Microsoft says Windows Server updates released on December Patch Tuesday will trigger errors when creating new virtual machines on some Hyper-V hosts.

The known issue only affects Windows Server/AzStack HCI hosts in SDN-enabled environments that are managed using System Center Virtual Machine Manager (SCVMM).

“You may receive an error on workflows involving the creation of a new network adapter (also known as a network interface card or NIC) attached to a VM network or a new virtual machine (VM) with a network attached network adapter VM”, Microsoft Explain.

“Existing VMs with existing network adapters should have no connection issues after installing this update, only new network adapters created after installing this update are affected.”

On affected systems, Windows administrators will see one of the following errors:

• Creating a new VM fails: Creating a new virtual machine with a network adapter fails.
• SLB load balancer or RAS SDN gateway failure with an ‘EthernetConnection’ error
• vNIC teaming fails: Attach a network adapter to an existing virtual machine.
• Warning message during live migration: Live migration of non-highly available VMs ends with an “EthernetConnection” warning message.

Microsoft added that only Windows Server 2019 and Windows Server 2022 should be impacted after installing this month’s Patch Tuesday updates (KB5021237 and KB5021249).

Workaround available

Microsoft has also provided administrators of affected SDN-based SCVMM deployments with a workaround for this newly acknowledged issue.

It requires running the following set of commands on all SCVMM-managed Hyper-V hosts from an elevated PowerShell window (immediately after installing updates KB5021237 and KB5021249):

$lang = (Get-WinSystemLocale).Name

C:\Windows\system32\wbem\mofcomp.exe C:\Windows\system32\wbem\en-US\VfpExt.mfl

C:\Windows\system32\wbem\mofcomp.exe C:\Windows\system32\wbem\VfpExt.mof

Administrators can also do this after installing updates using a dedicated script for bulk deployment of the workaround on the SCVMM management server if the number of affected hosts is too large and executing the commands manually would take too long.

Microsoft also provides a post-installation script that can be used with remediation tools when deploying updates KB5021237 and KB5021249.

You can find both scripts for large scale deployments and more details on applying the workaround in this support article.

Redmond said that they are currently working on a fix for the known issue and will provide an update with a future release.