Updating the Windows Server Operating System in a Disaster Recovery configuration can be performed without destruction of the DR pair. Cross-Operating System support within a DR pair is supported to allow you to step through the upgrades without a major outage.
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Validate Disaster Recovery Health prior to beginning any OS upgrades.
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Ensure the Primary and Secondary Disaster Recovery units are both at an equal and current patch level for Windows Operating System and SQL Server.
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Upgrade the Operating System on the Secondary unit in the Disaster Recovery pair (non-active).
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Apply latest Windows and SQL patches to Secondary unit to get to latest patch level for the new OS version.
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Verify .NET 4.7+ is installed and patched on the Secondary unit.
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Perform a Planned Disaster Recovery Failover from Primary to Secondary, making the recently upgraded server active.
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Upgrade the Operating System on the Primary unit in the disaster recovery pair (non-active following failover).
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Apply latest Windows and SQL patches to Primary unit to get to latest patch level for the new OS version
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Verify .NET 4.7+ is installed and patched on the Primary unit.
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Perform a Planned Disaster Recovery Failover from Secondary to Primary, returning the DR pair to a “normal” state.
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Verify both hosts have a matching local security policy for Encryption types allowed for Kerberos to ensure AD based authentication will function. You can check this under Local Security Policy > Network Security: Configure encryption types allowed for Kerberos.