Working with Sites, Administrative Groups, and Routing Groups

For Exchange 2003 and Exchange 5.x organizations to coexist, you will need to leave Exchange Server 2003 in mixed mode. Remember that there is no direct relationship between mixed mode and native mode in Windows Server 2003 and mixed mode and native mode in Exchange Server 2003. In other words, you can have all your servers running Windows Server 2003 in native mode but still have Exchange Server 2003 running in mixed mode to permit interoperability with one or more Exchange 5.x servers.

Mixed mode offers compatibility with previous versions of Exchange Server, but it also has its limitations. Be aware of these, which include the following:

• Exchange 5.x sites appear as administrative groups in the Exchange System snap-in.

• You cannot move mailboxes easily between administrative groups.

• You cannot move servers between administrative groups.

• Members of a routing group must come from the same administrative group.

• Exchange 2003 Administrative Groups will appear as sites in the Exchange 5.5 Administrator.

• The Exchange 5.5 Administrator will not recognize routing groups in Exchange Server 2003.

In other words, when running in mixed mode, Exchange 2003 administration is similar to Exchange 5.x administration, with the site boundaries defining your routing and administrative boundaries.

The last bullet point in the preceding list is particularly important. In mixed mode, each routing group must be created in the administrative group in which the servers exist. This is because Exchange 5.x sites have a one-to-one relationship with administrative groups. Although you can place your Exchange 2003 servers into routing groups that exist in different administrative groups, you cannot assign a server to a routing group that is held under a different administrative group than the administrative group of which the server is a member. For example, if Server1 was created in the Minneapolis administrative group, it cannot be a member of the Arizona routing group unless the Arizona routing group is created inside the Minneapolis administrative group. (See Chapter 12, “Using Administrative and Routing Groups,” and Chapter 13, “Connecting Routing Groups,” for a discussion about how to implement administrative and routing groups.)

Real World—Increasing Administrative Flexibility

One way to increase your administrative flexibility in mixed mode is to subdivide an Exchange 5.x site. Assume that sufficient bandwidth exists between three locations, each hosting three Exchange servers, to place all the servers in the same Exchange 5.5 site. Let’s further assume that of these nine servers, five are running Exchange Server 2003 and four are running Exchange Server 5.5.

When a user on an Exchange 5.5 server sends a message to a user on an Exchange 2003 server, the Exchange 5.5 Message Transfer Agent (MTA) routes the message directly to the Exchange 2003 server. From an Exchange 5.5 server’s perspective, the routing process works the same as it always has. Now suppose that a user on an Exchange 2003 server sends a message to a user on an Exchange 5.5 server. Because Exchange Server 2003 can operate under different rules, you can subdivide the administrative group into several routing groups and force the message from the Exchange 2003 user to travel along a certain path before it gets to the Exchange 5.5 server. This works because the ADC service and SRS do not replicate routing groups and intra-administrative group connectors to the Exchange 5.5 environment.

After switching your organization into native mode, your servers and routing groups will no longer be tied to the administrative group. Thereafter, you’ll have full flexibility in routing messages in your environment and in the kind of administrative model you implement.