Rolling upgrades let you update software on a passive node in a Microsoft cluster while other nodes in the cluster are actively hosting Exchange Virtual Servers (EVSs). Rolling upgrades are useful for reducing planned downtime for software upgrades such as security patches, service packs, and layered software (e.g., monitoring software). This type of upgrade is also useful for upgrading the server BIOS or firmware. Rolling upgrades do have some limitations: Some third-party products, such as virus scanners, don't support rolling upgrades, and updates must be performed on an active node while taking the EVS offline. Plus, failovers might not function correctly until all nodes in the cluster run at the same software-revision level.
Exchange 2000 Server lets you upgrade service packs using a rolling upgrade model. The following is a summary of the sequence of events to upgrade a two-node, active/passive cluster with an active Node1 and passive Node2, hosting one EVS (EVS1).
Step 3 is optional because at this stage all nodes in the cluster are running the new service pack. However, best practice is to perform a planned failover back to Node1 to verify that the service pack has been applied correctly to Node1 and that failover operates correctly.