The ROM Monitor is a bootstrap program that initializes the hardware and boots the Cisco IOS XE software when you power on or reload a router. When you connect a terminal to the router that is in ROM Monitor mode, the ROM Monitor command-line interface (CLI) prompt is displayed.
If your system (router, switch, or access server) does not find a valid system image to load when it is booting, the system will enter the ROM monitor mode. ROM monitor (ROMMON) mode can also be accessed by interrupting the boot sequence during startup.
The ROM monitor mode is used to:
specify config-register value to use for the next boot up
boot a valid IOS XE image
bypass NVRAM settings and config-register value for password recovery
The ROM Monitor software is known by many names. It is sometimes called ROMMON because of the CLI prompt in ROM Monitor mode. The ROM Monitor software is also called the boot software , boot image , or boot helper . Although it is distributed with routers that use the Cisco IOS XE software, ROM Monitor is a separate program from the Cisco IOS XE software. During normal startup, the ROM Monitor initializes the router, and then control passes to the Cisco IOS XE software. After the Cisco IOS XE software takes over, ROM Monitor is no longer in use.
Environmental Variables and the Configuration Register
Two primary connections exist between ROM Monitor and the Cisco IOS XE software: the ROM Monitor environment variables and the configuration register.
The ROM Monitor environment variables define the location of the Cisco IOS XE software and describe how to load it. After ROM Monitor has initialized the router, it uses the environment variables to locate and load the Cisco IOS XE software.
The configuration register is a software setting that controls how a router starts up. One of the primary uses of the configuration register is to control whether the router starts in ROM Monitor mode or Administration EXEC mode. The configuration register is set in either ROM Monitor mode or Administration EXEC mode as needed. Typically, you set the configuration register using the Cisco IOS XE software prompt when you need to use ROM Monitor mode. When the maintenance in ROM Monitor mode is complete, you change the configuration register so the router reboots with the Cisco IOS XE software.
Accessing ROM Monitor Mode with a Terminal Connection
When the router is in ROM Monitor mode, you can access the ROM Monitor software only from a terminal connected directly to the console port of the card. Because the Cisco IOS XE software (EXEC mode) is not operating, the nonmanagement interfaces are not accessible. Basically, all Cisco IOS XE software resources are unavailable.
Network Management Access and ROM Monitor Mode
ROM Monitor mode is a router mode, not a mode within the Cisco IOS XE software. ROM Monitor software and the Cisco IOS XE software are two separate programs that run on the same router. At any given time, the router is running one of these programs, but it never runs both at the same time.
One area that can be confusing when using ROM Monitor and the Cisco IOS XE software is the area that defines the IP configuration for the Management Ethernet interface. Most router users get comfortable with configuring the Management Ethernet interface in the Cisco IOS XE software. When the router is in ROM Monitor mode, however, the router is not running the Cisco IOS XE software, so that Management Ethernet interface configuration is not available.
When you want to access other devices, such as a TFTP server, while in ROM Monitor mode on the router, you must configure the ROM Monitor variables with IP access information.