how does the baseboard management controller (BMC) talk to the host machine?

Question

I can turn off a server by using ipmitool. But how does the BMC initiate the off command to the Host OS? How are the two connected internally?

And also when the host is powered down how does the bmc turn on the machine? I am assuming it's able to talk to the OS when the machine is on and it's not able to do so when it's turned off. In that case how does the BMC turn on the host machine

Answer 1

BMC does not use OS services. BMC is completely OS independent and it may monitor and control hardware even when no OS is running or installed. BMC power line is independent on the host power and BMC is powered even when the host is powered off. It is ensured by power source design. BMC can control the host power supply. BMC usually has a direct hardware link to the power supply. BMC sends to power-off command directly to the power supply and not to the host OS.

Answer 2

A general design is:

BMC connects to a CPLD, which controls the power sequence. When power off is needed, BMC will trigger the CPLD so that it is the same as if someone pushes the front panel power button.