bootup process

Boot Sequence of a Cisco switch

The switch loads the boot loading software from NVRAM.
The boot loader:
Performs low-level CPU initialization.
 
Performs POST (power-on self test) for the CPU subsystem.
 
Initializes the flash file system on the system board.
 
Loads a default OS image into memory and boots the switch.
 
The OS runs using config.text, stored on the switch flash.

The boot loader can help recover from an OS crash:

provides access into the switch if OS has problems.
provides access to the files stored on flash before the OS is loaded.
use the boot loader command line to perform recovery operations.

To modify where to load IOS: startup configuration or configuration register.

Cisco router configuration register

Cisco router configuration register:
http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a008022493f.shtml

Bit Number	Hex	Meaning
00-03	0x0000-0x000F	Boots Field Parameters 0x0000 Stays at the system bootstrap prompt 0x0001 Boots system image on EPROM 0x0002-0x000F Specifies a default netboot filename
06	0x0040	Ignore NVRAM contents
07	0x0080	Disable boot messages
08	0x0100	Break disabled
10	0x0400	IP broadcast with all zeros
5,11,12	0x0020, 0x0800, 0x1000	Console line speed
13	0x2000	Boots default ROM software if network boot fails
14	0x4000	IP broadcasts do not have net numbers
15	0x8000	Enables diagnostic messages Ignores NVRAM contents