Simple answer is if the system fails to boot up within the configured time, it will restart and try again.
How will it come to know whether OS is running or not, is there a record of the OS in BIOS.
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The system BMC starts a FRB timer when you push the power on button.
The OS turns the timer off when it boots.
If the OS fails to turn off the timer before the time runs out, the BMC reboots the system.
The OSes send the FRB timer disable command regardless, so if the timer is not enabled, has no impact.
Some OSes also use the FRB timer during run time to make sure nothing catastrophic has hung the OS.
They set the timer to expire at 90 seconds and then reset it every 30 seconds back to 90 seconds with a back ground command.
If the OS gets hosed up and the timer does not get reset, BMC does a hard reset and reboots the server.
The SEL log also records a entry when the OS boots or if the FRB timer times out.
Thank you sir.