Archive for the Config-Register Stuff!! Category

Oops! I changed the Config-register now what?

Posted in Config-Register Stuff!! on July 1, 2008 by itdaddy

OOPs I changed the Config-register now what do I do??

oops have you ever tried changing the config-register values for fun to see what they would do?
I have. I changed the config-register values to these:

0x2142 skip NVRAM
0x2102 boot to NVRAM
0x1  rommon (safe mode)
0x0  rom only (very limited IOS

The command to modify the configuration register value is o/r, followed by the new register value. After pressing enter, use

the command i to initialize the router. The i command reloads the router, using the new configuration register setting. In

this case, a configuration register value of 0×2102 ensures that the router will boot as per the factory default settings.

:Use this when in ROM mode only; great to change config register back to booting to NVRAM. I was stuff here awhile trying to find ROM mode only command to change config-register. Phewww! Here it is. This value 2102 changes it bakc to boot normally from NVRAM. Yeah!

>o/r 0x2102

If you issue the break sequence on a Cisco 2600 router, you’ll be presented with the rommon> prompt. Although the commands

from this prompt are a little different, they achieve the same result. In the example below, we’re setting the configuration

register to 0×2142, which tells the router to ignore the contents of the startup configuration file, since bit 6 has been

set. The command to change the configuration register from the rommon> prompt is confreg, followed by the new register value.

To reload the router, issue the reset command.

rommon 1>confreg 0x2142
rommon 2>reset

After issuing the reset command, the router will reboot using the new configuration register value of 0×2142.

Common configuration register settings and their meanings:

0×2102 The default configuration register setting. The break command is disabled, the contents of NVRAM are processed, and

the router will boot according to the commands stored in NVRAM

0×2101 The break command is disabled. The router will process the contents of NVRAM, but will boot into the RxBoot image

stored in ROM.

0×2100 The break command is disabled, and the router will boot into ROM Monitor mode.

0×2142 The break command is disabled, and the router will ignore the contents of NVRAM during the boot process.

0×2002 The break command is enabled, but otherwise the router will boot normally.


Stop Here.