Upgrade options and recommendations for Azure Kubernetes Service (AKS) clusters

This article gives you a technical foundation for Azure Kubernetes Service (AKS) cluster upgrades by covering upgrade options and common scenarios. For in-depth guidance tailored to your needs, use the scenario-based navigation paths at the end of this article.

What this article covers

This technical reference provides comprehensive AKS upgrade fundamentals on:

• Manual versus automated upgrade options and when to use each.
• Common upgrade scenarios with specific recommendations.
• Optimization techniques for performance and minimal disruption.
• Troubleshooting guidance for capacity, drain failures, and timing issues.
• Validation processes and pre-upgrade checks.

This hub is best for helping you to understand upgrade mechanics, troubleshoot issues, optimize upgrade settings, and learn about technical implementation.

For more information, see these related articles:

• To upgrade your production AKS clusters, see AKS production upgrade strategies.
• To get upgrade patterns for AKS clusters with stateful workloads, see Stateful workload upgrade patterns.
• To use the scenario hub to help you choose the right AKS upgrade approach, see AKS upgrade scenarios: Choose your path.

If you're new to AKS upgrades, start with the upgrade scenarios hub for guided, scenario-based assistance.

Quick navigation

Upgrade options

Perform manual upgrades

Manual upgrades let you control when your cluster upgrades to a new Kubernetes version. These upgrades are useful for testing or targeting a specific version:

• Upgrade an AKS cluster
• Upgrade multiple AKS clusters via Azure Kubernetes Fleet Manager
• Upgrade the node image
• Customize node surge upgrade
• Process node OS updates

Configure automatic upgrades

Automatic upgrades keep your cluster on a supported version and up to date. Use these upgrades when you want to automate your settings:

• Automatically upgrade an AKS cluster
• Automatically upgrade multiple AKS clusters via Azure Kubernetes Fleet Manager
• Use planned maintenance to schedule and control upgrades
• Stop AKS cluster upgrades automatically on API breaking changes (preview)
• Automatically upgrade AKS cluster node operating system images
• Apply security updates to AKS nodes automatically by using GitHub actions

Special considerations for node pools that span multiple availability zones

AKS uses best-effort zone balancing in node groups. During an upgrade surge, the zones for surge nodes in virtual machine scale sets are unknown ahead of time, which can temporarily cause an unbalanced zone configuration. AKS deletes surge nodes after the upgrade and restores the original zone balance.

To keep zones balanced, set surge to a multiple of three nodes. Persistent volume claims that use Azure locally redundant storage disks are zone bound and might cause downtime if surge nodes are in a different zone. Use a pod disruption budget (PDB) to maintain high availability during drains.

Optimize upgrades to improve performance and minimize disruptions

Combine planned maintenance window, max surge, PDB, node drain timeout, and node soak time to increase the likelihood of successful, low-disruption upgrades:

• Planned maintenance window: Schedule auto-upgrade during low-traffic periods. We recommend at least four hours.
• Max surge: Higher values speed upgrades but might disrupt workloads. We recommend 33% for production.
• Max unavailable: Use when capacity is limited.
• Pod disruption budget: Set to limit pods down during upgrades. Validate for your service.
• Node drain timeout: Configure pod eviction wait duration. The default is 30 minutes.
• Node soak time: Stagger upgrades to minimize downtime. The default is 0 minutes.

Validations used in the upgrade process

AKS performs pre-upgrade validations to ensure cluster health:

• API breaking changes: Detects deprecated APIs.
• Kubernetes upgrade version: Ensures a valid upgrade path.
• PDB configuration: Checks for misconfigured PDBs (for example, maxUnavailable=0).
• Quota: Confirms enough quota for surge nodes.
• Subnet: Verifies sufficient IP addresses.
• Certificates/service principals: Detects expired credentials.

These checks help to minimize upgrade failures and provide early visibility into issues.

Common upgrade scenarios and recommendations

Scenario 1: Capacity constraints

If your cluster is limited by product tier or regional capacity, upgrades might fail when surge nodes can't be provisioned. This situation is common with specialized product tiers (like GPU nodes) or in regions with limited resources. Errors such as SKUNotAvailable, AllocationFailed, or OverconstrainedAllocationRequest might occur if maxSurge is set too high for available capacity.

Recommendations to prevent or resolve

• Use maxUnavailable to upgrade by using existing nodes instead of surging new ones. For more information, see Customize unavailable nodes during upgrade.
• Lower maxSurge to reduce extra capacity needs. For more information, see Customize node surge upgrade.
• For security-only updates, use security patch reimages that don't require surge nodes. For more information, see Apply security and kernel updates to Linux nodes in Azure Kubernetes Service.

Scenario 2: Node drain failures and PDBs

Upgrades require draining nodes (evicting pods). Drains can fail when pods are slow to terminate or strict Pod Disruption Budgets (PDBs) block pod evictions.

Example error:

Code: UpgradeFailed
Message: Drain node ... failed when evicting pod ... Cannot evict pod as it would violate the pod's disruption budget.

Option 1: Force upgrade (bypass PDB)

Use force upgrade only when PDBs prevent critical upgrades and cannot be resolved. This will override PDB protections and potentially cause complete service unavailability during the upgrade.

Requirements: Azure CLI 2.79.0+ or AKS API version 2025-09-01+

az aks upgrade \
  --name $CLUSTER_NAME \
  --resource-group $RESOURCE_GROUP_NAME \
  --kubernetes-version $KUBERNETES_VERSION \
  --enable-force-upgrade \
  --upgrade-override-until 2023-10-01T13:00:00Z

Option 2: Handle undrainable nodes (honor PDB)

Use this conservative approach to honor PDBs while preventing upgrade failures.

Configure undrainable node behavior:

az aks nodepool update \
  --resource-group <resource-group-name> \
  --cluster-name <cluster-name> \
  --name <node-pool-name> \
  --undrainable-node-behavior Cordon \
  --max-blocked-nodes 2 \
  --drain-timeout 30

Behavior options:

• Schedule (default): Deletes blocked node and surges replacement
• Cordon (recommended): Cordons node and labels it as kubernetes.azure.com/upgrade-status=Quarantined

Max blocked nodes (preview):

• Specifies how many nodes that fail to drain are tolerated
• Requires undrainable-node-behavior to be set
• Defaults to maxSurge value (typically 10%) if not specified

Prerequisites for max blocked nodes

• The Azure CLI aks-preview extension version 18.0.0b9 or later is required to use the max blocked nodes feature.

  # Install or update the aks-preview extension
  az extension add --name aks-preview
  az extension update --name aks-preview
  

Example configuration with max blocked nodes

az aks nodepool update \
  --cluster-name jizenMC1 \
  --name nodepool1 \
  --resource-group jizenTestMaxBlockedNodesRG \
  --max-surge 1 \
  --undrainable-node-behavior Cordon \
  --max-blocked-nodes 2 \
  --drain-timeout 5

Recommendations to prevent drain failures

• Set maxUnavailable in PDBs to allow at least one pod eviction
• Increase pod replicas to meet disruption budget requirements
• Extend drain timeout if workloads need more time. (The default is 30 minutes.)
• Test PDBs in staging, monitor upgrade events, and use blue-green deployments for critical workloads. For more information, see Blue-green deployment of AKS clusters.

Verify undrainable nodes

• The blocked nodes are unscheduled for pods and marked with the label "kubernetes.azure.com/upgrade-status: Quarantined".

• Verify the label on any blocked nodes when there's a drain node failure on upgrade:

  kubectl get nodes --show-labels=true
  

Resolve undrainable nodes

1. Remove the responsible PDB:

   kubectl delete pdb <pdb-name>
   

2. Remove the kubernetes.azure.com/upgrade-status: Quarantined label:

   kubectl label nodes <node-name> <label-name>
   

3. Optionally, delete the blocked node:

   az aks nodepool delete-machines --cluster-name <cluster-name> --machine-names <machine-name> --name <node-pool-name> --resource-group <resource-group-name>
   

4. After you finish this step, you can reconcile the cluster status by performing any update operation without the optional fields as outlined in az aks. Alternatively, you can scale the node pool to the same number of nodes as the count of upgraded nodes. This action ensures that the node pool gets to its intended original size. AKS prioritizes the removal of the blocked nodes. This command also restores the cluster provisioning status to Succeeded. In the following example, 2 is the total number of upgraded nodes.

   # Update the cluster to restore the provisioning status
   az aks update --resource-group <resource-group-name> --name <cluster-name>
   
   # Scale the node pool to restore the original size
   az aks nodepool scale --resource-group <resource-group-name> --cluster-name <cluster-name> --name <node-pool-name> --node-count 2
   

Scenario 3: Slow upgrades

Conservative settings or node-level issues can delay upgrades, which affects your ability to stay current with patches and improvements.

Common causes of slow upgrades include:

• Low maxSurge or maxUnavailable values (limits parallelism).
• High soak times (long waits between node upgrades).
• Drain failures (see Node drain failures).

Recommendations to prevent or resolve

• Use maxSurge=33%, maxUnavailable=1 for production.
• Use maxSurge=50%, maxUnavailable=2 for dev/test.
• Use OS Security Patch for fast, targeted patching (avoids full node reimaging).
• Enable undrainableNodeBehavior to avoid upgrade blockers.

Scenario 4: IP exhaustion

Surge nodes require more IPs. If the subnet is near capacity, node provisioning can fail (for example, Error: SubnetIsFull). This scenario is common with Azure Container Networking Interface, high maxPods, or large node counts.

Recommendations to prevent or resolve

• Ensure that your subnet has enough IPs for all nodes, surge nodes, and pods. The formula is Total IPs = (Number of nodes + maxSurge) * (1 + maxPods).

• Reclaim unused IPs or expand the subnet (for example, from /24 to /22).

• Lower maxSurge if subnet expansion isn't possible:

  az aks nodepool update \
    --resource-group <resource-group-name> \
    --cluster-name <cluster-name> \
    --name <node-pool-name> \
    --max-surge 10%
  

• Monitor IP usage with Azure Monitor or custom alerts.

• Reduce maxPods per node, clean up orphaned load balancer IPs, and plan subnet sizing for high-scale clusters.

Frequently asked questions

Can I use open-source tools for validation?

Yes. Many open-source tools integrate well with AKS upgrade processes:

• kube-no-trouble (kubent): Scans for deprecated APIs before upgrades.
• Trivy: Security scanning for container images and Kubernetes configurations.
• Sonobuoy: Kubernetes conformance testing and cluster validation.
• kube-bench: Security benchmark checks against Center for Internet Security standards.
• Polaris: Validation of Kubernetes best practices.
• kubectl-neat: Clean up Kubernetes manifests for validation.

How do I validate API compatibility before upgrading?

Run deprecation checks by using tools like kubent:

# Install and run API deprecation scanner
kubectl apply -f https://github.com/doitintl/kube-no-trouble/releases/latest/download/knt-full.yaml

# Check for deprecated APIs in your cluster
kubectl run knt --image=doitintl/knt:latest --rm -it --restart=Never -- \
  -c /kubeconfig -o json > api-deprecation-report.json

# Review findings
cat api-deprecation-report.json | jq '.[] | select(.deprecated==true)'

What makes AKS upgrades different from other Kubernetes platforms?

AKS provides several unique advantages:

• Native Azure integration with Azure Traffic Manager, Azure Load Balancer, and networking.
• Azure Kubernetes Fleet Manager for coordinated multicluster upgrades.
• Automatic node image patching without manual node management.
• Built-in validation for quota, networking, and credentials.
• Azure support for upgrade-related issues.

Choose your upgrade path

This article provided you with a technical foundation. Now select your scenario-based path.

Ready to execute?

Still deciding?

Use the upgrade scenarios hub for a guided decision tree that considers your:

• Downtime tolerance
• Environment complexity
• Risk profile
• Timeline constraints

Next tasks

• Review AKS patch and upgrade guidance for best practices and planning tips before you start any upgrade.
• Always check for API breaking changes and validate your workload's compatibility with the target Kubernetes version.
• Test upgrade settings (such as maxSurge, maxUnavailable, and PDBs) in a staging environment to minimize production risk.
• Monitor upgrade events and cluster health throughout the process.