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The order in which the router looks for
system bootstrap information depends on the boot field setting in the
configuration register. The default configuration register setting can
be changed with the global configuration mode command
config-register.
Use a hexadecimal number as the argument for this command.
The configuration register is a 16-bit
register in NVRAM. The lowest four bits of the configuration register
form the boot field. To ensure that the upper 12 bits are not changed,
first retrieve the current values of the configuration register using
the show version
command. 
Then use the config-register
command, changing only the value of the last hexadecimal digit.
To change the boot field in the
configuration register, follow these guidelines:
- To enter the ROM monitor mode, set the
configuration register value to 0xnnn0, where nnn represents
the previous value of the non-boot field digits. This value sets the
boot field bits to 0000 binary. From ROM monitor, boot the operating
system manually by using the
b
command at the ROM monitor prompt.
- To configure the system to boot
automatically from ROM, set the configuration register to 0xnnn1,
where nnn represents the previous value of the non-boot field
digits. This value sets the boot field bits to 0001 binary.
- To configure the system to use the
boot system commands in NVRAM, set the configuration register to any
value from 0xnnn2 to 0xnnnF, where nnn
represents the previous value of the non-boot field digits. These
values set the boot field bits to a value between 0010 and 1111
binary. Using boot system commands in NVRAM is the default.
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Lab Activity
e-Lab Activity: Configuration Register
In this lab, the students will learn to change the boot procedure
of the router. 
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