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Microsoft releases Exchange Server updates every quarter. These updates include many fixes, but they can also break things.
Microsoft calls these quarterly releases cumulative updates. Each version lists the set of bugs the company fixed. Each cumulative update includes all the previous updates, so you only need the latest cumulative update with a new Exchange Server deployment to get caught up. A good rule of thumb is to keep Exchange one or two versions behind and test a rollup update for legacy versions or a cumulative update for newer versions of Exchange.
A cumulative update is a complete server build with enhancements in functionality that Microsoft started with Exchange 2013. A rollup update, also released each quarter, consists of new security updates, as well as security fixes from earlier rollups for Exchange 2010.
With each cumulative update or rollup release, there might be a new prerequisite, such as a newer .NET Framework version. Some admins don't update immediately when Microsoft releases a patch because they prefer to stay on a stable version.
This is where the challenges come in. Some admins think they need to install every rollup or cumulative update. If, for example, an Exchange 2013 system is on cumulative update 15, but cumulative update 21 is available, there is no need to install each cumulative update one by one until you reach the current version. You can go to the latest or the previous version. Microsoft only keeps two versions of rollups and cumulative updates available to download, so you can't download the old ones anyway.
Another challenge occurs when the update instructions say that the Exchange system needs to be on a certain cumulative update with a specific .NET Framework version before you can install the latest cumulative update. It might sound like a Catch-22 if Microsoft has already moved the specific cumulative update. Don't let that hold you back. Install the required version of the .NET Framework, and, after the reboot, install the new cumulative update. You must also be on the correct version of an Exchange rollup or cumulative update to get support from Microsoft.
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