Short answer: For a lab install of ESXi 8.0 on an unsupported CPU, add the kernel boot option allowLegacyCPU=true, either interactively with Shift+O or on the kernelopt= line of boot.cfg. This turns the CPU error into a warning you must accept. To avoid that prompt in a scripted install, add --forceunsupportedinstall to the kickstart install line. This is unsupported, and it is not for production.
Why the ESXi 8 installer stops on older CPUs
The ESXi installer checks the processor against Broadcom’s support list. KB 318697 (CPU Support Deprecation and Discontinuation) defines two states. For a deprecated CPU the installer shows a warning and continues, because it is still supported but will be dropped in a future release. For a discontinued CPU the installation is halted with an error. allowLegacyCPU=true targets the second case: it downgrades the hard stop to a warning that you have to acknowledge.
Important: allowLegacyCPU does not appear in Broadcom’s ESXi installation documentation. It is a community-known option (popularised by William Lam). A host installed this way runs on hardware outside the compatibility guide, so no Broadcom support applies. Future updates can also stop booting on it.
Interactive install: add allowLegacyCPU=true with Shift+O
- Boot the ESXi 8.0 installer ISO or USB.
- When the installer window appears, press Shift+O to edit boot options, as documented for scripted installs in TechDocs.
- Append
allowLegacyCPU=trueto the existing line and press Enter. - Accept the unsupported CPU warning when the installer displays it.
Automating the ESXi 8.0 install with kickstart
In a scripted install, the warning prompt is what breaks automation: the installer waits for a keypress. You need two things. The first is the boot option, so the installer does not halt. The second is a kickstart flag, so it does not prompt.
1. boot.cfg: kickstart location plus allowLegacyCPU
According to About the boot.cfg File, kernelopt= appends its string to the kernel boot options. Put both options there, in /EFI/BOOT/BOOT.CFG (UEFI) and /BOOT.CFG (BIOS) on your custom ISO, USB or PXE boot files:
kernelopt=ks=http://192.168.30.10/kickstart/KS.CFG allowLegacyCPU=true2. KS.CFG: suppress the CPU prompt
vmaccepteula
install --firstdisk=local --overwritevmfs --forceunsupportedinstall
rootpw <your-password>
network --bootproto=static --ip=192.168.30.13 --netmask=255.255.255.0 --gateway=192.168.30.1 --nameserver=192.168.30.1 --hostname=esx-lab01.example.local
reboot--forceunsupportedinstall is listed under the install command in the ESXi 8.0 script command reference, which describes it only briefly in relation to deprecated CPUs. William Lam’s original tip also adds --ignoreprereqwarnings and --ignoreprereqerrors, noting that some systems need all three. Those two flags are not in the TechDocs command reference, so treat them as undocumented. Also note that --overwritevmfs erases an existing datastore on the target disk.
Limits: ESXi 8.0 Update 2 and later, and ESX 9
- William Lam reported that ESXi 8.0 Update 2 requires the XSAVE CPU instruction even with
allowLegacyCPU=true. In practice that means Intel Sandy Bridge / AMD Bulldozer or newer. Older CPUs cannot be forced past this. - Each update is a new risk. An unsupported host may fail the precheck or not boot after patching, so keep the previous boot bank.
- For ESX 9.x, KB 318697 lists additional discontinued CPUs. Check it before you plan a lab refresh. It is not documented whether this boot option still works with the 9.x installer.
Related kickstart notes on vDan: failed to locate kickstart on a nested ESXi VM and disabling IPv6 in an ESXi kickstart. For post-install host configuration, see esxcli commands for ESXi 8.0. For network installs of ESX 9, my ESX PXE boot server generator builds a Linux PXE/UEFI HTTP/iPXE boot server with dnsmasq, boot.cfg and per-host kickstarts.
Further reading
- Original post: Quick Tip – Automating ESXi 8.0 install using allowLegacyCPU=true (William Lam)
- Heads Up – ESXi 8.0 Update 2 requires XSAVE CPU instruction even with allowLegacyCPU=true (William Lam)
- Broadcom KB 318697: CPU Support Deprecation and Discontinuation
- Broadcom TechDocs: ESXi 8.0 Installation and Upgrade Script Commands
