To determine which guest operating systems are compatible with v Sphere 5.1, use the ESXi 5.1 information in the VMware Compatibility Guide.Beginning with v Sphere 5.1, support level changes for older guest operating systems have been introduced.In addition, check this site for information about supported management and backup agents before installing ESXi or v Center Server.The v Sphere Client and the v Sphere Web Client are packaged with the v Center Server and modules ZIP file.To use such virtual machines on ESXi 5.1, upgrade the virtual machine compatibility. v Center Server 5.1 can exist in Linked Mode only with other instances of v Center Server 5.1.
The virtual machines can migrate to the already upgraded host.
Although the installations are straightforward, several subsequent configuration steps are essential.
In particular, read the following: You cannot directly migrate third-party solutions installed on an ESX or ESXi host as part of a host upgrade.
Reenable Fault Tolerance after the upgrade process is complete.
If you see the problem when multiple hosts are rebooting or in a small cluster, wait until several hosts in the cluster have completed the boot process, and reboot the affected host.