When I first started to seriously look at the Beta version of Exchange 2010 that has been released by Microsoft I saw the term CAS Array mentioned in various articles on the internet but not a clear definition of what an CAS Array is or how to configure it so I had a look into it in our Exchange 2010 lab.

In simple terms a CAS Array is a set of Exchange 2010 CAS servers that are load balanced using either Windows Network Load Balancing, Microsoft ISA Server or a third party load balancing appliance. For the vast majority of the CAS functions this is very similar to load balancing the CAS role in an Exchange 2007 Organization.

However with the advent of Exchange Server 2010 the MAPI endpoint has also now been moved onto the CAS role and is no longer on the Mailbox role as was the case with all previous versions of Exchange Server. Without configuring a CAS Array the Outlook client will connect to a single CAS server within the Active Directory Site and the connection would fail should that CAS server go offline.

So the array configuration takes two steps which are:

1. Configure the CAS servers in a Windows Network Load Balancing cluster.
2. Configure the CAS Array in Exchange.

It is also worth mentioning now that with Exchange 2010 that what we have known in the past for Exchange clustering has now been replaced by Database Availability Groups (DAG) and that CAS and HUB roles can now be collocated with the Mailbox role when a DAG is configured. However DAGs still utilize components of Windows failover-clustering and as such should not be mixed with the Network Load Balancing that comes with Windows Server. In these instances Microsoft ISA server or a 3rd party appliance would have to be used to provide the load balancing functionality and the CAS Array configured to point to these devices.

Step 1 – Configuring the NLB

I am not going to re-invent the wheel here as Henrik Walther already has a very good article on creating an NLB for CAS servers here. I created an NLB with two Exchange 2010 CAS servers using the FQDN internalarray2.2k10.dom as the NLB cluster address.

The only difference that I made over what Henrik Walther describes for the NLB setup is that in order to accommodate MAPI connectivity on the CAS role through the NLB I added in a port range for TCP port 135 to expose the RPC endpoints and for the port range TCP 1024-65535 as I did not know which port the MAPI endpoint was being published or if the port used by the Exchange 2010 CAS server for MAPI can be fixed.

Step 2 – Configuring the CAS Array

Configuring the CAS Array is quite straight forward and can be done with few Powershell commands.

1. Open up Exchange 2010 Powershell
2.Type in this command to create the CAS Array in the Exchange organization

syntax : new-clientaccessarray –name <name of array> –fqdn <DNS FQDN of the Array>

Example : new-clientaccessarray –name “InternalArray1” –fqdn internalarray1.2k10.dom

It is worth noting here that there can be only one CAS array configured per Active Directory Site. Also while creating a new Mailbox database in the AD site, the value for the CAS Array was automatically picked up.

3. The next Powershell command to run sets an attribute on the mailbox database its self. This attribute is used to tell connecting clients which FQDN they should connect to in order to access this database.

This example sets the value on all mailbox databases…

syntax : get-mailboxdatabase | set-mailboxdatabase –rpcclientaccessserver <DNS FQDN of the Array>

Example : get-mailboxdatabase | set-mailboxdatabase –rpcclientaccessserver internalarray1.2k10.dom

4. Finally the setting can be checked using Powershell as can be seen in the above example

Example : get-mailboxdatabase | select name,rpcclientaccessserver | fl

Checking client Access

The last step is to verify that client access is using the new URL so I created a new Microsoft Outlook profile using the CAS Array FQDN as the server name and typed in the details of a test user. As can be seen in the screen shot below, the profile creation was successful.

This article was written based on Microsoft Exchange Server 2010 Beta 1 and as such is subject to possible change as software features may be added or removed.