Cisco Equipment Operations - CCIE Routing and Switching Exam Certification Guide - page 66

Configuration Register
The configuration register is a 16-bit virtual register that specifies boot sequence and break
parameters and sets the console baud rate. The register is usually represented in hexadecimal.
Figure 3-1 shows one of the common values for the configuration register, 0x2102. Another
common value is 0x0101. The significance of each bit in the configuration register is described
in this section.

Configuration Register

Figure 3-1

11 1 1 1 1 98 7 654 3 1 0

2

Bit:

543210

0x2102 001000010000 0 0 1 0

You can change the configuration register by using the global configuration command config-
register . The following changes the configuration register to 0x10e:

Router(config)#config-register 0x10e

Boot Sequence
The last four bits (bits 3 to 0) of the configuration register specify the location of the boot file
that the router must use when booting up:
•

0x0000 specifies to go to ROM monitor mode.

•

0x0001 specifies to boot from ROM.

•

0x0002 to 0x000F specify to examine the configuration file in NVRAM for boot system
commands.

If no boot system commands are in the configuration file, the router attempts to boot the first
file in system flash memory. If no file is found in system flash memory, the router attempts to
boot a default file from the network whose name is derived from the value of the boot field (e.g.,
cisco2-4500) by using Trivial File Transfer Protocol (TFTP). If the attempt to boot from a
network server fails, the boot helper image in boot Flash boots up.

The default filename is constructed by the word cisco, the value of the boot bits, and the router
model or processor name. The format is cisco n-processor_name , where n is a value between 2
and 15 and equal to the value of the boot field. If the boot field is set to 3 (0011 binary) on a
Cisco 4500, the default boot name is cisco3-4500.

If several boot system commands are in the configuration, they are attempted in the order that
they appear. Example 3-7 shows the configuration for a router where it first attempts to boot an
image from flash, and if that fails, to boot from a TFTP server, and if that fails, to boot from
ROM. The IP address of the TFTP server is specified in the boot system tftp command.