As I mentioned earlier last week I had the privilege of going to a MS Exchange 2010 Partner Ignite training course.  I found the course to be quite interesting give the audience was mostly Exchange architects from around the country.  With this type of Audience there were a number of interesting conversations that occurred during the session and the following was one that was quite interesting.

During the class we were talking about the Client Access Server and how Outlook clients will now connect to a CAS rather than the mailbox server.  The question came up, “What happens if a user who normally resides in the New York office is visiting the Los Angeles office?”

This really sparked a great conversation because we were not sure.   We determined first the Outlook client machine will log on to a DC in LA (assuming the domain topology is simple) due to the configuration of Active Directory Sites and Services.  That domain controller will then inform the computer what site the computer is in.  Once the computer is alerted of its site the computer will then connect to a CAS server in that AD site. 

Now, the question is, what’s next?  The conversation went about a few ways.  Will the user remain off line?  Will the user connect to their CAS server back in NY rather than the LA CAS server?  Why won’t outlook connect directly to the mailbox server?

After doing some digging I believe I found the answer to this question and great conversation back in class.  When a user connects to a CAS in a different Site the CAS will perform an AD lookup.  This AD lookup will inform the CAS where the users mailbox resides.  If the users mailbox resides in the same site as the CAS then do nothing, the connection is established.  However, if the mailbox resides in another mailbox server in a different site the CAS will then “redirect” (yes, redirect) the user to the proper CAS server which will then connect the user to their mailbox.

So, what this means is that when the user visiting LA launches outlook, outlook will do a query and locate the CAS in the LA office.  That CAS will look up the user in AD and redirect that users request to a CAS in NY. 

Here is how MS explains it:

If the user’s mailbox is in the same Active Directory site as the Client Access server, the user is connected directly to their mailbox. If the user’s mailbox is in a different Active Directory site than the Client Access server that received the initial connection, the connection is redirected to a Client Access server in the remote Active Directory site.

I pulled that from this link:  

http://technet.microsoft.com/en-us/library/aa998561(EXCHG.140).aspx