Exchange Server 2007 tries to provide the
??? best ??? service by using a CAS located closest to the user ??™ s mailbox server.
There are architectural decisions on where to place a Client Access Server, and how to create a
namespace for external access. These design points are beyond the scope of this book, but understanding
the impact and how to configure the CAS to handle these configurations is not. This discussion focuses
on Outlook Web Access, but proxy and redirection affects the other CAS services too. Check with the
Exchange Server 2007 online help to learn in detail how the other services use proxy and redirection.
Only one Client Access Server must be Internet facing. Exchange will proxy OWA requests to the Client
Access Server in other sites if configured correctly. There must be at least one Client Access Server in
each AD site to use these services. For Exchange to proxy correctly, the Internet - facing CAS has the
ExternalURL property set and the other CAS has only the InternalURL set. (See Figure 6 - 12 .)
Site A Site B
CAS A
ExternalURL = webmail.exchangeexchange.com
CAS B
No ExternalURL
MBX
Figure 6-12
Part II: Working with Server Roles
182
Site A Site B
CAS A
ExternalURL = webmail.exchangeexchange.com
CAS B MBX
HTTP Redirect
ExternalURL = eurwebmail.
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