Cisco Router Boot Sequence

Here we’re going to discuss from a high level of how a Cisco Router boots
after you flip the power switch to on which will assist you in your CCNA studies. For the most part, the initial boot process is controlled by the BootROM which
permanently stores the startup diagnostic code(ROM Monitor). The main task of the BootROM is to run the Power On Self Test(POST), which are the hardware
diagnostics, & to load the Cisco IOS software from the Flash(in most cases) to memory. Note that the Boot ROM isn’t erasable.

1.
The BootROM will initialize the hardware & run through the POST to make certain there’re no hardware errors & the CPU & interfaces are all fully
functioning. If there is some sort of error, you’ll see it in the POST as the router begins to boot

2. Next the BootROM locates &
loads a valid Cisco IOS image. Below you’ll find the process that the router will go through to load the IOS image:

  • The router first checks the Flash for a
    valid IOS.
  • If there is none found in Flash & it has a separate Boot-Helper image, it can load the IOS via a TFTP server(netboot)
  • Finally, as a last resort it will
    load the RxBoot mini-IOS that is found in ROM.

    • 3. The router then checks the Configuration Register value (NVRAM) to see
    where to load the router configuration file from. By default this is stored on an EPROM named startup-config which doesn’t lose its contents when the router is
    powered off. If a valid startup-config file can not actually be found, the router will load the System Configuration Dialog, which basically drops you into Setup mode. Then you
    will be easily able to perform the initial configuration of the router via a menu. Finally, there’re various configuration-register options you can set to change the default
    behavior which will be covered in another article.

    Bob Bello, is the owner of http://www.CiscoKits.com At his site you’ll find over 200
    various articles dedicated to Cisco CCNA & CCNP study help, free CCNA & CCNP study
    exams, step by step labs along with CCNA & CCNP home lab kits.