Hayden

Have you encountered the frustrating “Unsupported CPU” error message while installing or upgrading VMware ESXi? This issue is particularly common with ESXi 7.0 and later versions. This guide provides practical, step-by-step solutions to resolve this problem effectively. VMware discontinues support for older CPU models with each new ESXi release. Fortunately, there are proven workarounds to get your system running.

 

 

1. ESXi “Unsupported CPU” : Understanding the Problem and Current Status

Discontinued CPU List

VMware ESXi’s CPU support policy has become increasingly strict. According to Broadcom’s official documentation, ESXi 8.0 has dropped compatibility for many older CPU models.

ESXi Version Discontinued Major CPUs
ESXi 7.0 Intel Westmere (6th Gen, 2010+)
ESXi 8.0 Intel Sandy Bridge, Ivy Bridge, Haswell, Broadwell<br>AMD Bulldozer generation
ESXi 8.0 Update 2+ All CPUs without XSAVE instruction support

Common Error Messages

VMB: 611: Unsupported CPU: Intel family 0x06, model 0x1a, stepping 0x5
Error: CPU_SUPPORT_ERROR
See http://www.vmware.com/resources/compatibility

ESXi 9 displays CPU compatibility warnings instead of errors in some cases, allowing users to continue with installation.

 

 

2. Temporary Solution – Adding Boot Parameters

Using SHIFT + O for Boot Option Modification

The quickest method to apply during ESXi installation or boot:

  1. Press SHIFT + O at the ESXi boot screen
  2. Add the following parameter at the end of the command line: 
    allowLegacyCPU=true
  3. Press Enter to continue booting

During Installation Process

Apply the following during ESXi installation:

<ENTER: Apply options and boot>
> cdromBoot runweasel allowLegacyCPU=true

When warning messages appear during installation, press “Enter” to continue.

 

 

3. Permanent Solution – Editing boot.cfg Files

Enabling SSH Service

After ESXi boots using the temporary solution, enable SSH for permanent configuration:

  1. Access the ESXi web interface
  2. Navigate to Manage → Services
  3. Start the TSM-SSH service

Locating and Editing boot.cfg Files

Connect to the ESXi host via SSH and locate the boot.cfg files:

  1. Find boot.cfg file locations: 
    find / -name "boot.cfg"
  2. Typical file locations: 
    /bootbank/boot.cfg
/altbootbank/boot.cfg
  3. Edit files using vi editor: 
    vi /bootbank/boot.cfg
vi /altbootbank/boot.cfg

Modifying Kernel Options

Locate the line starting with kernelopt in the boot.cfg file and append allowLegacyCPU=true:

Before modification:

kernelopt=autoPartition=FALSE

After modification:

kernelopt=autoPartition=FALSE allowLegacyCPU=true

Save Configuration and Reboot

  1. Save in vi editor: :wq
  2. Edit both files identically
  3. Reboot the ESXi host

 

 

4. Pre-configuration for USB Installation

Editing USB Drive on Another Computer

When installing ESXi from USB, you can pre-edit the boot.cfg file:

  1. Connect the ESXi-installed USB to another computer
  2. Access the partition with multiple files (two FAT partitions will be visible)
  3. Open BOOT.CFG file with a text editor
  4. Add allowLegacyCPU=true to the kernelopt line
  5. Save and connect the USB to the target server

 

 

5. Kickstart Automated Installation Configuration

Kickstart Configuration for Unattended Installation

For automated installations, add these options to your Kickstart file:

vmaccepteula
install --firstdisk=local --overwritevmfs --ignoreprereqwarnings --ignoreprereqerrors
reboot
network --bootproto=static --ip=192.168.1.10 --netmask=255.255.255.0
rootpw VMware1!

 

 

6. Important Considerations and Limitations

Production Environment Limitations

⚠️ Critical Considerations:

  • No Official Support: Not officially supported by VMware
  • Update Limitations: Some patch updates may fail
  • Stability Concerns: Unexpected behavior may occur

Recommended Use Cases

✅ Suitable Environments:

  • Homelab environments
  • Testing and development environments
  • Educational purposes

CPU Feature Limitations

Older CPUs may encounter additional issues such as:

This host does not support virtualizing real mode.
The Intel "VMX Unrestricted Guest" feature is necessary to run this virtual machine on an Intel processor.

This can cause VM power-on failures.

 

 

7. Version-Specific Additional Parameters

Additional Options for ESXi 8.0 and Later

Parameter Description Applicable Version
allowLegacyCPU=true Allow legacy CPUs 7.0+
cpuUniformityHardCheckPanic=FALSE Disable CPU uniformity check 8.0+
--ignoreprereqwarnings Ignore prerequisite warnings 8.0+
--ignoreprereqerrors Ignore prerequisite errors 8.0+

 

 

8. Compatibility Verification Methods

Using VMware Compatibility Guide

Always verify before installation:

  1. Access VMware Compatibility Guide
  2. Search for your CPU model
  3. Check support status for each ESXi version

Alternative Hardware Considerations

Practical advice for older hardware users:

  • Intel NUC: Low-power, compact form factor homelab solution
  • Refurbished Servers: Purchase servers with supported CPUs from platforms like eBay
  • LabGopher: Search engine for refurbished server hardware deals

 

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