One of the great changes in Exchange 2010 over Exchange 2007 is the ability to combine both CAS & Hub roles on mailbox servers configured as DAG, this gives you the flexibility to have a highly available solution with just 2 servers.
Note that you can still combine the UM role on the same servers however it’s not recommended because of the performance impact…
The challenge is that if you are having a requirement for load balancing the CAS or Hub traffic among the two servers you will not be able to use the Windows load balancing service “WLBS” and this is because simply it’s not supported to enable both WLBS and Clustering services on the same server. So the question now is how can you provide the required high availability to the CAS and Hub roles while maintaining the DAG and without using WLBS? to answer this question let’s split the roles.
Hub servers are automatically load balanced by Exchange 2010 for any servers to server submissions, you don’t need to configure any type of load balancing mechanism to load balance the mail submission traffic among the Exchange servers, however some of the environments are having SMTP clients or applications that use Hub servers to send/relay email to either internal users or to external users. To load balance these clients traffic there are two methods that you can use:
Note that by any method you will choose it’s a must that you don’t use the selected load balancing method to load balance the server to server communication…
CAS servers traffic can be load balanced using any of the below methods:
If we will have a quick analysis to the above options, using a hardware load balancer is definitely the recommended option if you are going to combine the CAS/Hub with a DAG.