Chapter 2: Exchange and the Active Directory

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Overview

Messaging systems depend on directories for a variety of needs. Fetching email addresses for users and knowing what server their mailboxes are located on is perhaps the most obvious, but directories also hold large amounts of configuration data, such as how servers connect together, permissions, and so on.

In the first generation of Exchange, the Directory Store (DS) held all of this data. The link between the DS and the other Exchange components, such as the Store and the MTA, that need to access directory data is simple, since all of the components reside on the same physical server. Thus, a network outage might prevent a first-generation Exchange server from contacting other servers to transfer email or stop users from connecting to mailboxes, but it will not stop the interaction between the different Exchange components. Because the DS is located on the same server, access to its data is also fast and reliable.

You cannot deny that having a local copy of the directory on every Exchange server has its advantages (such as always having a local directory available rather than having to depend on a network service), but it also implies a lot of replication traffic to keep all the copies of the directory synchronized. As the number of servers grows in an organization, the replication traffic expands dramatically. In addition, Microsoft designed the Exchange DS to be single purpose so other applications cannot use it. Exchange 2000 addressed these issues by supporting the AD and moving to a network-based directory model where servers do not maintain local copies of the directory. Instead, the AD comprises a set of Domain Controllers (DCs) and Global Catalog servers (GCs) distributed within the network to provide directory services to applications and the operating system. DCs hold a complete copy of all the objects belonging to a domain and a copy of objects replicated in the forest-wide configuration naming context (NC).

GCs hold a complete copy of all objects in their own domain plus partial copies of objects from all other domains within the forest.

Exchange accesses DCs and GCs for different purposes. DCs provide essential system configuration data, such as details of the servers and connectors installed within an organization. GCs provide information about user mailboxes and email addresses. The Routing Engine uses data taken from the GCs to route email, and MAPI clients, such as Outlook, use the same data as the Global Address List (GAL). Other clients use LDAP requests to search and retrieve information about recipients from the AD. Other examples of AD consumers include the Active Directory Connector, Recipient Update Service, and the Exchange System Manager (ESM) console.

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