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The pros and cons of a Network Load Balancing Service front-end Exchange Server cluster

Tutorial: How to set up a front-end Exchange Server cluster -- part 2 of 7.

The two most common methods for clustering front-end Exchange servers are network load balancing, and DNS-based load balancing. Both methods have their place, but I have a slight preference for using the Network Load Balancing Service (NLBS), because it offers a higher degree of fault tolerance.

NLBS allows you to create a cluster of between two and 32 Web servers. Each of those Web servers has its own computer name and static IP address (static IP addresses are a requirement and the addresses must belong to a common subnet); it also has a name and an IP address that it shares with every other node in the cluster.

Normally, if two or more computers on a network have the same IP address, they can't communicate on the network because of the IP address conflict. In this case though, NLBS prevents a conflict from ever occurring.

When you implement NLBS, the DNS server contains a record that associates the shared host name used by clustered nodes with the shared IP address. Whenever traffic is sent to that IP address, NLBS intercepts the traffic and routes it to whichever node in the cluster is least busy -- by referencing the server's individual IP address rather than its shared address (actually, there is a rather complex routing algorithm, but I will spare you the gory details).

NLBS monitors the nodes in the cluster, not only to tell how busy they are, but also to make sure that all nodes are online. If a cluster node becomes unavailable, NLBS will automatically shift that node's workload over to the other cluster nodes. When the node comes back online, it can pick up where it left off as though nothing ever happened.

Another benefit of NLBS is that it's expandable. You can always start out with a simple, two-node front-end Exchange Server cluster. Later, as your organization grows, you can add more nodes, and NLBS will dynamically adjust itself to accommodate them.

The only drawback to a Network Load Balancing Service front-end Exchange Server cluster is that it's more complicated to configure than a DNS-based front-end Exchange Server cluster.


 Home: Introduction
 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
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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

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