This post explains what happens once Outlook connects to the Exchange Client Access Server (CAS), how it accesses mailboxes and the Address Book service, and how it connects to internal URLs like Out of Office (OOF), Offline Address Book (OAB), and the Availability service.

Outlook 2007/2010 needs to connect to RPC and address book (AB) ports at the Exchange CAS server to access mailboxes and address books through the Global Catalog Server. In a load balancer environment, we need to make sure that we configure static ports for the Address Book and RPC services. This is important because if we don’t do so, the CAS server can assign any dynamic RPC port range (6005-59530) for its outgoing Outlook connection. If no static ports are configured on the CAS server, then the load balancer has to be configured to open with all dynamic ports in the range from 6005 to 59530. Configuring Exchange CAS servers with static ports for the AB and RPC services avoids this scenario, makes configuration faster, and improves Outlook performance.

See the Microsoft TechNet article for steps for configuring static ports for RPC and AB on a CAS server.

Let’s see how the Outlook client accesses a mailbox using these static ports.

  1. When Outlook starts, it talks to the Autodiscover service and requests a configuration XML file. This XML file will contain details such as the user’s display name, internal and external connection settings, location of the user’s mailbox, Outlook Anywhere settings, and the URLs of OOF, OAB, Availability service, and so on configured on the CAS servers.
  2. Outlook uses the details in the XML file to talk to one of the Exchange 2010 CAS servers behind the load balancer and connect the RPC Endpoint mapper on port 135 of the CAS server.
  3. Outlook requests the RPC and AB port numbers from the CAS server to access the mailbox and address book.
  4. If the CAS server is configured with RPC and AB static ports, it will return the same; otherwise, it returns anything between ports 6005 and 59530 for the RPC and AB services.
  5. Outlook uses these ports to establish a connection with the mailbox and address book.
  6. Outlook also uses other internal URLs from the XML file to access other services such as OAB, EWS, and the Availability service.

Registry changes have to be made on all CAS servers in the Active Directory site. If there are multiple AD sites, this Registry configuration has to be updated on all CAS servers in every AD site. Microsoft recommends setting this to a unique Registry value between 59531 and 60554. Once configurations are made, the “Microsoft Exchange RPC” and “Microsoft Exchange Address Book” services should be restarted in order to apply changes. It should be fairly simple to configure the load balancer; the vendor’s documentation should have details on how to do so. Once you have this configured, you can verify the connection using the tool netstat at the command prompt. Below is an example to check if the ports are listed on CAS servers and if static ports 59531 and 59532 are configured for the RPC and AB services, respectively.

netstat -an -p tcp | findstr 59531
netstat -an -p tcp | findstr 59532

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I hope you find this article informative and helpful in understanding Outlook connections to RPC and AB service ports on Exchange 2010 CAS servers.

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