An ESXi host might disconnect from vCenter Server or all ESXi hosts might disconnect from vCenter Server at the same time.
An ESXi host disconnects from vCenter Server or all ESXi hosts disconnect from vCenter Server and you receive a licensing-related error message.
You cannot add hosts to the vCenter Server inventory. The hosts and the virtual machines on the hosts continue to run.
The 60-day evaluation period of the host is expired or the host license is expired.
The 60-day evaluation period of vCenter Server is expired or the vCenter Server license is expired.
Obtain a vSphere license key and assign it to the ESXi host.
Obtain a vCenter Server license key and assign it to vCenter Server. If the vCenter Server system is managing ESXi 3.5, it must have access to a license server. You can download the VMware License Server from the VMware Web site.
When you assign a license key to an ESXi host and vCenter Server, the license edition must be compatible with all of the features you configured. If the license edition and the configured features are incompatible, you cannot assign the license key.