Check the external cable connection. Suspect the processor or software. System boots, but console screen is frozen. Check the external console connection. Verify the console baud rate in the terminal's documentation.
System powers on and boots with a particular interface disconnected.
Suspect the network interface. System powers on and boots with a particular network interface or cable disconnected. Suspect the network interface or cable. Note the activity of the LEDs to judge the activity of the network interface to which it corresponds. When facing the chassis, the LED indicator lights represent the following, depending on which options are installed on your router.