Explanation/Reference:
Explanation:
Starting with the update to Microsoft SQL Server 2012 Service Pack 1, customers with SQL Server 2012 failover cluster instances can greatly benefit from the fully automated update process provided by Cluster- Aware Updating (CAU).
CAU causes minimal down time for SQL Server 2012 client connectivity. While the primary node is being updated, the SQL Server clustered role (also called the resource group) goes through two failovers. The first failover happens before updating the primary node when CAU brings the original primary node into maintenance mode. During the update process on the original primary node, the SQL clustered role is temporarily brought online on a new primary node. The second failover happens once the update process is complete on the original primary node, and CAU fails back the SQL Server clustered role to the original primary node and brings it online.
Reference:
http://technet.microsoft.com/en-us/library/ms191009.aspx
http://technet.microsoft.com/en-us/library/ms191295.aspx