With the release of Exchange 2013 Microsoft has adopted a new update process. It’s been almost two and a half years that this update process has been in place, and there is still a considerable lack of understanding in the community about the process. There is so little understanding that I’ve seen conversations between several different groups of really smart Exchange experts this week that included tons of misinformation and misunderstanding. In this post I am going to try to clear up all the questions and lay it all out for you.

Exchange cumulative updates (CUs) are scheduled to be released quarterly. That means there should be four CUs released each calendar year, but it does not mean that a new CU will be release every three months. Microsoft may release a CU at any point during the quarter in which it is scheduled. It may happen that CU n comes out on the first day of the quarter and CU n+1 comes out on the last day of the following quarter, meaning nearly 6 months between CU releases. Microsoft’s priority is to release a new CU when they are ready rather than hitting an arbitrary schedule.

Exchange CUs are full installs of Exchange 2013. You can download and install a new Exchange 2013 server from any CU, or you can apply the same CU download to any existing Exchange 2013 server.

It is supported to install any CU over any running CU. It is supported by Microsoft to install CU7 right on an Exchange 2013 server that is running Exchange 2013 RTM. In the last sentence I used the word “supported”. When talking about Microsoft software supported can be a bit of a sticky wicket. Supported can mean several different things in several different circumstances. In this case it means Microsoft will take your support call and work to resolve your issue without requiring that you get to some different “supported configuration” before proceeding. In this case supported does not mean the upgrade from any CU to any other CU has been tested. The information I have been able to gather from the Exchange team is that they test each new CU with the last couple of CUs. This means upgrades to CU7 are tested from CU6 and CU5, but probably not from CU4 or earlier.

Each CU is very likely to include schema updates. I believe every Exchange 2013 CU to date has included schema updates, and I expect that trend to continue going forward.

There is no requirement that all Exchange 2013 servers in an organization be running the same CU, but it is recommended.

The supported version of Exchange is CU n-1. This means that if you call Microsoft for an Exchange support issue they reserve the right to ask you to update to the current CU or one CU back to resolve the issue.

Did I miss anything? If you still have questions about the CU process for Exchange, let me know in the comments below.