Posted by: Anthony Drewery | January 18, 2006

Load balancing Exchange front-end servers

If you have an enterprise and/or high volume environment you may want to consider load balancing your front-end servers to improve resilience and performance. In the current organisation that I manage we use three HP blade servers as load balanced front-end boxes in each routing group. We use the load-balanced addresses for offering OWA, HTTP-RPC and ActiveSync, as well as for SMTP and POP3.

Microsoft provide an excellent FAQ on load balancing with Windows 2000/2003 which includes information on configuration and best practices.

Some points worth considering:

  • You need to specify the ports that you wish to load balance so if you are going to use it for SMTP, POP3, HTTPS etc you’ll need to know the port numbers. You can configure a range of ports but I prefer to be specific.
  • A host entry for the load balanced address is not automatically created in DNS.
  • Your routing group and SMTP connectors don’t support the load balanced address as a bridgehead so you’ll have to specify your front-end servers individually.
  • As touched on in a previous post you may have issues when accessing a load balanced address through a firewall. Specifically, one particular server is favoured and if it goes offline then the other nodes do not take over. These issues have been documented by Cisco.


  1. […] Keeping the above in mind the front-end/back-end principle works perfectly. We have several shipping applications that use POP3 to access mailboxes. By pointing them to the network load balanced address of our front-end servers we don’t need to reconfigure the application if we move the mailboxes around. We still have to follow strict change control with any move but it makes our lives and those of the application admins a whole lot easier. […]

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