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)

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:

Router#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-JS-L), Version 12.1(5), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2000 by cisco Systems, Inc.
Compiled Wed 25-Oct-00 05:18 by cmong
.................................................
.................................................
.................................................
1 Token Ring/IEEE 802.5 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read ONLY)
Configuration register is 0x2102

The factory-default setting for the configuration register is 0×2102. 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

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.

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.

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:

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 0×2102, 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:

Router#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-JS-L), Version 12.1(5), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2000 by cisco Systems, Inc.
Compiled Wed 25-Oct-00 05:18 by cmong
Image text-base: 0x03071DB0, data-base: 0x00001000
ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE
BOOTFLASH: 3000 Bootstrap Software (IGS-RXBOOT), Version 10.2(8a), RELEASE SOFTWARE (fc1)
Router uptime is 7 minutes
..........................................................................................
..........................................................................................
..........................................................................................
TN3270 Emulation software.
1 Token Ring/IEEE 802.5 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read ONLY)
Configuration register is 0x2142

Set the Configuration Register

Use the table in Configuration Register Settings and their Meaning to determine the desired configuration register setting (usually 0×2102).
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
Router(config)#config-register 0x2102
Router(config)#end
Router#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-JS-L), Version 12.1(5), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2000 by cisco Systems, Inc.
Compiled Wed 25-Oct-00 05:18 by cmong
Image text-base: 0x03071DB0, data-base: 0x00001000
............................................................................
............................................................................
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read ONLY)
Configuration register is 0x2142 (will be 0x2102 at next reload)

The new configuration register setting 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.

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