Router Boot Sequence

Router Boot Sequence1) The router performs the POST.2) The bootstrap searches for and loads the Cisco IOS software.3) The IOS software looks in NVRAM for a valid configuration file.4) If there is a valid configuration file in NVRAM, then this file will be loaded and executed.
POST: When first powered up, a router will carry out a power-on self-test (POST). Recall that the POST is used to check whether the CPU and router interfaces are capable of functioning correctly.

Boot Strap:After a successful POST, the router will execute the Bootstrap program from ROM. The bootstrap is used to search Flash memory for a valid Cisco IOS image.

IOS:Once the IOS image is loaded, it will search for a valid startup configuration in NVRAM. If a valid startup configuration file cannot be found, the router will load the System Configuration Dialog, or what is sometimes called setup mode. This mode allows you to perform the initial configuration of the router.

Check Configuration Register value (NVRAM) which can be modified using the config-register command 0 = ROM Monitor mode1 = ROM IOS2 - 15 = startup-config in NVRAM
Boot System Commands
Router(config)# boot system flash IOS filename – boot from FLASH memoryRouter(config)# boot system tftp IOS filename tftp server ip address – boot from a TFTP serverRouter(config)# boot system rom – boot from system ROM
Configuration Register Command
Router(config)# config-register 0×10x (where that last x is 0-F in hex)
When the last x is:0 = boot into ROM Monitor mode1 = boot the ROM IOS2 – 15 = look in startup config file in NVRAM

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When you turn the router on, it runs through the following boot process.
The Power-On Self Test (POST) checks the router's hardware. When the POST completes successfully, the System OK LED indicator comes on.
The router checks the configuration register to identify where to load the IOS image from. A setting of 0x2102 means that the router will use information in the startup-config file to locate the IOS image. If the startup-config file is missing or does not specify a location, it will check the following locations for the IOS image:
Flash (the default location)
TFTP server
ROM (used if no other source is found)
The router loads the configuration file into RAM (which configures the router). The router can load a configuration file from:
NVRAM (startup-configuration file)
TFTP server
If a configuration file is not found, the router starts in setup mode.