With the vSphere Auto Deploy ESXi feature, you can provision and reprovision large numbers of ESXi hosts efficiently with vCenter Server.
When you provision hosts by using Auto Deploy, vCenter Server loads the ESXi image directly into the host memory. Auto Deploy does not store the ESXi state on the host disk.
vCenter Server makes ESXi updates and patches available for download in the form of an image profile. Optionally, the host configuration is provided in the form of a host profile. You can create host profiles by using the vSphere Web Client. You can create custom image profiles by using ESXi Image Builder CLI. See Using vSphere ESXi Image Builder and vSphere Host Profiles.
The first time you provision a host by using Auto Deploy, the host PXE boots and establishes contact with the Auto Deploy server, which streams the image profile and any host profile to the host. The host starts using the image profile, and Auto Deploy assigns the host to the appropriate vCenter Server system.
When you restart the host, the Auto Deploy server continues to provision the host with the appropriate image and host profile. To provision the host with a different image profile, you must change the rule that specifies the image profile, and perform a test and repair compliance operation. To propagate changes to all hosts that the rule specifies, change the rule and perform the test and repair operation. The ability to propagate changes to multiple hosts makes Auto Deploy an efficient way to provision and reprovision large numbers of hosts, and to enforce compliance to a master ESXi image.