Robert’s Rules of Exchange: Multi-Role Servers

Ilantz, in the situation where you have the CAS roles on the DAG servers, you are correct, WNLB is not supported.  There has been a lot of internal discussion around DNS "round robin" as a possible solution, but the problem there is that when your TTL times out, the client will query DNS for the addresses again and reconnect.  When the client does that, they have a 50/50 chance of connecting to a different server.  For some clients, this is "masked" from the user (Outlook in RPC mode and EAS clients will both resend the credentials silently), but for some clients (OWA comes to mind), this will cause the user to be prompted for credentials.  It is the opinion of Microsoft that this is a poor user experience, so we don't recommend doing this.

The recommended solution in this case is to look to our hardware load balancer partners and find an agressively low priced solution.  For instance, KEMP (http://www.kemptechnologies.com/) has a purpose built load balancer for Exchange 2010 that costs less than US$1600 list price AND supports a highly available architecture by having two devices.  That's HA load balancing in hardware for US$3200 list - not a bad solution.  KEMP also has virtualized versions of their load balancing devices in both Hyper-V and VMware versions...  

(NOTE:  I work for Microsoft, not KEMP, and I am not advocating KEMP over any other hardware load balancer vendor - if there is another vendor that wants equal billing, I am more than willing to work with you as well!!!)

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rgillies at Microsoft