How would shutting down both nodes help you to get access to node A? Really curious
To boot diagnostics you need access to console. It is available either locally via serial port or remotely via SP. If SP network is configured, SP is accessible even if node is shut down, as long as power is applied to the system.
To configure SP use "sp setup" command on filer. It shares e0M port with Data ONTAP, so you have to connect it to LAN. It does not support tagged VLANs.