Summary

Exchange Server 2003 computers can be clustered to provide a highly available solution in environments demanding near 100 percent messaging system uptime. Clustering is not a feature of Exchange Server 2003 itself but instead a feature provided by the operating system: Windows 2000 Server or Windows Server 2003. Exchange, however, is clustering aware, meaning that it can recognize that it is being installed into a cluster and will configure itself appropriately during the installation to support operation in this environment.

Windows Server 2003 provides support for two different clustering methods: Network Load Balancing and the Microsoft Clustering Service. Each of these methods has different purposes and, as you might expect, different hardware requirements.

Network Load Balancing is installed on all participating members as an additional network interface driver that uses a mathematical algorithm to equally distribute incoming requests to all members of the NLB cluster. Incoming client requests are assigned to cluster members based on their current loading level but can be modified through the use of filtering and an affinity for applications that require session state data to be maintained , such as an e-commerce application that uses cookies to place items in a shopping cart for purchase.

The Microsoft Clustering Service is the second clustering method available in Windows Server 2003 and the only one that Exchange Server 2003 supports. MSCS provides for highly available server solutions through a process known as failover. An MSCS cluster consists of two more nodes (members) that are configured such that upon the failure of one node, any of the remaining cluster nodes can transfer the failed node ‚ s resources to itself, thus keeping the resources available for client access. During this time, clients will see little, if any, interruption in service as the resources failover from the nonresponsive node to a remaining functional node.

Clusters using the Microsoft Clustering Service can be created in one of two clustering modes: Active/Active and Active/Passive. When using Exchange Server 2003 on Windows Server 2003 Enterprise Edition or Datacenter Edition, you can create up to eight-node Active/Passive clusters.

When Active/Passive clustering is used, a cluster can contain between two and eight nodes. At least one node must be active and at least one node must be passive. You can have any other combination as well, as long as you meet these minimum requirements. Due to its flexibility and lack of restriction, the Active/Passive clustering mode is the preferred mode of operation for Exchange Server clusters.

When Active/Active clustering is used, each node in the cluster runs one instance of the clustered service. Should a failure of the clustered service occur, that instance is transferred to the other active node. Although you will be able to use both nodes of the cluster in Active/Active mode, you are limited in how many storage groups and Exchange mailboxes each node can host because a single Exchange server can hold only four production storage groups. In addition to this restriction, the nodes in an Active/Active cluster can have only 1,900 active mailboxes each, which is several thousand less than an Exchange server might typically be able to hold.

You should deploy your Exchange servers in a front-end/back-end configuration for increased security and greater flexibility. The front-end servers, which are located in your screened subnet, provide only a secure connection point for those users located on the Internet to connect to your Exchange organization. SSL is used to secure the traffic in this connection. Front-end servers then pass information back and forth between back-end servers and clients. Therefore, clients never directly connect to a back-end server, which is located in your private, protected network. IPSec can be used to secure the connections between the front-end servers and back-end servers, domain controllers, and global catalog servers for further security. Front-end servers can also be set up in a Network Load Balancing cluster for improved availability.