How a Cisco device locates and loads IOS

[zillah2004]

Cisco says :

"The default source for Cisco IOS software depends on the hardware platform. Most routers use the boot system commands saved in NVRAM. Cisco IOS software allows several alternatives to be used. Other sources can be specified for the software, or the router can use its own fallback sequence to load the software."

My understanding to what cisco says, that :

"Most routers use the boot system commands saved in NVRAM."

What happend if we erase startup ?

We are not going to see the boot system command in the "show startup-config" command ?

Do we need to rely on Cisco IOS to boot first from Flash then tftp , then ROM ?

[amit-singh]

Hi,

The whole thing depends on the Boot path settings which are set and stored in the NVRAM and also configuration register settings. You can set the boot path to boot the router from flash, tftp, RCP depends from where you want it to boot.

If you erase the startup-config then it would boot default from the flash which depends on theconfiguration-register settings. Like 0x2102 which signifies that the router will boot from the first available file in the flash.

Here is the link which will tell you the different configuration register settings.

http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a008022493f.shtml

HTH,

-amit singh