Understand Configuration Register Usage On All Routers - Cisco

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)

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 versionCisco IOS XE Software, Version 16.10.01aCisco IOS Software [Gibraltar], ASR1000 Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 16.10.1a, RELEASE SOFTWARE (fc2)Technical Support: http://www.cisco.com/techsupportCopyright (c) 1986-2018 by Cisco Systems, Inc.Compiled Thu 29-Nov-18 03:47 by mcpreCisco IOS-XE software, Copyright (c) 2005-2018 by cisco Systems, Inc.All rights reserved. Certain components of Cisco IOS-XE software arelicensed under the GNU General Public License ("GPL") Version 2.0. Thesoftware code licensed under GPL Version 2.0 is free software that comeswith ABSOLUTELY NO WARRANTY. You can redistribute and/or modify suchGPL code under the terms of GPL Version 2.0. For more details, see thedocumentation or "License Notice" file accompanying the Cisco IOS-XE software,or the applicable URL provided on the flyer accompanying the Cisco IOS-XEsoftware.ROM: IOS-XE ROMMONRouter uptime is 2 minutesUptime for this control processor is 5 minutesSystem returned to ROM by Reload CommandSystem image file is "bootflash:asr1002x-universalk9.16.10.01a.SPA.bin"Last reload reason: Reload CommandThis product contains cryptographic features and is subject to UnitedStates and local country laws governing import, export, transfer anduse. Delivery of Cisco cryptographic products does not implythird-party authority to import, export, distribute or use encryption.Importers, exporters, distributors and users are responsible forcompliance with U.S. and local country laws. By using this product youagree to comply with applicable laws and regulations. If you are unableto 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.htmlIf you require further assistance please contact us by sending email toexport@cisco.com.License Type: Smart License is permanentLicense Suite: AdvUCSuiteK9 Next reload License Suite: AdvUCSuiteK9 The current throughput level is 10000000 kbps Smart Licensing Status: UNREGISTERED/EVAL MODEcisco ASR1002-X (2RU-X) processor (revision 2KP) with 1189381K/6147K bytes of memory.Processor board ID FOX1719GE2814 Gigabit Ethernet interfaces1 Ten Gigabit Ethernet interface32768K 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.

Note: The command config-register cannot change the console baud rate. In legacy Cisco IOS, the config-register command was indeed used to change console baud rate, however, for Cisco IOS XE, you need to configure the line console speed to change the baud rate.

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	OEM mode (to report different product names and copyrights)
08	0x0100	Break disabled.
09	0x0200	Causes the system to select the secondary bootstrap. This is typically not used (set to 0).
10	0x0400	Use 0.0.0.0 as the broadcast IPv4 address.
5,11,12	0x0020, 0x0800, 0x1000	Console line speed
13	0x2000	Boots default ROM software if network boot fails.
14	0x4000	Use the highest address in the subnet as the broadcast IPv4 address.
15	0x8000	Enables diagnostic messages Ignores NVRAM contents

The correct way to configure the console baud rate setting is by entering the line console configuration mode and modifying the speed, as shown in the next examples:

In non-Controller mode Router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router(config)#line console 0 Router(config-line)#speed 19200 Router(config-line)#end In Controller (SDWAN) mode Router#config-transaction Router(config)#line con 0 Router(config-line)#speed 19200

After the reload of the router, in both of the previous scenarios, the config-register is changed to 0x2122.

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 versionCisco IOS XE Software, Version 16.10.01aCisco IOS Software [Gibraltar], ASR1000 Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 16.10.1a, RELEASE SOFTWARE (fc2)Technical Support: http://www.cisco.com/techsupportCopyright (c) 1986-2018 by Cisco Systems, Inc.Compiled Thu 29-Nov-18 03:47 by mcpreCisco IOS-XE software, Copyright (c) 2005-2018 by cisco Systems, Inc.All rights reserved. Certain components of Cisco IOS-XE software arelicensed under the GNU General Public License ("GPL") Version 2.0. Thesoftware code licensed under GPL Version 2.0 is free software that comeswith ABSOLUTELY NO WARRANTY. You can redistribute and/or modify suchGPL code under the terms of GPL Version 2.0. For more details, see thedocumentation or "License Notice" file accompanying the Cisco IOS-XE software,or the applicable URL provided on the flyer accompanying the Cisco IOS-XEsoftware.ROM: IOS-XE ROMMONRouter uptime is 1 minuteUptime for this control processor is 4 minutesSystem returned to ROM by Reload CommandSystem image file is "bootflash:asr1002x-universalk9.16.10.01a.SPA.bin"Last reload reason: Reload CommandThis product contains cryptographic features and is subject to UnitedStates and local country laws governing import, export, transfer anduse. Delivery of Cisco cryptographic products does not implythird-party authority to import, export, distribute or use encryption.Importers, exporters, distributors and users are responsible forcompliance with U.S. and local country laws. By using this product youagree to comply with applicable laws and regulations. If you are unableto 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.htmlIf you require further assistance please contact us by sending email toexport@cisco.com.License Type: Smart License is permanentLicense Level: adventerpriseNext reload license Level: adventerpriseThe current throughput level is 10000000 kbps Smart Licensing Status: UNREGISTERED/EVAL MODEcisco ASR1002-X (2RU-X) processor (revision 2KP) with 1189381K/6147K bytes of memory.Processor board ID FOX1719GE2814 Gigabit Ethernet interfaces1 Ten Gigabit Ethernet interface32768K 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#configure terminal Enter configuration commands, one per line. End with CNTL/Z.Router(config)#config-register 0x2102Router(config)#endRouter#show versionCisco IOS XE Software, Version 16.10.01aCisco IOS Software [Gibraltar], ASR1000 Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 16.10.1a, RELEASE SOFTWARE (fc2)Technical Support: http://www.cisco.com/techsupportCopyright (c) 1986-2018 by Cisco Systems, Inc.Compiled Thu 29-Nov-18 03:47 by mcpreCisco IOS-XE software, Copyright (c) 2005-2018 by cisco Systems, Inc.All rights reserved. Certain components of Cisco IOS-XE software arelicensed under the GNU General Public License ("GPL") Version 2.0. Thesoftware code licensed under GPL Version 2.0 is free software that comeswith ABSOLUTELY NO WARRANTY. You can redistribute and/or modify suchGPL code under the terms of GPL Version 2.0. For more details, see thedocumentation or "License Notice" file accompanying the Cisco IOS-XE software,or the applicable URL provided on the flyer accompanying the Cisco IOS-XEsoftware.ROM: IOS-XE ROMMONRouter uptime is 6 minutesUptime for this control processor is 9 minutesSystem returned to ROM by Reload CommandSystem image file is "bootflash:asr1002x-universalk9.16.10.01a.SPA.bin"Last reload reason: Reload CommandThis product contains cryptographic features and is subject to UnitedStates and local country laws governing import, export, transfer anduse. Delivery of Cisco cryptographic products does not implythird-party authority to import, export, distribute or use encryption.Importers, exporters, distributors and users are responsible forcompliance with U.S. and local country laws. By using this product youagree to comply with applicable laws and regulations. If you are unableto 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.htmlIf you require further assistance please contact us by sending email toexport@cisco.com.License Type: Smart License is permanentLicense Level: adventerpriseNext reload license Level: adventerpriseThe current throughput level is 10000000 kbps Smart Licensing Status: UNREGISTERED/EVAL MODEcisco ASR1002-X (2RU-X) processor (revision 2KP) with 1189381K/6147K bytes of memory.Processor board ID FOX1719GE2814 Gigabit Ethernet interfaces1 Ten Gigabit Ethernet interface32768K 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

• Why Does My Router Lose Its Configuration During Reboot?
• Troubleshoot Password Recovery in Cisco IOS and Cisco IOS XE Routers
• Technical Support & Documentation - Cisco Systems