The Configuration Register

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The Cisco Router use configuration register setting to determine how the router boots up. In some rare cases, such as password recovery or trouble-shooting, you may need to change the configuration register settings to bypass the startup configuration file or enter ROM Monitor mode.

The configuration register is a 16-bit register stored in NVRAM. For example, the factory default value is hexadecimal 0x2102 or binary 0010 0001 0000 0010. The lowest 4 bits of the configuration register (0x2 or 0010 in our example) form the boot field. The boot field have the following meanings:

• 0x0 -- Upon boot, this setting directs the router to enter ROM Monitor mode (ROMMON).


• 0x1 -- Upon boot, this setting allows the router to boot from the image (Mini-IOS) in ROM. This is also known as Boot mode or RXBOOT mode.


• 0x2 to 0xF -- Specifies a default netboot filename. May enable boot system commands that override the default netboot filename.

Whereas the lower 4 bits of configuration register control the boot characteristics, other bits control other functions. For example, Bit 8 controls the console Break key. Bit 10 controls the host portion of the Interet broadcast address...Check your hardware document for details.

Common configuration register settings are:

• 0x2102: The default. Router looks for a startup configuration file in NVRAM and for a valid IOS image in Flash.


• 0x2142: NVRAM contents are bypassed, startup configuration is ignored.


• 0x2100: Router boots into ROM Monitor mode.

Use command "show version" or "show hardware" to see the current configuration register setting.

Use command "config-regiter 0x2100" to change configuration register setting.

----------------The following example is tested on Cisco Packet Tracer with a Cisco 2620XM Router---------------------
Router>enable
Router#show version
Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2005 by cisco Systems, Inc.
Compiled Wed 27-Apr-04 19:01 by miwang
Image text-base: 0x8000808C, data-base: 0x80A1FECC

ROM: System Bootstrap, Version 12.1(3r)T2, RELEASE SOFTWARE (fc1)
Copyright (c) 2000 by cisco Systems, Inc.
ROM: C2600 Software (C2600-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)

System returned to ROM by reload
System image file is "flash:c2600-i-mz.122-28.bin"

cisco 2620 (MPC860) processor (revision 0x200) with 60416K/5120K bytes of memory
.
Processor board ID JAD05190MTZ (4292891495)
M860 processor: part number 0, mask 49
Bridging software.
X.25 software, Version 3.0.0.
1 FastEthernet/IEEE 802.3 interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read/Write)

Configuration register is 0x2102

Router#
Router#config t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#config-register ?
WORD  Config register number
Router(config)#config-register 0x2100
Router(config)#^Z
%SYS-5-CONFIG_I: Configured from console by console
Router#

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Notice any change you made to the configuration register setting is not immediate, you have to reload the Cisco router to let the change take effect.

As a result of the break sequence, the router will go into ROM Monitor mode. The commands here are totally different than the ones you're used to working with in the router's IOS. Use the command ? to see all the available commands.

---------------------------------------

Router#System Bootstrap, Version 12.1(3r)T2, RELEASE SOFTWARE (fc1)
Copyright (c) 2000 by cisco Systems, Inc.
cisco 2620 (MPC860) processor (revision 0x200) with 60416K/5120K bytes of memory

rommon 1 > ?
boot                boot up an external process
confreg             configuration register utility
dir                 list files in file system
help                monitor builtin command help
reset               system reset
set                 display the monitor variables
tftpdnld            tftp image download
unset               unset a monitor variable
rommon 2 > confreg 0x2102
rommon 3 > boot

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Be very careful, if you change register to an incorrect value and then reload the router, you can cripple the router and even Cisco can't bring it back!