Although the Exchange 2013 Client Access Server (CAS) role maintains the same name as in the previous versions of Exchange, it has changed significantly. The CAS role in Exchange 2010 performed proxy/redirection, authentication, and data rendering for Internet Protocols. In Exchange 2013, this role does not perform data rendering; it only performs proxy/redirection and authentication and offers support for the client Internet Protocols, e-mail transport, and part of the Unified Messaging stack (it redirects SIP traffic generated from incoming calls to the Mailbox server). As a result of this architectural change, from a protocol session perspective, the CAS role is now stateless, and because all processing for a mailbox happens on the Mailbox server, it does not matter which CAS receives each client request; that is, there is no session affinity.

Before we start exploring how to achieve high availability for the CAS role, there are...