Cisco - Changing the Configuration Register

To change the configuration register while running the system software, follow these steps:

 

Step 1 - At the privileged EXEC prompt (Router#), enter the configure terminal command to enter global configuration mode.

Router# configure terminal

Router(config)#

 

Step 2 - Set the contents of the configuration register by entering the config-register value configuration command, where value is a hexadecimal number preceded by 0x as in the following example:

Router(config)# config-register 0x2142

 

Step 3 - Press Ctrl-Z to exit configuration mode.

 

Step 4 -Display the current configuration register value, which will be used at the next system reload, by entering the show version command.

 

The value is displayed on the last line of the screen display, as in the following example:

Configuration register is 0x2102 (will be 0x2142 at next reload)

 

Step 5 - Restart the router. Changes to the configuration register take effect only when the system reloads.

 

 

 

Configuration Register Boot Field

Boot Field

Meaning

Used For:

00

ROM monitor mode

To boot to ROM monitor mode, set the configuration register to 2100. You must then manually boot the router with the b command. The router will show a

rommon> prompt.

01

Boot image from ROM

To boot an IOS image stored in ROM, set the configuration register to 2101. The router will show the router(boot)>

prompt.

02 to 0F

Specifies a default boot filename

Any value from 2102 to 210F tells the router to use the boot commands specified in NVRAM.

 

 

 

 

  

Configuration Register Setting

Router Behavior

0x102
  • Ignores break
  • 9600 console baud
     

0x1202
  • 1200 baud rate
     

0x2101
  • Boots into bootstrap
  • Ignores break
  • Boots into ROM if initial boot fails
  • 9600 console baud rate
     

0x2102
  • Ignores break
  • Boots into ROM if initial boot fails
  • 9600 console baud rate default value for most platforms
     

0x2120
  • Boots into ROMmon
  • 19200 console speed
     

0x2122
  • Ignores break
  • Boots into ROM if initial boot fails
  • 19200 console baud rate
     

0x2124
  • NetBoot
  • Ignores break
  • Boots into ROM if initial boot fails
  • 19200 console speed
     

0x2142
  • Ignores break
  • Boots into ROM if initial boot fails
  • 9600 console baud rate
  • Ignores the contents of Non-Volatile RAM (NVRAM) (ignores configuration)
     

0x2902
  • Ignores break
  • Boots into ROM if initial boot fails
  • 4800 console baud rate
     

0x2922
  • Ignores break
  • Boots into ROM if initial boot fails
  • 38400 console baud rate
     

0x3122
  • Ignores break
  • Boots into ROM if initial boot fails
  • 57600 console baud rate
     

0x3902
  • Ignores break
  • Boots into ROM if initial boot fails
  • 2400 console baud rate
     

0x3922
  • Ignores break
  • Boots into ROM if initial boot fails
  • 115200 console baud rate

 

 

 

 

Troubleshoot Configuration Register Issues

 

An inappropriately set configuration register can cause many problems, such as:

 

  • The configuration file is ignored.
  • There is no output or garbage output from the console.
  • Booting into ROMmon.

 

Change the configuration register to an appropriate setting, such as the factory default 0x2102, in order to solve these problems.

 

Troubleshooting when the Configuration Register Value is Not Known

 

If the configuration register value is not known, try to establish a Telnet or console session with the router. You can then check the show version output to determine the value of the configuration register:

 

Router#show version

Cisco Internetwork Operating System Software

IOS (tm) 2500 Software (C2500-JS-L), Version 12.1(5), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-2000 by cisco Systems, Inc.

Compiled Wed 25-Oct-00 05:18 by cmong

Image text-base: 0x03071DB0, data-base: 0x00001000

ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE

BOOTFLASH: 3000 Bootstrap Software (IGS-RXBOOT), Version 10.2(8a), RELEASE SOFTWARE (fc1)

Router uptime is 7 minutes

System returned to ROM by reload

System image file is "flash:c2500-js-l_121-5.bin"

cisco 2500 (68030) processor (revision D) with 16384K/2048K bytes of memory.

Processor board ID 03867477, with hardware revision 00000000

Bridging software.

X.25 software, Version 3.0.0.

SuperLAT software (copyright 1990 by Meridian Technology Corp).

TN3270 Emulation software.

1 Token Ring/IEEE 802.5 interface(s)

2 Serial network interface(s)

32K bytes of non-volatile configuration memory.

16384K bytes of processor board System flash (Read ONLY)

Configuration register is 0x2142