Coexisting with Previous Versions

Understanding Exchange's Modes of Operation

An Exchange 2000 organization can operate in two modes: native mode and mixed mode. Native mode offers full Exchange 2000 functionality, while mixed mode offers interoperability between Exchange 2000 and previous versions of Exchange. When you install Exchange 2000, your Exchange organization is in mixed mode by default. This default setting ensures future interoperability with previous versions of Exchange (for example, the installation of an Exchange 5.5 server at a later time), even if no Exchange 5.x servers exist in the organization at the time of installation.

The concept of mixed mode and native mode organizations is similar to the concept of mixed mode and native mode domains in Active Directory. With Exchange 2000, you can select native mode and mixed mode only at an organizational level. No direct relationship exists between the mode of the domain and the mode of an Exchange organization. The similarity exists only in terms of naming and restrictions on earlier versions.

For Exchange Server 5.5 and Exchange 2000 to coexist and replicate directory information, the Exchange 2000 configuration must remain in a state that can be recognized by Exchange Server 5.5. Active Directory Connector (ADC) is critical to ensure coexistence with previous versions of Exchange.

Sites, Administrative Groups, and Routing Groups

Much of the added functionality of Exchange 2000 is a result of the Exchange 4.x and 5.x concept of site a being divided into administrative groups and routing groups.

Note In mixed-mode organizations, Exchange 5.5 sites are represented as administrative groups in Active Directory.

In earlier versions of Exchange, the site concept defined three boundaries: single-hop routing, a collective administration unit, and a namespace hierarchy. To provide a more flexible deployment and administration structure, all three boundaries are separate in Exchange 2000.

Single-hop routing is now defined by a routing group.
Collective administration is now defined by an administrative group.
Namespace hierarchy has moved to Active Directory in the form of a domain.

The ability to operate an Exchange organization in mixed mode exists for backward compatibility with previous versions of Exchange and other software that relies on previous versions of Exchange. Operating an Exchange organization in native mode is useful when all servers are running the latest version of Exchange and the restrictions imposed by earlier versions no longer apply.

After all previous versions of Exchange are been upgraded or removed from the organization, you can configure Exchange 2000 to no longer support previous versions of Exchange by switching to native mode. This allows you to configure your administrative groups and routing groups independently.

This section includes the following topics: