Exchange 2010 Coexistence

An Exchange 2007 CAS Server/Exchange 2003 Front End Server can’t proxy requests to Exchange 2010 mailboxes. Nor can an Exchange 2010 CAS Server proxy requests to Exchange 2003/2007 mailboxes (the one exception is Exchange ActiveSync). While an Exchange 2010 CAS Server can’t proxy, it can transparently redirect clients to an Exchange 2007 CAS Server / Exchange 2003 Front End Server though. Typically we would do this as follows.

The current configuration looks like this…

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We would then stand the new Exchange 2010 environment in parallel to the Exchange 2007 environment. Note that I drew the ASA and ISA Server as two icons for clarity but in both cases we’re just talking about a second IP address on the same device.

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At this point everything would be published and testable but no redirection would be occurring. Here comes the first step were we actually impact users. We would change the External URL values for OWA, EWS and Offline Address book on the Exchange 2007 CAS Server to point to legacy.domain.com (for Exchange 2003 we would add the Exchange2003URL value to the Exchange 2010 CAS Server) so Exchange 2010 would know where to redirect traffic. We would then swap IP addresses between the Exchange 2007 and Exchange 2010 Servers…

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Now, when clients type owa.domain.com they would hit the Exchange 2010 server. If their mailbox is here they would stay put however if their mailbox is still on Exchange 2007/2003 their browser would be redirected to legacy.domain.com which would send them into the Exchange 2007 Server. We can now move mailboxes at our leisure. Once they are all moved we can tear out the temporary legacy stuff and we’re left with this…

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