What is a Configuration register? What is the Purpose? What are the values and the meaning?

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What is a Configuration Register?

The configuration register is a 16-bit number, represented in hexadecimal, which controls everything from the way in which a CISCO router boots to whether or not it will process the contents of the startup configuration file. By default, the configuration register on a router is set to a value of 0×2102. The “0x” portion represents the value as hexadecimal, while the “2102” portion is the hexadecimal representation of the 16-bit value.

 

What is a Configuration register? What is the Purpose? What are the values and the meaning?

What is the Purpose of 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)
  • Set and display the configuration register value
  • Force the router into the ROM monitor (bootstrap program)
  • Select a boot source and default boot filename
  • Enable or disable the Break function
  • Control broadcast addresses
  • Load operating software from ROM

 

To view the setting of the Configuration Register, use the following

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
System returned to ROM by reload
System image file is "flash:c2500-js-l_121-5.bin"
cisco 2500 (68030) processor (revision D) with 16384K/2048K bytes of memory.
Processor board ID 03867477, with hardware revision 00000000
Bridging software.
X.25 software, Version 3.0.0.
SuperLAT software (copyright 1990 by Meridian Technology Corp).
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 0x2102

 

To get the CISCO Router into Setup mode you need to enter global Configuration mode by typing: config 

Router(config)#config-register 0x2142

 

For CCNA exam scope, you need to know two configuration register settings. These are:
0x2102 is the factory-default configuration register value.
0x2142 boots from flash without using NVRAM contents good for password recovery.

When the config. register is set to 2142, the router will start in setup-mode, where you can interactive select some options.
The “normal” config. register is 2102.

The various Values for the Configuration Registers and it’s meaning are as follows

 

Configuration Register Setting Router Behavior
0x102
  • Ignores break
  • 9600 console baud
0x1202
  • 1200 baud rate
0x2101
  • Boots into bootstrap
  • Ignores break
  • Boots into ROM if initial boot fails
  • 9600 console baud rate
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 rate
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 rate
  • Ignores the contents of Non-Volatile RAM (NVRAM) (ignores configuration)
0x2902
  • Ignores break
  • Boots into ROM if initial boot fails
  • 4800 console baud rate
0x2922
  • Ignores break
  • Boots into ROM if initial boot fails
  • 38400 console baud rate
0x3122
  • Ignores break
  • Boots into ROM if initial boot fails
  • 57600 console baud rate
0x3902
  • Ignores break
  • Boots into ROM if initial boot fails
  • 2400 console baud rate
0x3922
  • Ignores break
  • Boots into ROM if initial boot fails
  • 115200 console baud rate

 

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:

  • 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
  • Disable boot messages
08 0x0100
  • Break disabled
10 0x0400
  • IP broadcast with all zeros
5,11,12 0x0020, 0x0800, 0x1000
  • Console line speed
13 0x2000
  • Boots default ROM software if network boot fails
14 0x4000
  • IP broadcasts do not have net numbers
15 0x8000
  • Enables diagnostic messages
  • Ignores NVRAM contents

table via: cisco.com

 

In summary these Boot Registers are quite handy:

  • The value range is from 0x0 to 0xFFFF.
  • 0x2102 is the factory-default configuration register value.
  • 0x2142 boots from flash without using NVRAM contents good for password recovery.
  • 0x2101 boots from boot prom image not flash, good for upgrading image on flash.
  • 0x2141 boots from boot prom and ignores NVRAM contents.
  • 0x141, which disables the Break key, ignores the NVRAM configuration, and
    boots the default system image from ROM.
We will be writing an article about how to reset password for a CISCO Router using configuration register very soon.

The DNetWorks Team

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