ESXi CPU Not Supported Error: How to Install on Unsupported CPUs

1. Understanding the “CPU Not Supported” Error

When installing VMware ESXi—especially in nested or older server environments—you may encounter:

CPU_SUPPORT_ERROR: The CPU on this host is not supported by ESXi <version>. Please refer to the VMware Compatibility Guide… KB 82794.

This indicates the CPU model is not officially supported by that ESXi version

2. Lab Workaround: Using `allowLegacyCPU=true`

To bypass this in lab or test setups:

Boot from the ESXi installation media, F11 to reboot

At the “Loading ESXi Installer” screen, press Shift + O (within ~5 seconds).

Append allowLegacyCPU=true to the boot command line (e.g., runweasel cdromBoot allowLegacyCPU=true).

At the end of the command line in the bottom left corner, add allowLegacyCPU=true (do not delete any default generated fields, just append it directly at the end).

Press Enter to continue. The installer will downgrade the error to a warning, allowing the installation to proceed.

This method is widely deployed for home labs and POC environments.

3. Caveats & Production Environment Warnings

Important: This workaround is not recommended for production due to potential instability and lack of official support.

Moreover, starting with ESXi 8.0 Update 2, CPUs must support the XSAVE instruction (Intel Sandy Bridge or AMD Bulldozer or newer). Even with allowLegacyCPU=true, installation will fail on older CPUs.

4. Best Practice: Always Check CPU Compatibility First

Before installation—especially in production—always verify CPU compatibility:

Consult the official VMware Compatibility Guide (Broadcom).
Search by CPU series or model to ensure support for the specific ESXi version.
Only use legacy hardware with the override option in strictly non‑production contexts.

5. Automating Installation: Kickstart Options for Legacy CPUs

For automated or unattended installations (e.g., using Kickstart), use these scripts to suppress boot-time prompts:

--ignoreprereqwarnings
--ignoreprereqerrors
--forceunsupportedinstall

Example snippet:

install --firstdisk=local --overwritevmfs --ignoreprereqwarnings --ignoreprereqerrors

These flags help bypass warnings/errors during installation on legacy CPUs.

FAQ:

Q. Will allowLegacyCPU=true survive reboots?
Not by default. To persist, edit boot.cfg under /bootbank/ and /altbootbank/ to include the parameter.

Q. What if installation still fails despite allowLegacyCPU=true?
Starting with ESXi 8.0 U2, unsupported CPUs without XSAVE support will not install—even with overrides.

Q. Is there any way to override CPU checks during upgrades?
Yes, use ESXCLI with --no-hardware-warning during profile updates, though success may vary.

Wrap-Up

Using allowLegacyCPU=true can help tech enthusiasts and lab administrators get ESXi running on older or unsupported hardware. Just remember: it’s a temporary lab-friendly workaround, not a solution for production setups. Always plan production deployments around officially supported CPUs.

"For official guidance, refer to the VMware Compatibility Guide and KB 82794.”

Check the ESXi version or search there.

You can also learn more tips on poweradm.com

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