In this blog, I will talk about transitioning your Active Directory site while upgrading your messaging Infrastructure to Microsoft Exchange Server 2010. The transition to Exchange 2010 should be performed one Active Directory site at time. When transitioning a site, the order of Exchange 2010 server role deployment should be as follows:

  1. Client Access server
  2. Hub Transport server
  3. Mailbox server
  4. Edge Transport server (Can be introduced at any time during the migration)

The first Exchange 2010 role that must be installed is the Client Access server. The new Client Access servers will handle the front end protocols at same time act as a proxy if the destination mailbox is in Exchange 2007. Below are some guidelines for transitioning to the Exchange 2010 CAS; however it is recommended that each organization performing an Exchange Server 2010 upgrade test the steps in an isolated environment:

  1. Verify the existing environment by running ExBPA on CAS server and analyzing the output
  2. Install Exchange 2010 CAS role
  3. Installing & Configuring Load Balancing Solution
  4. Install Exchange 2010 software updates
  5. Obtain SSL certificate from a Commercial CA/trusted CA
  6. Replace self-signed certificate with trusted SSL certificate
  7. Configure Exchange 2010 CAS
  8. Configure Exchange 2010 CAS to have proper internal and external URL settings.
  9. Creating DNS records for CAS array
  10. Creating CAS Array
  11. Change externalURLs and certificate on Exchange 2007 Internet facing CAS servers to Legacy URLs and certificate.
  12. Reconfigure external DNS and/or reverse proxy by reconfiguring Autodiscover and webmail.companyname.com namespace to point to Exchange 2010 servers.

The detailed upgrade steps are outlined in TechNet at http://technet.microsoft.com/en-us/library/aa998604.aspx