Microsoft recommends that you have one front-end Exchange server for every four back-end Exchange servers. Having one front-end server for every four back-end servers is a good recommendation, but I think you should use it as a general guideline rather than a hard and fast rule.
For example, if you have 12 back-end Exchange servers, but only a couple hundred users who use Outlook Web Access (OWA), does it really make sense to have the recommended three front-end servers? In reality, you could probably easily get by with two.
On the flip side, I have seen organizations with only one or two back-end servers, but almost everybody accesses Exchange Server through OWA. If that's your scenario, you may need more front-end servers.
In my opinion, the best thing you can do is use Performance Monitor to test each node in your front-end Exchange Server cluster to see whether you have enough servers to adequately handle the workload.
If you determine that your front-end Exchange Server cluster is going to require more than 32 nodes, you will need to use an alternative type of cluster, such as a hardware-based cluster, which is beyond the scope of this tutorial.
Cluster configuration consistency
When you cluster Internet Information Servers, each server functions as an independent entity (at least to some extent). As such, you must ensure that each IIS server in the cluster is configured identically. Otherwise, users may not have a consistent experience when they use OWA.
Suppose that OWA existed in a custom virtual directory named Mail. The Mail virtual directory would then have to exist on each server within the cluster. Otherwise, OWA would be inaccessible to some users. Active OWA sessions might also suddenly stop working for some users.
Service Pack version consistency
Maintaining consistent versions of Service Packs is also important. Let's pretend that all front-end cluster servers are running Exchange Server 2003 Service Pack 2, and Service Pack 3 is being released tomorrow with some sort of change to the OWA user interface.
Of course, Exchange Server 2003 Service Pack 3 is a long way off, and I have no idea what it's going to do. But, if you were to load Exchange 2003 SP3 onto some but not all front-end Exchange servers, users would have an inconsistent experience, depending on which front-end server serviced their requests. The layout of the OWA user interface could potentially change from one screen to the next.
Lastly, I have seen several Exchange Server deployments in which IP filtering is used to make sure that only the front-end Exchange server is allowed to communicate with the back-end Exchange server over a particular interface. If such a security mechanism is used, you may have to create rules that allow each server in the cluster to access the back-end server.
Whether or not you have to do this depends on your cluster setup. For example, if you are using a DNS round-robin cluster, each server in the cluster will have a unique IP address. The DNS server rotates which IP address (and therefore which server) is assigned to the organization's domain name at a given moment. That being the case, the back-end servers must be able to communicate with each IP address in use on the front end.
TUTORIAL: HOW TO SET UP A FRONT-END EXCHANGE SERVER CLUSTER
A lesson in cluster node configuration and consistency
Pros and cons of a Network Load Balancing Service front-end cluster
Requirements for a Network Load Balancing Service front-end Exchange cluster
How to set up a Network Load Balancing Service front-end Exchange cluster
The pros and cons of a DNS-based front-end Exchange cluster
How to set up a DNS-based front-end Exchange cluster
Related links from SearchExchange.com
|ABOUT THE AUTHOR:|
| Brien M. Posey, MCSE
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Exchange Server, and has previously received Microsoft's MVP award for Windows Server and Internet Information Server (IIS). Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at http://www.brienposey.com.