Recall from Chapter 6 that the read-only memory (ROM) on a Cisco router also includes a limited IOS version that can be used to boot the router in cases where an IOS image is not present. This limited IOS version is commonly referred to as the boot image (or RxBoot) and provides an environment from which you can access a TFTP server to install a new IOS image to your router. If you ever accidentally erase the contents of flash, you’ll definitely need to be familiar with the boot image.
There are two common methods of purposely accessing a router’s boot image stored in ROM. The first is to add the boot system rom command to a router’s startup configuration. However, it’s much more common to access RxBoot by changing the value of the router’s configuration register, as we’ll explore shortly. If your router automatically loads the boot image, that usually indicates that you need to change the configuration register on your router, or that a valid IOS image could not be found in Flash memory or on a network TFTP server. In all cases, accessing the boot image will require that you reboot the router.
You can recognize that the router has loaded the boot image according to the prompt displayed. The prompt when the boot image is loaded will be:
No matter how badly you have misconfigured a router, you should always be able to access the boot image since it is stored in ROM. If you can’t, that likely indicates a hardware-related issue.