Before upgrading an ESXi host, it is best practice to to look at VMware HCL and check compatibility of host and IO devices. The combination of driver version, firmware version and ESXi release is crucial for compatibility. Even minor updates might lead to loss of HCL compatibility. A system that used to be HCL compliant at time of deployment, might no longer be compatible after e.g. the third ESXi update release. Updates can bring new driver versions which in turn might require higher firmware versions.

If you’re lucky you may have a software solution that keeps track of all your firmware and driver versions. Runecast Analyzer for example does a pretty good job and shows you current compatibility issues with a single click. Furthermore you can simulate updates/upgrades to any higher vSphere version and the resulting HCL status.

Unfortunately many customers do not have a software solution like that. In these cases you need to go back to the roots (literally) and gather all information on the ESXi shell. To do so you need to enable SSH service on all hosts you want to verify. That can be done in the vSphere-Client or more elegant and faster by a PowerCLI command.