Hi,
Windows Server 201 2R2 and Windows Server 2016 nodes can run in the same Cluster as long as it is at the 2012 R2 Cluster functional level.
Windows Server 2016 and Windows Server 2019 hosts can run in the same cluster as long as it is at the 2016 Cluster functional level. These are the
only combinations you can have when there is a mix of OS's.
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Upgrading the operating system on an existing failover cluster is only supported when going from Windows Server 2016 to Windows 2019.
If the failover cluster is running an earlier version, such as Windows Server 2012 R2 and earlier, upgrading while the cluster services are running will not allow joining nodes together. If using the same hardware, steps can be taken to get it to the newer
version.
You'll find more information here:
Upgrading Failover Clusters on the same hardware
Best regards,
Leon
Blog:
https://thesystemcenterblog.com
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