The Purpose of the Configuration Register

The configuration register can be used to change router behavior in several ways, such as:

• how the router boots (into ROMmon, NetBoot)

• options while booting (ignore configuration, disable boot messages)

• console speed (baud rate for a terminal emulation session)

Note: If the baud rate is set different than the default rate, odd characters are seen on CLI. Check table1 to set baud rates for different Confreg values.

The configuration register can be set from configuration mode using the config-register command. From ROMmon, use the confreg command. Issue the show version command to view the current setting 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 0x2102

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.

Configuration Register Values and their Meaning

configreg

If you know the value of your configuration register, you can determine its meaning. For information on the meaning of your configuration register setting, including potential issues and fixes, collect the output of the show version command, or the show tech-support command, and input into the Output Interpreter (registered customers only) tool. In order to use Output Interpreter (registered customers only) , you must be a registered customer, be logged in, and have JavaScript enabled.

This table contains some common settings which are valid on most platforms.

Note: Check the appropriate hardware installation guide to verify that the configuration register can be used before you change the configuration register on your router to one of the values in this table.

 

If the value you have for the configuration register is not in the table, then determine which bits are set in order to compute the value:

 

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

If you cannot establish a console session, or if you see only garbage characters, a speed mismatch between the router and the terminal emulation software could be the cause. Try to change the baud rate of your terminal emulation software. Possible settings include 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200. Once you establish a session, you can issue the show version command to determine the setting. If the router is in ROMmon mode, you can try to issue the boot command to boot the operating system manually. For information on the meaning of your configuration register setting, including potential issues and fixes, collect the output of the show version command, or the show tech-support command, and input into the Output Interpreter (registered customers only) tool. In order to use Output Interpreter (registered customers only) , you must be a registered customer, be logged in, and have JavaScript enabled.

Troubleshooting when the Configuration Register Value is Known

If you know the value of your configuration register, use the table in Configuration Register Settings and their Meaning to determine the behavior. For information on the meaning of your configuration register setting, including potential issues and fixes, collect the output of the show version command, or the show tech-support command, and input into the Output Interpreter (registered customers only) tool. In order to use Output Interpreter (registered customers only) , you must be a registered customer, be logged in, and have JavaScript enabled. If you can access the router through Telnet, establish a session with the router. If not, set your terminal emulation program to the baud rate indicated by the configuration register setting to establish a console session.

Set 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.

 

 

Show version command

Issuing the show version command on a Cisco network router displays hardware unique information. Review the output of the show version command on a router and try to locate the following information. The output gives you insight into the router’s capabilities, and overall gives you practice reading the output of many Cisco commands:

•     IOS version
•     System uptime
•     Image filename
•     Type of processor
•     Amount of RAM
•     Number of ports on the switch
•     Amount of flash memory
•     Current configuration register

Here is what the show version command displays for one of my routers:

Router1#show version
Cisco IOS Software, C2600 Software (C2600-ADVIPSERVICESK9-M), Version 12.3(4)T4,  RELEASE SOFTWARE (fc2)
Technical Support: https://www.cisco.com/techsupport
Copyright (c) 1986-2004 by Cisco Systems, Inc.
Compiled Thu 11-Mar-04 19:57 by eaarmas
ROM: System Bootstrap, Version 12.2(8r) [cmong 8r], RELEASE SOFTWARE (fc1)
Router1 uptime is 20 minutes
System returned to ROM by power-on
System image file is "flash:c2600-advipservicesk9-mz.123-4.T4.bin"
This product contains cryptographic features and is subject to United
States and local country laws governing import, export, transfer and
use. Delivery of Cisco cryptographic products does not imply
third-party authority to import, export, distribute or use encryption.
Importers, exporters, distributors and users are responsible for
compliance with U.S. and local country laws. By using this product you
agree to comply with applicable laws and regulations. If you are unable
to comply with U.S. and local laws, return this product immediately.
A summary of U.S. laws governing Cisco cryptographic products may be found at:
https://www.cisco.com/wwl/export/crypto/tool/stqrg.html
If you require further assistance please contact us by sending email to
[email protected].
Cisco 2621XM (MPC860P) processor (revision 0x300) with 125952K/5120K bytes of memory.
Processor board ID JAE081160XR (3618058385)
M860 processor: part number 5, mask 2
2 FastEthernet interfaces
1 Virtual Private Network (VPN) Module
32K bytes of NVRAM.
32768K bytes of processor board System flash (Read/Write)
Configuration register is 0x2102

 

 

The show version command provides a lot of information in addition to the version of software that is running on the router. The following information can be collected with the show version command:

The following is a sample output of a show version command.

Router# show version
Cisco Internetwork Operating System Software
IOS (tm) 3600 Software (C3640-J-M), Version 11.2(6)P, SHARED PLATFORM,
RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1997 by cisco Systems, Inc.
Compiled Mon 12-May-97 15:07 by tej
Image text-base: 0x600088A0, data-base: 0x6075C000

ROM: System Bootstrap, Version 11.1(7)AX [kuong (7)AX], EARLY DEPLOYMENT
RELEASE SOFTWARE (fc2)

Router uptime is 1 week, 1 day, 38 minutes
System restarted by power-on
System image file is "flash:c3640-j-mz_112-6_P.bin", booted
via flash
Host configuration file is "3600_4-confg", booted via tftp
from 171.69.83.194

cisco 3640 (R4700) processor (revision 0x00) with 107520K/23552K bytes
of memory.
Processor board ID 03084730
R4700 processor, Implementation 33, Revision 1.0
Bridging software.
SuperLAT software copyright 1990 by Meridian Technology Corp).
X.25 software, Version 2.0, NET2, BFE and GOSIP compliant.
TN3270 Emulation software.
Primary Rate ISDN software, Version 1.0.
2 Ethernet/IEEE 802.3 interface(s)
97 Serial network interface(s)
4 Channelized T1/PRI port(s)
DRAM configuration is 64 bits wide with parity disabled.
125K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read/Write)

Configuration register is 0x2102