The Software Components of a Typical Router

A typical Cisco router (or switch) utilizes a combination of the following three types of software: the ROM Monitor, which is stored in EPROMs; the boot loader or boot software image, which is present in selected Cisco routers; and the operating system, which can be Cisco IOS software (another Cisco operating system prominent in Catalyst Switches). The rxboot software is another component that is found in most routers. However, newer generations of modular routers and switches are doing away with rxboot. The following sections highlight the characteristics of each software component.

Boot ROM System (ROM Monitor)

Boot ROM software is contained in an EPROM which permanently stores platform-specific startup diagnostic software. The boot ROM is also known as ROM Monitor (ROMMON) or bootstrap code. At power up, the boot ROM software verifies that the hardware is fully functional and that a more intelligent software image exists. In some routers, the more intelligent software image can be the boot image or the full IOS image. For example, in the 2500 series, there is not a boot image, so the ROMMON loads the Cisco IOS image. However, on the 7500 series or the 4500 series, the ROMMON will look for the boot image instead. In any case, if ROMMON fails to identify a valid software image (boot image or IOS image), it has the ability to handle those exceptions and take appropriate remedial action or provide the user with a prompt for further troubleshooting. If all is well, it proceeds to load the next software component, the boot loader or the fully qualified Cisco IOS image.