In order to make your CAS infrastructure highly-available, you need to load balance traffic across the different servers in the CAS pool. That pool of Client Access Servers by itself doesn't necessarily create high availability. There are other components and mechanisms that play an equal important part in the process. For example, the load balancing solution should be able to determine whether or not a Client Access Server is still a viable endpoint. If not, you risk ending up sending traffic to a non-functional server, therefore impacting on an end user's experience.

Unlike Exchange 2010, Exchange 2013 offers you the ability to load balance traffic between two or more Client Access Servers over layer 4 (L4). This means that load balancing is now effectively performed at the transport layer (remember the OSI model?!), opening the door for solutions like DNS round robin.

Load balancers operating at layer 4 are content-agnostic...