On ISR2900 router as well as Catalyst switch's boot process can be interrupted using CTL-C, and device goes ROMMON prompt. However, ISR4000 router such as ISR4431 does not wait or prompt when to send the interrupt during the boot process. Tried pressing CTRL-C or sending BREAK using tools like PuTTY throughout the boot process, but did not help. Router just initializes hardware, loads IOS XE image and boot. Does anyone know how to interrupt the boot process and take router to ROMMON mode?
That's a good thought. However, it is tedious on ISR 4000 series because CF is inside now. You need to take out the fan assembly to reach to CF.
Here is an update - CTRL-C does not break the boot operation but by sending a BREAK (for example, from putty menu, Special Command --> Break) does work, but it works when console port is directly connected to PC. It did not work over a remote console (using console management servers).
This boot sequence did not work for me. There are several docs out there with other methods that didn't work either. :/. Putting the old flash back in until we can figure out what the issue is. New flash stays in continuous boot that cannot be broken.