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Managing Cisco Router Startup and Configuration 437
How a Device Locates and Loads Cisco IOS Image and
Configuration Files
When a Cisco router boots, it searches for the Cisco IOS image in a speci?¬?c sequence: the
location speci?¬?ed in the con?¬?guration register, ?¬‚ash memory, a TFTP server, and ROM.
The bootstrap code is responsible for locating the Cisco IOS Software. It searches for the
image according to the following sequence:
1. The bootstrap code checks the boot ?¬?eld of the con?¬?guration register. The boot ?¬?eld is
the lower 4 bits of the con?¬?guration register and is used to specify how the router boots.
These bits can point to ?¬‚ash memory for the Cisco IOS image, the startup-con?¬?g ?¬?le
(if one exists) for commands that tell the router how to boot, or a remote TFTP server.
Or, these bits can specify that no Cisco IOS image is to be loaded and to just start
the Cisco IOS subset image in ROM. The con?¬?guration register bits perform other
functions as well, such as selecting the console baud rate and determining whether to
use the saved con?¬?guration ?¬?le (startup-con?¬?g) in NVRAM.
For example, a con?¬?guration register value of 0x2102 (the 0x indicates that the digits
that follow are in hexadecimal notation) has a boot ?¬?eld value of 0x2 (the rightmost
digit in the register value is 2 and represents the lower 4 bits of the register).
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