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Some of the features included with Exchange 2003 are only available if you are operating in an environment that includes only Exchange 2003 servers and are running in native mode. However, if you are migrating from Exchange 5.5 to Exchange 2003, it is unlikely that you will simultaneously upgrade all of your existing Exchange servers. Instead, you will be left with an Exchange organization that includes a mixture of Exchange 5.5 and Exchange 2003 servers.
Even if you only have a small Exchange 5.5 environment, the duration of your migration to a native mode Exchange 2003 environment may take longer than you expect because some of the connectors that allow you to exchange e-mail with other systems may not be available for Exchange 2003. You will need to keep these connectors running on Exchange 5.5 servers.
As long as you have Exchange 5.5 servers in your organization, you must run your Exchange 2003 servers in mixed mode to provide full compatibility and coexistence with the older version of Exchange. In mixed mode, you can take advantage of some new Exchange 2003 features, but some features can only be used in native mode.
Windows also has a mixed mode and a native mode. However, the Exchange mode concept is independent of the Windows mode.
In mixed mode, your use of Exchange 2003 administrative groups and routing groups is limited so that the Exchange 2003 servers can work cooperatively with the Exchange 5.5 sites. In mixed mode, all Exchange 2003 administrative groups are mapped to Exchange 5.5 sites, and each administrative group has a single routing group that contains the servers installed in the administrative group. This combination of administrative group and routing group is the conceptual equivalent of an Exchange 5.5 site. When you install an Exchange 2003 server in an existing Exchange 5.5 organization, an administrative group will be displayed in ESM for each of the existing Exchange 5.5 sites.
Mixed mode also affects the routing protocols that are used to transfer information between Exchange servers. Exchange 2003 servers that are in the same routing group will always use SMTP to route messages to each other. If you install two Exchange 2003 servers in an existing Exchange 5.5 site, the two Exchange 2003 servers will continue to use SMTP to route messages between each other, even if there are Exchange 5.5 servers in the site.
However, when the Exchange 2003 servers need to route messages to or from the Exchange 5.5 servers, they will automatically revert to the X.400-over-RPC protocol that is understood by Exchange 5.5.
Because Exchange 5.5 servers do not understand Exchange 2003's Link State Algorithm, mixed mode also affects the routing mechanism used by Exchange 2003. The Exchange 5.5 Gateway Address Resolution Table is replicated to the Exchange 2003 server through the Active Directory Connector. This allows messages to be routed between the Exchange 2003 and Exchange 5.5 servers.
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