Oops! I changed the Config-register now what?

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.



2 Responses to “Oops! I changed the Config-register now what?”

  1. have you seen a config register setting of 0x2? I have this on my 4507 and 4510R SupV. I have done some research and cisco is not clear as to what this means. 0x2102 is the standard default “out of the box” config register for routers and according to the 4500 IOS config guide 0x0102 is the default for a 4500. I tried this cool link: http://www.marcuscom.com/confregdecode.html but it is still confusing as to what it means in practical terms, and what it does exactly. I don’t have the luxury of a lab 4500 to test with, Any input you can provide would be greatly appreciated.

    We are not running redundant sup modules just the one it seems pretty straight forward but I would like to understand the differences and nuances of this setting.

  2. amir not sure why it isnt working? huh? in GNS3 not sure if you can even do that kind of thing
    since it is really not a true setup. i mean it is emulating a router but not a true thing. keep looking
    there are mnay guys who are beter than me who can help. me I like the real thing 😉

    have you noticed that this command is not working in gns3 under no circumstances. please test it & update me

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