Active/passive clustering has one or more nodes in the cluster that is online providing services. The other nodes in the cluster are online but do not provide any services or applications to clients.
Active/active clustering has one instance of the clustered service or application running on each node in the cluster. If a failure of a node occurs, that instance is transferred to one of the running nodes.
An active/passive cluster must contain at least one active node and at least one passive node however, you cannot exceed eight nodes total.
Clusters on Windows 2000 require SPF4. Only Exchange Server 2003, Enterprise Edition supports clustering.
Exchange Server 2003 supports two-node active/active clusters and up to eight-node active/passive clusters.