Idea of Cisco Router – Managing Configuration Registers

Managing Configuration Registers

All Cisco routers have a 16-bit software register, which is written into NVRAM. By default, the configuration register is set to load the Cisco IOS from memory and to look for and load the startup-config file from NVRAM.

Binary Version of configuration Register. Value Hex 2102

15  14  13  12  0    0     1    0

11  10  9  8  0     0   0   1

7  6  5  4  0  0  0  0

3  2  1  0  0  0   1  0

Software Configuration Meanings

Bit	Hex	Description
0-3	0*0000-0*000F	Boot field ( see Table 7.4)
6	0*0040	Ignore NVRAM contents
7	0*0080	OEM bit enabled
8	0*0100	Break disabled
10	0*0400	IP broadcast with all zeros
11-12	0*0800-0*1000	Console line speed
13	0*2000	Boot default ROM software if network boot fails
14	0*4000	IP broadcasts do not have net numbers
15	0*8000	Enable diagnostic messages and ignore NVM contents

The Boot Filed (configuration Register Bits 00-03)

Boot Field	Meaning	Use
00	ROM monitor mode	To boot to ROM monitor mode, set the configuration register to 2100. You must manually boot the router with the b command. The router will show the common> prompt.
01	Boot image from ROM	To boot an IOS image stored in ROM, set the configuration register to 2101. The router will show the router (boot)> promot.
02-F	Specifies a default boot filename	Any value from 2102 through 210F tells the router to use the boot commands specified in NVRAM.

Checking the Current Configuration Register Value

You can see the current value of the configuration register by using the show version command ( sh version or show ver for short), as in the following example:

Router#sh version
Cisco internetwork Operating System Software
IOS ™ C2600 Software (C2600-I-M), Version 12.0(3)T3,
RELEASE SOFTWARE (fc 1)
[output cut]
Configuration register is 0×2102

The last information given from this command is the value of the configuration register. In this example, the value is 0×2102, which is the default setting. Notice theshow version command provides the IOS version. In the example above, it shows the IOS version as 120(3)T3.

You can change the configuration register by using the   config-register command. For example, the following commands tell the router to boot from ROM monitor mode and then show the current configuration register value:

Router (config)# config-register 0×0101
Router (config)#^Z
Router#sh ver
[cut]
configuration register is 0×2102 (will be 0×0101 at next reload)

Notice that the show version command shows the command shows the current configuration register value, as well as what it will be when the router reboots. Any change to the configuration register will not take effect until the router is reloaded.