INTERPRETING CONFIGURATION REGISTERS ON CISCO ROUTERS

The Purpose of the Configuration Register

The configuration register can be used to change router behavior such as how the router boots whether you would like it to boot into rommon mode for recovery purpose such as copying an IOS into the router via xmodem protocol, ignore the configuration or disable boot messages and change the console speed settings.

The configuration register can be verified through the "show version" command:

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 0x2102


If you want to change the configuration register, you can use the command "config-register" or "confreg" if it's in rommon.The factory-default setting for the configuration register is 0x2102. This indicates that the router should attempt to load a Cisco IOS® software image from Flash memory and load the startup configuration with a console speed of 9600 baud.

For More information for the list of configuration register values and their meaning, check this cisco site:

http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech
_note09186a008022493f.shtml

Some mistakes made with configuration register is that the configuration file is ignored, there is no output or garbage output from the console and the device boots into rommon. To initially troubleshoot is to check the configuration register.

Setting the Configuration Register

Use the table in Configuration Register Settings and their Meaning to determine the desired configuration register setting (usually 0x2102).

Set the Configuration Register from Configuration Mode

Issue the config-register command to set the configuration register:

Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#config
Router(config)#config-register 0x2102
Router(config)#end
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 11 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 (will be 0x2102 at next reload)

The new configuration register setting becomes active once the router reloads.

Router#reload
System configuration has been modified. Save? [yes/no]: n
Proceed with reload? [confirm]

Set the Configuration Register from ROMmon

Set the configuration register with the confreg command if the router is in ROMmon mode:

rommon 1 >confreg 0x2102

You must reset or power-cycle for the new configuration register to take effect.