Exchange High Availability
Exchange Server high-availability options and features have changed with the evolution of the Exchange product. Microsoft went from stand-alone servers to two-node clustering, and over time and new releases of the Exchange product, it offered four-node clustering.
Exchange Server 2003 and the Windows Server product increased the clustered node numbers to eight nodes, with a seven-active-and-one-passive clustered configuration. Although clustering could provide server and service availability, it was still not considered by the industry as a true highly available solution.
In a true highly available solution, you will find the following characteristics: