A Bridge Too Far…. Exchange 2013 CU15

Update – 2/15/2018
There may be a change in the way that Microsoft will support upgrades from older CU’s to the latest CU of Exchange 2013 or 2016. If we review the Exchange Server Supportability Matrix we see that in the .NET section, there is this note:

    “When upgrading Exchange from an unsupported CU to the current CU and no intermediate CUs are available, you should upgrade to the latest version of .NET that’s supported by Exchange first and then immediately upgrade to the current CU. This method doesn’t replace the need to keep your Exchange servers up to date and on the latest, supported, CU. Microsoft makes no claim that an upgrade failure will not occur using this method, which may result in the need to contact Microsoft Support Services. “

Reading this it tells me that while an upgrade from an old, unsupported CU to a newer supporter is allowed, there is no guarantee it will work. If it fails, a call to Microsoft support is needed. This clarification seems to eliminate the need for the Bridge CU (CU15) and may allow for a direct path for an upgrade. I would take this note with a grain of salt as there is no guarantee it would work. This upgrade path is a hard one for Microsoft to predict as there are quite a few CUs for Exchange 2013 now and there are endless possibilities for this to fail when upgrading .NET and then upgrading to the latest CU. Proceed with caution.
Update – 11/15/2017
So a client of mine is upgrading Exchange 2013 from CU5 to CU18 this weekend. He asked what he could do to prep in order to reduce our upgrade time. The obvious one is to download all update beforehand and stage them on the servers to be upgraded. However, he was only able to get CU18 as CU15 is no longer available. So the bridge has washed out. If you need this update, you may be able to contact Microsoft? By YMMV on this one.
End of Update
Update – 9/29/2017
It appears that Microsoft has decided to leave the CU15 update as downloadable for now. Even though, as of this update, CU18 is out and only CU16 to CU18 should be out, CU15 is still available. CU14, however, has joined the ranks of the removed downloads…. I wonder how long this will last?
End of Update
For those of you who support Exchange 2013, you may be aware of the availability of CUs for Exchange 2013. If you are not, then simply put, when a new CU comes out, only that CU and the previous CU are available for Public download. There will be some interim time when the previous CU (CU-2) is also available, but that too shall pass. Currently Microsoft is now up to CU17 for Exchange 2013.

    CU – Cumulative Update – Microsoft’s quarterly update for Exchange server

What this means is that CU17 and CU16 will eventually be the only updates for Exchange 2013 out there. While CU15 is still available at the time of this post, that again shall disappear.
The Bridge
If you are running any version of Exchange 2013 that is CU14 or lower, this is going to cause you some headache if you need to update to the latest CU. Microsoft put in a .NET requirements that makes it impossible to go directly from CU14 and before to CU16 or CU17. These latest Cumulative Updates require .NET 4.6.2 BEFORE you can install them. However, CU14 and before only support up to .NET 4.6.1. See the Exchange 2013 Supportability Matrix for more information on what is supported.
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