Developing an Interoperability and Migration Strategy

You can migrate a messaging environment to Exchange 2003 in a single phase or in multiple phases. A third option is long-term coexistence, although this is not a true migration strategy.

Your assessment of the existing messaging infrastructure should give you a clear understanding of your migration options. You can use the flowchart shown in Figure 1 as a guide when choosing an appropriate migration strategy for your company.

Figure 1   Developing a migration strategy

Single-Phase Migration

Single-phase migration is straightforward. The day before the migration, all users are on a non-Exchange messaging system. The next day, they are all on Exchange 2003, and the legacy system is decommissioned. Existing user data might or might not be migrated. Some organizations reduce migration costs by not transferring existing messages. When this occurs, users must save memos and other important information on their local computers. Most organizations, however, prefer to migrate existing data to avoid lost productivity.

The most significant advantages of a single-phase migration to Exchange 2003 are:

• All users are migrated at once, which yields quick results.

• If you deploy Outlook on all desktops before migration and advise your users to store all messages on their client computers, it is not necessary to move server-based user data to Exchange 2003. After migration, it is necessary only to reconfigure Outlook profiles to connect to Exchange 2003, rather than connecting to the legacy messaging system.

• There is no need for messaging connectivity between the legacy messaging system and Exchange 2003. You are not required to install or configure an Exchange 2003 connector.

• Preserving existing e-mail addresses is straightforward, because the new Exchange 2003 messaging system replaces the legacy messaging system.

• You can establish a new messaging infrastructure that perfectly mirrors the existing recipient information.

The most significant disadvantages of a single-phase migration to Exchange 2003 are:

• The migration of large numbers of users or large amounts of data results in unacceptable downtime.

• You must establish the entire Exchange organization before you migrate users. It is difficult to stage server and client deployments.

• You cannot control the pace of the migration. You cannot migrate divisions or departments individually, for example.

• You have limited flexibility. For example, it is not possible to leave a particular group of users on the legacy system for any reason.

Multiphase Migration

For a small company with one or two messaging servers, single-phase migration is an advantageous option for migrating to Exchange 2003. However, for larger companies with a more complicated messaging infrastructure, single-phase migration is rarely possible, because these companies typically cannot react quickly enough to issues that can arise in a single-phase migration. Your company might have multiple messaging servers, which might be located in different places. You might have several different messaging clients that connect to messaging systems from multiple locations. It might not be feasible for all of the mailboxes to be migrated to Exchange 2003 and for all of the clients to be reconfigured in one giant step. In these situations, the migration process takes an extended period of time and includes multiple phases.

One of the most important issues that you must address in multiphase migrations is interoperability between the messaging systems, because users on the legacy system must be able to exchange messages with users who have already migrated to Exchange 2003. Because users tend to exchange e-mail messages primarily with other users in the same workgroup or department, it is recommended that you migrate users according to workgroup or department. This can help to reduce the number of messages that a messaging connector must transfer from one messaging system to the other.

The most significant advantages of a multiphase migration to Exchange 2003 are:

• You can complete the migration in incremental and manageable steps. In a large company, you can migrate departments, business units, or teams at one time. It is recommended that you migrate users who require delegate access to each other's calendars and mailboxes in the same cycle.

• You can continue to support complex workgroup applications on the legacy system until new versions are available for Exchange 2003. However, it is important to evaluate and test your legacy application's ability to support users with mailboxes in the Exchange 2003 organization.

• You can migrate heterogeneous messaging environments to Exchange 2003 without consolidating the resources in the old environment before migration.

• You can minimize risks. If one particular operation in the multiphase migration is not successful, a limited number of people are affected, and you can back out of the operation fairly quickly. If migration of a group of users fails for any reason, users can continue to work with their old mailboxes until you fix the problem.

• You can minimize the system downtime for messaging users. If you choose to perform the migration during non-business hours, you might be able to nearly eliminate downtime for users.

• You can synchronize the reconfiguration of the messaging client and end-user training with the migration of mailboxes.

• You have control over the pace of the migration. You do not need to establish the entire Exchange organization before you migrate users. You can stage server and client deployments according to your migration phases.

The most significant disadvantages of a multiphase migration to Exchange 2003 are:

• Compared to single-phase migrations, multiphase migrations are more time consuming and therefore more expensive.

• The computer network experiences an increased amount of data traffic, which results from the need to communicate with users on the legacy messaging system, as well as from directory synchronization and calendar interoperability.

• The legacy messaging system and the Exchange 2003 organization must interoperate as seamlessly as possible. You must deploy a messaging connector and configure directory synchronization between the systems.

• You cannot establish a new messaging infrastructure that perfectly mirrors the existing recipient information. Preserving existing e-mail addresses is difficult, because message transfer processes use address information to distinguish the legacy system from the Exchange organization. Therefore, the Exchange organization cannot reuse the address information that already exists in the legacy environment.

• You must maintain both the legacy messaging system and Exchange 2003 for a period of time.

Long-Term Coexistence

Long-term coexistence is essentially a multiphase migration without an end point. After you migrate users to Exchange 2003, you do not decommission the legacy messaging system. Some companies might want to choose this strategy to preserve investments in existing technologies, such as complex business applications that rely on the legacy messaging system. With long-term coexistence, users must work with multiple clients, for example, Outlook to participate in the Exchange 2003 organization and another client (such as a Web-based interface) to work with the legacy business application. Coexistence is costly, support-intensive, and requires administrator knowledge of multiple messaging systems. For this reason, many companies consider standardizing their communication infrastructure on a single messaging system to be a key element in their messaging strategy.

Long-term coexistence and interoperability are also required in distributed environments with autonomous sites that use diverse messaging systems that are connected with each other over a corporate messaging backbone or central message switch. If this describes your situation, consult your infrastructure documentation, and collaborate with administrators in charge of the messaging backbone to determine appropriate technologies to connect the new Exchange 2003 organization to the existing infrastructure. Most backbones support X.400 or SMTP, and message switches might even support native Exchange communication protocols.