By Bob Marshall

Cisco Router Boot Sequence

Here we are 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, and to load the Cisco IOS software from the Flash(in most cases) to memory. Note that the Boot ROM is not erasable.

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

2. Next the BootROM locates and loads a valid Cisco IOS image. Below you will 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 and 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 does not lose its contents when the router is powered off. If a valid startup-config file cannot be found, the router will load the System Configuration Dialog, which basically drops you into Setup mode. Then you will be able to perform the initial configuration of the router via a menu. Finally, there are various configuration-register options you can set to change the default behavior which will be covered in another article.

At http://www.CiscoKits.com you will find over 200 various articles dedicated to Cisco CCNA and CCNP study help, free CCNA and CCNP study exams, step by step labs along with CCNA and CCNP home lab kits.

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