Chapter 16: Coexisting with Previous Versions of Exchange

Overview

Many of you who pick up this book are facing coexistence—that is, interoperability—with previous versions of Microsoft Exchange. You might be migrating from Microsoft Exchange Server 5.5 to Microsoft Exchange Server 2003, or upgrading from Exchange Server 2003. In either case, the upgrades or migrations will at some point take place over a period of time, and you will need to contend with coexistence issues.

To make the migration go more smoothly and enable a seamless coexistence with Exchange Server 5.x, Microsoft has created a couple of services— enhanced connectors that make it possible for two different versions of Exchange to exist together in the same environment. Knowing how to use these two tools will go a long way toward making the migration straightforward and relatively painless for both you and your users.

In this chapter, the word “coexistence” describes a configuration in which different versions of Exchange Server are installed in the same Exchange organization at the same time. This type of configuration is known as a mixed-mode configuration. Running Exchange Server 2003 in mixed mode means that Exchange Server 2003 can interoperate with previous versions of Exchange Server and accommodate the differences between the various versions.

When running in mixed mode, rules that apply to earlier versions of Exchange Server also apply to Exchange Server 2003. Moreover, after you install Exchange Server 2003, you can install earlier versions of Exchange into the organization and experience interoperability between the platforms.

If you move Exchange 2003 to native mode, you’re removing interoperability between previous versions of Exchange and Exchange 2003 in your organization. Because this is a one-time, no-going-back decision, before you switch to native mode, you must be certain that you’ll never again need to interoperate with an earlier version of Exchange.

Real World—Mixed Mode vs. Native Mode

Although running Exchange Server 2003 in mixed mode allows you to interoperate with Exchange 5.5 systems, running Exchange 2003 in native mode means that you’ve closed the door on interoperability with previous versions of Exchange. Here are some guidelines to determine whether you are ready to switch to native mode:

• You no longer have Exchange 5.5 servers in your organization.

• You have no plans to add Exchange 5.5 servers to your organization in the future—for example, as a result of a merger or the acquisition of a company with Exchange 5.5 servers.

• Your organization will never require interoperability between your Exchange 2003 servers and previous versions of Exchange Server. (Connectors can provide connectivity between your servers; however, administration of servers is limited to Exchange 2003 servers.)

• Your organization does not use any connectors or gateway applications that run only on Exchange Server 5.5.

To switch to native mode in the Exchange System snap-in, navigate to the organization that you want to switch to native mode, right-click the organization’s name, and select Properties. On the General tab of the organization property sheet, change the operation mode to Native Mode.

It is important to remember that once you have switched to native mode, your Exchange 2003 organization is no longer interoperable with Exchange 5.5 systems and the change cannot be reversed.

If you are currently running Exchange 5.5, during the Exchange Server 2003 setup, you are given the option of either joining the existing site or creating a new organization. Because you can have only one Exchange 2003 organization per Active Directory forest, you’ll need to decide whether you should install your first Exchange 2003 server into your Exchange 5.x site. If you do, the existing Exchange 5.x organization name will be replicated to Active Directory as the Exchange 2003 organization name. If you choose not to install Exchange Server 2003 into your Exchange 5.x organization, you’ll need to choose a unique organization name, and you’ll be forced to use a connector to transfer messages between the two organizations. In this case, you will have two separate Exchange organizations, and they will connect to each other as foreign e-mail systems.

In a mixed-mode organization, the Exchange System snap-in shows all Exchange servers installed in the organization, but non–Exchange 2003 servers appear as transparent objects. For example, in Figure 16-1, the PDC1 server is an Exchange 5.5 server.

Figure 16-1: An Exchange 5.5 server (PDC1) in the Exchange System snap-in.

When the first Exchange 2003 server is installed into an existing Exchange 4.x or 5.x organization, two additional services are installed automatically: Site Replication Service (SRS) and the Active Directory Connector (ADC) service. These two components work together to provide replication between the Exchange 4.x and 5.x servers and Exchange Server 2003. Intrasite replication occurs using remote procedure calls (RPCs), and intersite replication is handled via directory services (mail-based) transfer. A connection agreement is automatically established in the ADC service between Active Directory and the SRS database to allow directory replication to occur. (Connection agreements are discussed later in this chapter, in the section “Active Directory Connector.”) The other Exchange 4.x and 5.x servers will see the Exchange 2003 server as just another 5.x server.