Installing your first Exchange 2010 CAS – Part 2

A few days back I have talked about Installing your first Exchange 2010 Client Access Server when migrating from Exchange 2003 to Exchange 2010. Today I will discuss the requirements of installing your first Exchange 2010 CAS if migrating from Exchange 2007.

First we have to agree that that an Exchange 2010 CAS cannot communicate directly with an Exchange 2007 mailbox servers, so Exchange 2007 CAS are still required.

We will need to differentiate 2 scenarios when mixing Exchange 2007 & Exchange 2010 CAS

Exchange 2010 CAS and Exchange 2007 CAS in the same Active Directory Site

The first Exchange 2010 CAS server should be installed in your internet facing site, and it should be the first server people are directed to when they try to access your organization from the internet using, say, https://webmail.yourdomain.com/owa

Since Exchange 2010 Client Access Server cannot proxy the requests to an Exchange 2007 CAS server in the same active directory site, when the request reaches the 2010 CAS it will query Active Directory for the mailbox server of the user, if that mailbox is located on an Exchange 2007 mailbox server in the same Active Directory site the CAS will look for the ExternalURL & InternalURL parameter of that CAS server.

• If ExternalURL is set on your exchange 2007 CAS the user will be directly redirected to it
• If ExternalURL is not set but InternalURL and user is an internal user then the redirection will happen to the internalURL
• If ExternalURL is not set but InternalURL and user is an external user then the user will have an error.

In short to give access for external users who's mailboxes are still on Exchange 2007 you will need to configure both the Exchange 2007 and the Exchange 2010 CAS as internet facin.

Just like the case of Exchange 2010 CAS and Exchange 2003 mailbox server, for this redirection to work seamlessly Form Based Authentication should be enabled on the OWA virtual directory on the 2007 CAS.

However, unlike the case of Exchange 2010 CAS and Exchange 2003 mailbox, if FBA is not enabled on the 2007 CAS the user will be prompted twice for authentication but he will be able to access his mailbox with no errors.

Exchange 2010 CAS and Exchange 2007 CAS in the different Active Directory Site

If, upon querying Active Directory, the Exchange 2010 CAS finds that the user's mailbox is situated on an Exchange 2007 mailbox server in a different Active Directory site and

• That 2007 CAS has the ExternalURL parameter set, then the user will be presented with a page that contains a link to the correct Client Access server for their mailbox.
• That 2007 CAS doesn't have the ExternalURL parameter set, then the 2010 CAS will proxy the request to the Exchange 2007 CAS in the other site.

Even though, as I just stated, the Exchange 2010 CAS will try to proxy the requests to the 2007 CAS by default this will fail with the following error

and Event ID 46 from MSExchange OWA will be logged in  application log.

To solve this problem you should find on your Exchange 2007 CAS the the OWA directory (C:\Program Files\Microsoft\Exchange Server\ClientAccess\Owa by default). Inside this folder you will see many sub-folders with different version numbers like 8.1.375.2, 8.2.176.2, 8.2.176.2 etc...

Pick the one with the highest number (8.2.176.2 in that case) and copy it to the OWA directory on your Exchange 2010 CAS (C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\Owa by default).

Don't forget to restart your IIS using the IISReset /noforce command and you're set.

Part 1: Installing your first Exchange 2010 CAS in an Exchange 2003 organization
Part 2: Installing your first Exchange 2010 CAS in an Exchange 2007 organization
Part 3: Installing your first Exchange 2010 CAS in a mixed Exchange 2003/2010 organization

About Antoine Khater

I have been working in IT consultancy and solution integration since 1998 and I consider myself lucky to be, one in a few, making a living out of my passion. I am also member of the famous Experts Exchange (profile here) online community where I try my best to share what I have learned along the road.