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Windows Server nodepools not upgraded to Gen2 during cluster node image upgrade

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This article discusses how to troubleshoot a scenario in which Windows Server nodepools don't get upgraded automatically from Gen1 to Gen2 virtual machines (VMs) when a cluster node image is upgraded in Microsoft Azure Kubernetes Service (AKS).

Note

This scenario doesn't apply to Gen2 VMs on Linux nodepools.

Prerequisites

Symptoms

Existing Windows Server nodepools don't get upgraded from Gen1 to Gen2 when you upgrade the node image by using one of the following methods in Azure CLI:

Cause

Cause 1: Existing nodepools aren't automatically upgraded to Gen2 VMs

By design, a node image upgrade doesn't support updating or upgrading existing nodepools. The az aks upgrade and az aks nodepool upgrade commands upgrade only the node image (to a later node image version). Those commands don't upgrade the corresponding VM generation.

Cause 2: Cluster upgrade to Kubernetes version 1.25 or a later version upgrades only the OS

When you upgrade your cluster to Kubernetes version 1.25 or a later version, only the operating system (OS) is upgraded (to Windows Server 2022). Existing nodepools aren't affected. The associated VM scale set will contain VMs that have the same Gen1 VM.

Cause 3: Cluster upgraded and new nodepool created by using Windows Server 2022 without specifying a valid VM size

After you upgrade the cluster to Kubernetes version 1.25 or a later version, you specify Windows Server 2022 as the OS to use on the nodepool's VMs. However, the VMs don't use a Gen2 node image reference because of one of the following reasons:

  • You don't specify a VM size, and the default VM size in the region doesn't support Gen2 VMs.

  • You specify a Gen1-only VM size.

When you upgrade the default OS from Windows Server 2019 (Windows2019) to Windows Server 2022 (Windows2022), the existing nodepools aren't automatically upgraded to a different VM generation.

Solution

Upgrade the cluster, and then create a new Windows Server nodepool that supports Gen2 VM sizes on that cluster according to the following guidelines.

Kubernetes cluster upgrade version Cluster creation guidelines
1.25 or a later version When you run the az aks create command to create a cluster, set the --node-vm-size parameter to a Gen2 VM size.
Earlier version than 1.25 When you run the az aks create command to create a cluster, set the --os-sku parameter value to Windows2022, and set the --node-vm-size parameter value to a Gen2 VM size.

Note

If you specify a Gen2 VM size and the operating system as Windows Server 2019, you receive an ErrorCode_Windows2019NotSupportedWithGen2VM error code that's accompanied by the following error message:

<virtual-machine-size> is a Gen 2-only VM. Windows2019 does not support Gen 2 VMs. However, you can select a Gen 1 VM size or set os_sku to 2022.

To avoid this problem, choose one of the following options when you create the cluster:

  • Pick a Gen1 VM size to use together with Windows Server 2019.

  • Set the operating system SKU to Windows Server 2022 to use together with your Gen2 VM.

Contact us for help

If you have questions or need help, create a support request, or ask Azure community support. You can also submit product feedback to Azure feedback community.