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Microsoft to Fix New Bug Breaking Hyper-V VMs in Windows Server

Rabia Noureen

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Earlier this week, Microsoft released the December 2022 Patch Tuesday updates for various supported versions of Windows. Now, the company has warned that the latest Windows Server updates could prevent IT admins from creating new virtual machines (VMs) in some Hyper-V hosts.

“After installing KB5021249 on Hyper-V hosts managed by Software Defined Networking (SDN) configured System Center Virtual Machine Manager (VMM), you might receive an error on workflows involving creating a new Network Adapter (also called a Network Interface Card or NIC) joined to a VM network or a new Virtual Machine (VM) with a Network Adapter joined to a VM network,” Microsoft explained on the Windows Health Dashboard.

Specifically, IT admins could encounter Ethernet connection errors while creating a new virtual machine or a network adapter on an existing virtual machine. Additionally, these errors might appear when the SLB Load Balancer or SDN RAS Gateway services fail. Administrators may also see Ethernet connection warning messages during live migrations of non-highly available VMs.

The issue only affects new network adapters created after installing the latest Patch Tuesday updates (KB5021237 and KB5021249) on Windows Server 2019 and Windows Server 2022. However, it doesn’t cause connection issues for existing VMs with network adapters.

Microsoft provides a workaround to address Hyper-V issues in Windows Server

Thankfully, Microsoft says that its engineers are currently working on a fix that should be arriving in the near future. As a workaround, Microsoft suggests users to run the following commands in PowerShell with administrator privileges:

$lang = (Get-WinSystemLocale).Name
C:\Windows\system32\wbem\mofcomp.exe C:\Windows\system32\wbem\$lang\VfpExt.mfl
C:\Windows\system32\wbem\mofcomp.exe C:\Windows\system32\wbem\VfpExt.mof

Microsoft has provided a dedicated script to help organizations with large-scale deployments on the SCVMM Management Server. Moreover, there is also a post-installation script that can be used with patching tools. IT admins can download both scripts from this support page.

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