The Auto Deploy rules that are created are not activated by default. They have to be manually added to the active set. Only rules in the active ruleset are referenced by the Auto Deploy server when it receives an HTTP boot request. When a machine boots for the first time, the Auto Deploy servers select the image profile based on the lowest number deploy rule in the active ruleset. Once the image profile has been identified, it will be cached at the Auto Deploy server and reused during future reboots.

Now that we know how to prepare the environment for a PXE boot and then Auto Deploy the ESXi hosts, let's dive a bit deeper into what happens in the background during the first and subsequent boot-up operations:

• First boot: When a machine has been chosen to be provisioned with ESXi on, it does a PXE boot by fetching an IP address from the DHCP server. DHCP scope configuration options 66 and 67 will direct the server to contact the TFTP server and load the bootable gPXE image and an accompanying configuration file. The configuration will direct the server to send an HTTP boot request to the Auto Deploy server. The HTTP boot request will contain server attributes such as the IP address, MAC address, vendor details, and so on. On receipt of the HTTP boot request, the Auto Deploy server will check its rule engine to see whether there are any rules that match the host attributes. If it finds a corresponding rule, then it will use that rule from the active ruleset to load an appropriate ESXi image from an image profile.
• Subsequent boot: After the host has been provisioned and added to the vCenter Server, the vCenter Server holds the details of the image profile and the host profile associated with the host object in its database. Hence, during a subsequent reboot, the Auto Deploy server does not have to leverage its rule engine again. Instead, it uses the information from the vCenter database. If, for whatever reason, the vCenter service/database becomes unavailable, then the Auto Deploy rule engine is engaged to determine the image profile and the host profile to be used. However, the host profile cannot be applied to the host since vCenter is unavailable.