I should explain a little perhaps - and then explain a few options.
When a computer is configured in it’s firmware to boot to the network first, It will try that. DHCP hears the request and sends info. The PC confirms it’s received info - there’s a fraction of a second’s wait - then DHCP sends a “Use it” command once no other computer reports “no don’t use that, I’m using that IP”.
Then the computer network boots to FOG - as it was told. If it is registered - it will look for any active tasks and do those - if there are no tasks it will either (A) Show the boot menu for a specified time and then try to boot to the HDD or (B) not show a menu at all and immediately boot to the HDD.
Just depends on how you have it configured.
By default, non-registered hosts will show the menu so that you see it’s not registered. This menu also has a countdown that elapses - after it elapses, the machine tries to boot to the HDD.
Now - if you don’t like seeing the menu for registered machines - the very best that can be done is to set the timeout to 1 second. You can do that here:
FOG Configuration -> iPXE Boot Menu -> Menu Timeout (in seconds):*
Now, do not set this to 0 because 0 makes the menu stay up forever… 1 is as low as you can go.
@Developers is that normal? 0 causing the menu to just stay up?