If I have a NLB or hardware load balancer – why do I need a CAS array?

This is an excellent question and it’s all about client connectivity, transparency of the connection to the user and automagic cutover in the event of a CAS array member failing. this becoems even more important now that ALL client connections including RPC MAPI connections are now going through the CAS role. Heck if you look at the MAPI account config for an exchange mailbox user it now shows the CAS server name, not the Mailbox role server name. To avoid client reconfiguration or disconnection and forcing a re-query for available CAS boxes, we have CAS arrays to assist in this.

The following information was found in this TechNet page Understanding RPC Client Access. NOTE – this is for RPC high availability, if you still want OWA / EAS  / ECP high availability MS recommends a solution like Forefront Threat Management Gateway.

When a Client Access server array is defined in an Active Directory site, it serves as a single contact point for all client connections within that Active Directory site. A Client Access server array can include one or many Client Access servers.

Each Active Directory site can have a single Client Access server array. A Client Access server array doesn’t provide load balancing. A separate load balancing solution is still needed. For more information about load balancing, see Understanding Load Balancing in Exchange 2010.

Microsoft recommends that you create a Client Access server array even if you only have a single Client Access server within your organization. When a Client Access server array is created, clients connect through the virtual name of the Client Access server array rather than directly to the fully-qualified domain name (FQDN) of your single Client Access server. If a single Client Access server needs to be replaced within an Active Directory site or a second Client Access server is added, no profile updates are necessary on the clients.

After a Client Access server array is defined within an Active Directory site, all Client Access servers within that Active Directory site are automatically part of the Client Access server array.

The high level steps are here..

  1. Create a Client Access array
  2. Configure load balancing
  3. Configure IP ports
  4. Configure RPC encryption settings
  5. Configure your Mailbox databases
  6. Ensure low latency and sufficient network speed

     Create a Client Access Array

You can create a Client Access array within your Active Directory site by using the following command.

New-ClientAccessArray -Name name -Site site_name -FQDN internal_only_CAS_Array_FQDN


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.

It’s important that the (FQDN) specified in the command be only resolvable internally. If the name is also resolvable externally, these external clients will attempt to connect to the array via a TCP connection instead of HTTPS.

  Configure Load Balancing

Load balancing is recommended for high availability, failover, and for spreading the traffic load over multiple servers to help performance. When you choose a load balancing solution, consider the following:

  • Windows Network Load Balancing isn’t supported on Windows failover cluster servers.
  • You can’t use a Client Access array across multiple Active Directory sites. Instead, create two Client Access arrays and load balance separately within the sites.
  • Hardware load balancers typically monitor return traffic, port availability, or service availability to ensure that servers that can’t answer client requests aren’t given network connections.
  • Some load balancing solutions, such as ISA 2006 or TMG 2010, can’t do RPC load balancing or monitor RPC services. These solutions aren’t recommended unless all clients are connecting via Outlook Anywhere and all traffic is encapsulated inside HTTP.

For more information about load balancing, see Understanding Load Balancing in Exchange 2010.

Configure IP Ports

An IP port is an opening through which information can pass from the originating computer to the destination computer. By default, the dynamic port range for outgoing connections on Windows Server 2008 R2 is 49152 to 65535. Exchange 2010 Client Access changes this range to 6005 through 59530. The range was expanded to provide sufficient scaling for large deployments. This is a large range of ports to balance through your firewall between the client and the Client Access servers or Client Access array.

By fixing the MAPI and directory endpoints, you can greatly reduce the number of ports that need to be load balanced. The MAPI endpoint can be statically configured in the registry and the directory endpoint can be fixed in a configuration file.

To fix the MAPI endpoint, use the following setting in the registry.

HKLMSYSTEMCurrentControlSet ServicesMSExchangeRPCParametersSystemTCP/IP Port [DWORD] is the value for the IP port to use

To fix the directory services endpoint, edit the RpcTcpPort value in the configuration file Microsoft.Exchange.AddressBook.Service.Exe.config.


Microsoft doesn’t recommend that you change the default value of the Outlook Anywhere ports.


  Configure RPC Encryption Settings

In Exchange 2010, the RPC endpoint is encrypted by default. However, Outlook 2003 doesn’t enforce encrypted MAPI connections. When you upgrade your organization to Exchange 2010, your clients running Outlook 2007 or later versions will automatically be compatible with the change to RPC Client Access, since they support RPC encryption by default. Outlook 2003 doesn’t use RPC encryption, however, and RPC Client Access requires it by default. If you haven’t turned off RPC encryption, which we don’t recommend, your users will need to configure Outlook 2003 for RPC encryption or you’ll need to use a Group Policy to force Outlook 2003 to use RPC encryption.

Symptoms of this problem include the following error messages:

  • Cannot start Microsoft Office Outlook. Unable to open the Office window. The set of folders could not be opened.
  • Unable to open your default e-mail folders. The information store could not be opened.

If your users are using Cached Exchange Mode, Office won’t display an error, but will start in disconnected mode.

For more information about this issue, including workarounds, see Outlook Connection Issues with Exchange 2010 Mailboxes.


  Configure Outlook 2003 to Use RPC Encryption

To configure Outlook 2003 to use RPC encryption, use the following steps.

  1. Click Tools > E-Mail Accounts > View or Change an Existing Account.
  2. Select the account and click More Settings.
  3. Select the Security tab.
  4. Select Encrypt data between Microsoft Office Outlook and Microsoft Exchange Server.
  5. Click OK

In addition to the RPC encryption requirement, UDP notification support was removed from Exchange 2010. As a result, Outlook 2003 can only use polling notifications in online mode. This will result in a slight delay in updates to item status (30 seconds on average with up to a one-minute delay) when changes are made to items in a mailbox accessed by Outlook 2003. There are two workarounds for this issue:

  • Use Outlook 2003 in Cached Exchange Mode.
  • Adjust the polling interval on the Client Access server. This will impact the performance of the Client Access server.

For more information about this issue, see E-mail messages take a long time to send and receive.

  Configure Your Mailbox Database

Each Mailbox database contains an RPCClientAccessServer value. This value is established when the database is created and it determines the Client Access server or Client Access array that the clients with mailboxes on that Mailbox server will use. This value also determines the location of the RPC end point. For Outlook 2007 and Outlook 2010 clients, this value is obtained from the Autodiscover service.

The default value of the RPCClientAccessServer is determined by the following rules:

  • If you have configured a Client Access Server array within your Active Directory site, the address of that array will be used.
  • If an array does not exist within the Active Directory site and if you have both the Client Access server role and the Mailbox server role on the same physical server, the value of RPCClientAccessServer property for a particular Mailbox server will be the same as the Mailbox server.
  • Otherwise, the value of the RPCClientAccessServer property for a particular Mailbox server will be set to a random Client Access server within the Active Directory site.


    We don’t recommend that you install all the server roles on a single computer that’s also a domain controller. Although this configuration is supported, it’s not recommended.

  • If you created a Mailbox database before the creation of a Client Access array or the installed a Client Access server within the Active Directory site, you’ll need to reconfigure the value of the RPCClientAccessServer property. If no Client Access server exists in the Active Directory site when the Mailbox database is created, the value of the RPCClientAccessServer property will be set to the FQDN of the Mailbox server. To configure the value of the RPCClientAccessServer property, use the following command.
    Set-MailboxDatabase <name> -RPCClientAccessServer <internal_only_CAS_Array_FQDN>

  Latency and Bandwidth Requirements

For users running Outlook without Cached Exchange Mode, high latency times between the client and the server directly affect how frequently Outlook is unresponsive. In general, a latency of greater than 200 milliseconds (ms) to the home Mailbox server will result in poor client performance.

Because latency between the Client Access server and the mailbox should be less than 10 ms, we recommend that the value of the RPCClientAccessServer property always be configured to a Client Access array in the active Mailbox database site.


Changing the value of the RPCClientAccessServer property will force all clients to reconnect.

  Configuring the Address Book Service

The Address Book service is configured through the Microsoft.Exchange.AddressBook.Service.config file. This file allows you to configure the following:

  • The number of concurrent connections per user (the default limit is 50).
  • Disable or enable logging.
  • The location, size, and retention period for the log files.

To set the value of the maximum number of sessions per user, use the following value: <add key=“MaxSessionsPerUser” value=“50” />

To enable logging, use the following value: <add key="ProtocolLoggingEnabled" value="true" />

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