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 0Ã—2102. The “0x” portion represents the value as hexadecimal, while the “2102” portion is the hexadecimal representation of the 16-bit value.
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
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: configÂ
For CCNA exam scope, you need to know two configuration register settings. These are:
0x2102Â is the factory-default configuration register value.
0x2142Â 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,Â 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|
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:
|00-03||0x0000-0x000F||Boots Field Parameters:
|5,11,12||0x0020, 0x0800, 0x1000||
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.