Configure Client Access Server Array Exchange 2010 using WNLB

In this blog im going to configure client access server array, Previously MAPI Clients used to connect to direct Mailbox Server. But with Microsoft Exchange 2010 all the MAPI Users are now connected to Client Access Server.

Every Mailbox database is associated with Client Access Server service called RPC Client Access

Microsoft Recommends for CAS array Load Balancing with WNLB or Hardware NLB.

Note: You cannot do WNLB with DAG Failover cluster  on the same server

As you can refer the following KB article

We have option to use either WNLB or Hardware Load Balancers. In this scenario im going to configure CAS Server with Windows Network Load balancing Feature in Windows 2008 R2.

Actually Windows Network Load Balance come with the OS so its very reasonable and easy to configure, It can be a good solution for mid-sized companies wanting a cheap load balancing solution.

Before we start the WNLB we should make sure the IP settings in the server

Each Cas server should have minimum 2 Network Connections connected to LAN,

Configure the network adapter with Static Ips


Next Step to install and configure WNLB.

  1. Add the roles from Server Manager WNLB

Now we need to create the cluster in WNLB.

Create New Cluster


In New cluster wizard we need to enter the Cluster name , In host section we need to enter the ip of our Exchange Client access server node 1 click connect , The network adapters will be displayed select NLB adapter next.


Note: Priority value should be configured as 1 for the first Exchange server member

Same settings we need to configure for Second Exchange server to add NLB adapter as 2 priority.

Enter the Full Server Name:  for Ex :

Note: This is Virtual Name where all outlook users are going to connect to use since we have created FQDN for CAS array

 Cluster operation mode:

Unicast mode

The unicast mode changes the cluster adapter’s MAC address to the cluster MAC address. This cluster address is the same MAC address that is used on all cluster hosts. When this change is made, clients can no longer address the cluster adapters by their original MAC addresses.

 Multicast mode

When using multicast mode, NLB will add a multicast MAC access to the cluster adapters on all of the cluster hosts. At the same time, the cluster adapters retain their original MAC addresses. This way each host could be addressed individually

By using the multicast method with Internet Group Membership Protocol (IGMP), you can limit switch flooding, if the switch supports IGMP snooping. IGMP snooping allows the switch to examine the contents of multicast packets and associate a port with a multicast address. Without IGMP snooping, switches might require additional configuration to tell the switch which ports to use for the multicast traffic. Otherwise, switch flooding occurs, as with the unicast method.

For more info you can refer TechNet

Well, a best practice recommendation is to install two network adapters and use unicast mode, so that the host and cluster network traffic are separated on their own respective network interface. However, if you only have the option of installing one NIC in each CAS server or if you’re forced to using multi-cast mode because of the switches used in your organization, you should pick multicast mode.

Next step

Default  Settings for Ports rules,


To improve the security we should edit port and rules for certain protocols to allow access to Client access server.

Once we are done we need to add the host machines in the existing cluster.

Imp Note : By Default windows 2008 disable Ip forwarding so it will cause problem for WNLB we can enable by using

Refer this article for enabling the IP forwaring.

netsh interface ipv4 set interface “NLB” forwarding=enabled


Now lets Create CAS Array

Once we are done with the MS Exchange 2010 CAS setup we need to create CAS Array in order to make it one.

First Step.

Open the Exchange Management Shell for creating cas array

Run below Cmdlet

New-ClientAccessArray -Name CASArray -FQDN “” -Site “Default-First-Site-Name”

Name Parameter Specify the Client access Array name

Site Parameter Specify the active directory site which belongs to Client access server

Note  : After the Client Access array has been created, you’ll also need to create the address in DNS and associate it with the virtual IP address used for the Client Access array.

Once we are done we need to associate RPCClientAccessServer with Mailbox database on which clients will connect

Using CmdLets

Adding Mailbox Databases to Client Access Server Array

Set-MailboxDatabase MDB03 -RpcClientAccessServer “”

Verifying the Mailbox Database RPC Settings using Get-MailboxDatabase

Now our outlook clients will connect to NLB Cluster name

We can verify the Server connection from Outlook by using Press Ctrl + Right Click Outlook icon in system tray and check the

Connection status.

Hope this article helps you thanks for viewing.


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