Understand Configuration Register Usage on all Routers

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Updated:July 11, 2023
Document ID:50421

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    Introduction

    This document describes how to update the configuration register (config-register) to change router behavior.

    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, ensure that you understand the potential impact of any command.

    Conventions

    Refer to Cisco Technical Tips Conventions for more information on document conventions.

    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)

    • Boot Options (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 the Common Platform Parameters table to set baud rates for different confreg values.

    The configuration register can be set from configuration mode with the config-register command. From ROMmon, execute the confreg command. Issue the show version command to view the current setup of the configuration register:

    Router#show version
    Cisco IOS XE Software, Version 16.10.01a
    Cisco IOS Software [Gibraltar], ASR1000 Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 16.10.1a, RELEASE SOFTWARE (fc2)
    Technical Support: http://www.cisco.com/techsupport
    Copyright (c) 1986-2018 by Cisco Systems, Inc.
    Compiled Thu 29-Nov-18 03:47 by mcpre

    Cisco IOS-XE software, Copyright (c) 2005-2018 by cisco Systems, Inc.
    All rights reserved. Certain components of Cisco IOS-XE software are
    licensed under the GNU General Public License ("GPL") Version 2.0. The
    software code licensed under GPL Version 2.0 is free software that comes
    with ABSOLUTELY NO WARRANTY. You can redistribute and/or modify such
    GPL code under the terms of GPL Version 2.0. For more details, see the
    documentation or "License Notice" file accompanying the Cisco IOS-XE software,
    or the applicable URL provided on the flyer accompanying the Cisco IOS-XE
    software.

    ROM: IOS-XE ROMMON

    Router uptime is 2 minutes
    Uptime for this control processor is 5 minutes
    System returned to ROM by Reload Command
    System image file is "bootflash:asr1002x-universalk9.16.10.01a.SPA.bin"
    Last reload reason: Reload Command

    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:
    http://www.cisco.com/wwl/export/crypto/tool/stqrg.html

    If you require further assistance please contact us by sending email to
    export@cisco.com.

    License Type: Smart License is permanent
    License Suite: AdvUCSuiteK9 
    Next reload License Suite: AdvUCSuiteK9 
    The current throughput level is 10000000 kbps 

    Smart Licensing Status: UNREGISTERED/EVAL MODE

    cisco ASR1002-X (2RU-X) processor (revision 2KP) with 1189381K/6147K bytes of memory.
    Processor board ID FOX1719GE28
    14 Gigabit Ethernet interfaces
    1 Ten Gigabit Ethernet interface
    32768K bytes of non-volatile configuration memory.
    4194304K bytes of physical memory.
    6684671K bytes of eUSB flash at bootflash:.
    0K bytes of WebUI ODM Files at webui:.

    Configuration register is 0x2102

    The factory-default setup for the configuration register is 0x2102. This indicates that the router must 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 Significance

    configreg

    If you know the value of your configuration register, you can determine its significance. For information on the configuration register, potential issues, and fixes, collect the output of the show version command, or the show tech-support command, and input into the Cisco CLI Analyzer tool.

    Note: Only registered Cisco users can access internal tools and bug information.

    This table contains some common parameters 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.

    Platform Common Parameters

    Configuration Register Set At

    Router Behavior

    0x102
    • Ignores break
    • 9600 console baud rates

    0x1202
    • 1200 baud rates

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

    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 rates

    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 rates
    • Ignores the contents of Non-Volatile RAM (NVRAM) (ignores configuration)

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

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

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

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

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

    Note: Also remember that configuration register 0x2142 is used for Password Recovery procedures as it can ignore the contents of NVRAM.

    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 Order Values

    Bit Number

    Hex

    Significance

    00-03

    0x0000-0x000F

    Boots Field Parameters:

    • 0x0000 - Stays at the system bootstrap prompt.

    • 0x0001 - Boots the first system image in onboard Flash memory (EPROM).

    • 0x0002-0x000F- Specifies a default netboot filename. Enables boot system commands that override the default netboot filename.

    06

    0x0040

    • Ignore NVRAM contents.

    07

    0x0080

    • Disable boot messages.

    08

    0x0100

    • Break disabled.

    09

    0x0200

    • Causes the system to select the secondary bootstrap. This is typically not used (set to 0).

    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.

    • Boot into ROMmon.

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

    Troubleshoot the Unknown Configuration Register Value 

    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 IOS XE Software, Version 16.10.01a
    Cisco IOS Software [Gibraltar], ASR1000 Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 16.10.1a, RELEASE SOFTWARE (fc2)
    Technical Support: http://www.cisco.com/techsupport
    Copyright (c) 1986-2018 by Cisco Systems, Inc.
    Compiled Thu 29-Nov-18 03:47 by mcpre

    Cisco IOS-XE software, Copyright (c) 2005-2018 by cisco Systems, Inc.
    All rights reserved. Certain components of Cisco IOS-XE software are
    licensed under the GNU General Public License ("GPL") Version 2.0. The
    software code licensed under GPL Version 2.0 is free software that comes
    with ABSOLUTELY NO WARRANTY. You can redistribute and/or modify such
    GPL code under the terms of GPL Version 2.0. For more details, see the
    documentation or "License Notice" file accompanying the Cisco IOS-XE software,
    or the applicable URL provided on the flyer accompanying the Cisco IOS-XE
    software.

    ROM: IOS-XE ROMMON

    Router uptime is 1 minute
    Uptime for this control processor is 4 minutes
    System returned to ROM by Reload Command
    System image file is "bootflash:asr1002x-universalk9.16.10.01a.SPA.bin"
    Last reload reason: Reload Command

    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:
    http://www.cisco.com/wwl/export/crypto/tool/stqrg.html

    If you require further assistance please contact us by sending email to
    export@cisco.com.

    License Type: Smart License is permanent
    License Level: adventerprise
    Next reload license Level: adventerprise
    The current throughput level is 10000000 kbps 

    Smart Licensing Status: UNREGISTERED/EVAL MODE

    cisco ASR1002-X (2RU-X) processor (revision 2KP) with 1189381K/6147K bytes of memory.
    Processor board ID FOX1719GE28
    14 Gigabit Ethernet interfaces
    1 Ten Gigabit Ethernet interface
    32768K bytes of non-volatile configuration memory.
    4194304K bytes of physical memory.
    6684671K bytes of eUSB flash at bootflash:.
    0K bytes of WebUI ODM Files at webui:.

    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 rates 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 setup. If the router is in ROMmon mode, you can try to issue the boot command to boot the operating system manually.

    rommon 4 > boot bootflash:asr1002x-universalk9.16.10.01a.SPA.bin

    For information on the significance of your configuration register set, and potential issues and fixes, collect the output of the show version command, or the show tech-support command, and input into the Cisco CLI Analyzer tool.

    Note: Only registered Cisco users can access internal tools and bug information.

    Troubleshoot the Known Configuration Register Value 

    If you know the value of your configuration register, review the Configuration Register table to determine the behavior. 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 set to establish a console session, where 9600 baud rate is the default.

    Set the Configuration Register

    Review the the Configuration Register table  to determine the desired configuration register set (usually 0x2102).

    Set the Configuration Register from Configuration Mode

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

    Router#
    Router#configure terminal 
    Enter configuration commands, one per line. End with CNTL/Z.
    Router(config)#config-register 0x2102
    Router(config)#end
    Router#show version
    Cisco IOS XE Software, Version 16.10.01a
    Cisco IOS Software [Gibraltar], ASR1000 Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 16.10.1a, RELEASE SOFTWARE (fc2)
    Technical Support: http://www.cisco.com/techsupport
    Copyright (c) 1986-2018 by Cisco Systems, Inc.
    Compiled Thu 29-Nov-18 03:47 by mcpre

    Cisco IOS-XE software, Copyright (c) 2005-2018 by cisco Systems, Inc.
    All rights reserved. Certain components of Cisco IOS-XE software are
    licensed under the GNU General Public License ("GPL") Version 2.0. The
    software code licensed under GPL Version 2.0 is free software that comes
    with ABSOLUTELY NO WARRANTY. You can redistribute and/or modify such
    GPL code under the terms of GPL Version 2.0. For more details, see the
    documentation or "License Notice" file accompanying the Cisco IOS-XE software,
    or the applicable URL provided on the flyer accompanying the Cisco IOS-XE
    software.

    ROM: IOS-XE ROMMON

    Router uptime is 6 minutes
    Uptime for this control processor is 9 minutes
    System returned to ROM by Reload Command
    System image file is "bootflash:asr1002x-universalk9.16.10.01a.SPA.bin"
    Last reload reason: Reload Command

    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:
    http://www.cisco.com/wwl/export/crypto/tool/stqrg.html

    If you require further assistance please contact us by sending email to
    export@cisco.com.

    License Type: Smart License is permanent
    License Level: adventerprise
    Next reload license Level: adventerprise
    The current throughput level is 10000000 kbps 

    Smart Licensing Status: UNREGISTERED/EVAL MODE

    cisco ASR1002-X (2RU-X) processor (revision 2KP) with 1189381K/6147K bytes of memory.
    Processor board ID FOX1719GE28
    14 Gigabit Ethernet interfaces
    1 Ten Gigabit Ethernet interface
    32768K bytes of non-volatile configuration memory.
    4194304K bytes of physical memory.
    6684671K bytes of eUSB flash at bootflash:.
    0K bytes of WebUI ODM Files at webui:.

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

    The new configuration register set 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.

    Related Information

    Revision History

    Revision Publish Date Comments
    3.0
    11-Jul-2023
    Updated PII, Branding Requirements, Article Description, Contributed By and Formatting.
    2.0
    01-Jul-2022
    Recertification
    1.0
    06-Apr-2004
    Initial Release
    TAC Authored

    Contributed by Cisco Engineers

    • Michelle Staggs
      Cisco TAC Engineer

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