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How to use the configuration register (config register) on a Cisco router

By Brad Reese on Tue, 04/29/08 - 4:36pm.

Cisco How-To Tutorials

The configuration register can be used to change Cisco 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 Cisco How-To Tutorial explains how to interpret the config register setting and how to solve problems resulting from an improper setting.

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 Cisco 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 Summary

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 further information on using other settings refer to:

The Purpose of the Cisco Configuration Register


View more Cisco How-To Tutorials

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