Active/Passive configurations are enforced in clusters with more than two nodes (more than two Exchange nodes in Exchange 2003). This means that the scaling and monitoring limitations required for two-node Active/Active do not apply to clusters with more than two nodes. In clusters with more than two nodes, the interface prevents creating additional EVS such that there would not be at least one passive node.
Exchange clusters with more than two nodes also prevent multiple EVS from coming online on the same node concurrently. This can affect your failure planning. For instance, if you design a four-node A/A/A/P cluster and a single node is lost, the affected EVS will relocate successfully to the Passive node. However, if another node should fail, there will not be any passive node to receive the failed EVS and the EVS will go offline – potentially causing a service interruption. Due to this behavior, you may wish to consider designing clusters that consist of multiple passive nodes (A/A/P/P or A/A/A/A/A/P/P/P are two example designs).