Server load balancing is a way to manage which servers receive traffic. Server load balancing provides failover redundancy to ensure users continue to receive service in case of failure. It also enables your deployment to handle more traffic than one server can process while offering a single host name for clients.

Server load balancing serves two primary purposes. It reduces the impact of a single Client Access Server (CAS) failure within one of your Active Directory sites. In addition, server load balancing ensures that the load on your CAS and Transport servers is optimally distributed.

Server load balancing reduces the impact of a single CAS failure within one of your Active Directory sites and ensures that the load on your servers is evenly distributed. Architectural changes with respect to earlier versions of Exchange make server load balancing even more important than in the past. A load-balanced array of CASs is recommended for each Active Directory site and for each version of Exchange. It is not possible to share one load-balanced array of CASs for multiple Active Directory sites or to mix different versions of or service pack versions of within the same array.

Several aspects of Exchange 2010 make server load balancing important. The RPC CAS on the CAS role improves the user's experience during mailbox failovers by moving the connection endpoints for mailbox access from Outlook and other MAPI clients to the CAS role instead of to the mailbox server. In earlier versions of Exchange, Outlook connected directly to the mailbox server hosting the user's mailbox, and directory connections were either proxied through the mailbox server role or referred directly to a particular Active Directory global catalog server. Now that these connections are handled by the CAS role, both external and internal Outlook connections must be load-balanced across the array of CASs in a deployment to achieve fault tolerance and optimal performance.

For more information, please refer to the Microsoft documentation on this subject matter available on the web at http://technet.microsoft.com/en-us/library/ff625247.aspx.

When a LoadMaster-based CAS array has been configured, all servers in the array can be represented by a single VIP address and an FQDN (Fully Qualified Domain Name). When a client request comes in, it is sent to an Exchange 2010 CAS server in the CAS array using any available LoadMaster scheduling (distribution) method that you select. The scheduling method is defaulted to round robin as the preferred method because it does a better job of balancing traffic in many situations.