Use of the Configuration Register on All Cisco Routers

From Cisco
Use of the Configuration Register on All Cisco Routers
Document ID: 50421
Introduction
This document provides a description of the configuration register (config register).
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
This document is not restricted to specific software and hardware versions.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Conventions
Refer to Cisco Technical Tips Conventions for more information on document conventions.
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)
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
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.

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

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:

Bit Number
Hex
Meaning
00-03
0x0000-0x000F
Boots Field Parameters 0x0000
Stays at the system bootstrap prompt 0x0001
Boots system image on EPROM 0x0002-0x000F
Specifies a default netboot filename
06
0x0040
Ignore NVRAM contents
07
0x0080
Disable boot messages
08
0x0100
Break disabled
10
0x0400
IP broadcast with all zeros
5,11,12
0x0020, 0x0800, 0x1000
Console line speed
13
0x2000
Boots default ROM software if network boot fails
14
0x4000
IP broadcasts do not have net numbers
15
0x8000
Enables diagnostic messages
Ignores NVRAM contents

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.