If the device did not have a valid IOS image in the internal flash memory, it will go directly in that mode.
Once we have this resource, we must enter into ROM Monitor mode (rommon). The obvious prerequisite of this procedure is to have a valid IOS image, which is suitable for the device you want to put into operation, stored on a USB flash drive. This mode gives a reduced set of commands that essentially allow the administrator to manually run the boot sequence.įor such cases, and using commands in the ROMMON mode, the Cisco ISR routers have 1 or 2 USB ports that can be used to load the IOS image from a USB flash drive. In this case the device does not have a valid image to load and therefore the router boots into ROM monitor mode (rommon). However, it may happen that for various reasons the operating system image may not be available, maybe due to file corruption, flash memory corruption, accidental deletion, etc.
The flash memory of Cisco routers is usually internal or can be a removable flash card on higher end routers. Cisco routers typically store a copy of the device’s operating system (Cisco IOS) in their flash memory, and load this operating system image into RAM during the boot-up process.
