently queried—for example, a user's first and last names. However, for greater control over the defined attributes for a particular domain, Windows 2000 provides a means to modify the default settings. The system administrator can utilize the Schema Manager snap-in to update the attributes included in the Global Catalog replication.

When the first Active Directory is installed, it creates a default Global Catalog. More than one Global Catalog server can exist, depending on the size of the enterprise, the number of physical sites, and the quality of network connectivity. Global Catalog servers are added through the Sites and Servers Management snap-in of the Microsoft Management Console (MMC). Moving the GC to another domain controller is accomplished by modifying the NTDS Setting Properties in the Sites and Server Management snap-in.

In selecting a system to become the Global Catalog server, it is important that both capacity and network connectivity be considered. The system should have sufficient storage capability to support the management of a million or more objects. The CPU system speed should be sufficient to permit the processing of a steady flow of queries.

GCs and Sites

Network connectivity to the Global Catalog server must be fast and of high quality because access to a GC is required for successful network logon. Given that a site is bounded by rapid and reliable network connectivity, at least one GC domain controller per site is recommended.

Master Operation Roles

Multimaster domain replication assumes that all domain controllers eventually receive synchronized Active Directory information. However, there are master domain controller relationships to handle certain Active Directory information within a domain or forest. The master roles are defined below:

  • Domain naming master. This domain controller manages the addition and removal of domains in the forest. A forest can have only one domain naming master, which can be transferred to another domain controller through the Active Directory Domains and Trusts snap-in.

  • Infrastructure master. The infrastructure master is responsible for managing group and user references. Expect a delay in changes to user g when they are made across domains. Updates to other domains are made by the infrastructure master domain controller via a process called multimaster replication. This master role can be transferred to another domain controller through the Active Directory Users and Computers snap-in.

  • PDC Emulator master. In a mixed Windows 2000 and Windows NT environment, the PDC Emulator master supports the BDCs. Thus, it manages user account and password changes, and forwards that information to the Windows NT BDC. In a native mode Windows 2000 environment, the PDC Emulator master receives preference in the replication of user account passwords. Before a logon fails, it is checked for updated information. This master role can be transferred to another domain controller through the Active Directory Users and Computers snap-in.

  • Relative ID master. A single relative ID master in each domain of a tree manages the allocation of sequential relative IDs (RIDs) to each of the domain controllers. This makes all security IDs (SIDs) created in a domain relative to the domain controller. This master role can be transferred to another domain controller through the Active Directory Users and Computers snap-in.

  • Schema master. The schema master controls updates to the domain schema data. There is one schema master in the entire forest. It can be transferred to another domain controller through the Active Directory Schema Master snap-in.


Microsoft issues a word of caution regarding potential conflicts between the infrastructure master and the Global Catalog. In environments where more than one domain controller exists, the Global Catalog should not be hosted on a controller that also hosts the infrastructure master. Because the infrastructure master compares its data with the Global Catalog, there may be significant replication impacts, and full replication may fail. In particular, outdated information will not be seen. The exception to this rule about separating the Global Catalog and the infrastructure master is an environment where every domain controller retains a copy of the GC.

In the next part of this series, we examine the open standards and naming conventions embraced by Active Directory.

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