Read Only Memory

ROM stores a scaled-down version of a router's IOS image in the event that the Flash system becomes corrupted or no current IOS image is stored in Flash. ROM also contains the bootstrap program (sometimes referred to as the rxboot image in Cisco documentation) and a device's power-up diagnostics. You can perform a software upgrade (that is, perform a software image upgrade on ROM) only by replacing ROM chips, because ROM is not programmable.

The bootstrap program enables you to isolate or rule out hardware issues. For example, suppose that you have a faulty Flash memory card and, subsequently, the router cannot boot the IOS image. The power diagnostics program tests all the hardware interfaces on the router. ROM mode contains a limited number of IOS commands, which enable the administrator or the Technical Assistance Center (TAC) to help troubleshoot and ascertain any hardware or configuration issues on a Cisco router. Cisco TAC is available 24 hours a day, 7 days a week. You must pay Cisco for this service and have a valid contract number to open any cases.

Unfortunately, not all Cisco routers have the same ROM code, so the commands might vary, but the principle remains the same. You can always issue the ? command in ROM mode to identify the available commands used to troubleshoot a Cisco IOS-based router. Newer Cisco hardware models now contain a new boot program stored in boot Flash rather than in ROM. The program is a little more user-friendly. Menu-driven options are available to change the configuration register, for example.

Example 3-3 provides all the available options on a Cisco 3800 router when the ? command is used in ROM mode.

Example 3-3 ? Command Used When in ROM Mode

System Bootstrap, Version 12.0(3)T, RELEASE SOFTWARE


Copyright 1999 by

cisco Systems, Inc.

C1700 platform with

49152 Kbytes of main memory

rommon 1 > ?


set and display aliases command


boot up an external process


set/show/clear the breakpoint


configuration register utility


continue executing a downloaded



display the context of a loaded



display contents of cookie PROM

in hex




list the device table


unset an alias


unset a monitor variable


x/ymodem image download

The options in Example 3-3 include the ability to initialize a router with the i command after you have finished ROM mode. ROM mode enables you to recover lost passwords by altering the configuration registers (covered in the "Password Recovery" section, later in this chapter).

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