How to use the configuration register (config register)

[TCC_2]

Core Issue

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

• How the router boots (into ROM Monitor (ROMmon) or NetBoot)
• Options while booting (ignore configuration, disable boot messages)
• Console speed (baud rate for a terminal emulation session)

This document explains how to interpret the config register setting and how to solve problems resulting from an improper setting.

Resolution

To view the current setting of the configuration register, establish a console or Telnet session with the router. Issue the show version command, and input the contents into the Output Interpreter.  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. Change the baud rate of your terminal emulation software. 

These are possible settings:

• 1200, 2400, 4800,9600,19200,38400, 57600,115200

If the router is in ROMmon mode, issue the confreg command. You receive an explantion of your current setting similar to this:

      
rommon 7 > confreg
Configuration Summry
enabled are:
console baud: 9600
boot: the ROM Monitor

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. For most purposes, the factory default setting of the configuration register is the most appropriate. To change the configuration register to this setting, issue the configuration-register 0x2102 command, as shown:

     Router(config)#config-register 0x2102

From ROMmon mode, issue the confreg 0x2102 command, as shown:

     rommon 1 >confreg 0x2102

For information on using other settings, refer to the The Purpose of the Configuration Register section of Use of the Configuration Register on All Cisco Routers.