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)
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:
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 (registeredcustomers 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 |
|
| 0x1202 |
|
| 0x2101 |
|
| 0x2102 |
|
| 0x2120 |
|
| 0x2122 |
|
| 0x2124 |
|
| 0x2142 |
|
| 0x2902 |
|
| 0x2922 |
|
| 0x3122 |
|
| 0x3902 |
|
| 0x3922 |
|
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:
|
| 06 | 0x0040 |
|
| 07 | 0x0080 |
|
| 08 | 0x0100 |
|
| 10 | 0x0400 |
|
| 5,11,12 | 0x0020, 0x0800, 0x1000 |
|
| 13 | 0x2000 |
|
| 14 | 0x4000 |
|
| 15 | 0x8000 |
|
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:
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:
The new configuration register setting becomes active once the router reloads.
Set the Configuration Register from ROMmon
Set the configuration register with the confreg command if the router is in ROMmon mode:
You must reset or power-cycle for the new configuration register to take effect.
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