config-register and boot feild

Answered Question
Mar 1st, 2009

Hi every body!

I have few questions

My book says:

config-register 0x2100 tells router to boot into rmmon mode.

config-register 0x2101 tells router to boot into rx boot mode.

Config-register 0x210x where x is 2---f

tells router to load full featured ios.The router reads startup-config looking for boot system command which will in turn guides the router how to load ios.

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while researching on google, i came across a link that says " config-register 0x2142" instructs the router to load ios and ignore the nvram.

But the book says for any value 2-f in boot field means router checks the startup--config for boot system command to load ios. But the link says 2142 means load the ios and ignores the startup-config.

thanks a lot!

I have this problem too.
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Correct Answer by Mark Yeates about 7 years 10 months ago

Sarah,

Here's a good link that shows all the config-register values and what they do.

http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a008022493f.shtml

HTH,

Mark

Correct Answer by Giuseppe Larosa about 7 years 10 months ago

Hello Sarah,

the config-register 0x2142 is the special value to be used for password recovery.

Also notice that the digits are hex digits so the 4 here is not in the less meaning 4 bits.

So there is no contradiction values among 2-F are meant in the least significant digits (=the last hex digit).

A very important point is the 1 in the second position:

if you put a zero there the router can dump to rommon by simply removing the console cable !

It happened to me recently in a 2620XM that I was prepearing for an installation.

this bit should be the one that inhibits this behaviour not desirable in a production router.

Hope to help

Giuseppe

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sarahr202 Sun, 03/01/2009 - 08:43

Well it looks like there are few exceptions. For example i configured the router as follows

config-register 0x2142

boot system rom

The router boots into Ios without looking at startup-config. How? because if router had looked at startup-config, it wolud have found boot system rom and would have booted into rxbott mode. since it did not boot into rxboot mode, it reinforces the fact that config-register 0x2142 means router will simply boot into ios( full featured) without look at startup-config.

thanks a lot!

Correct Answer
Giuseppe Larosa Sun, 03/01/2009 - 09:19

Hello Sarah,

the config-register 0x2142 is the special value to be used for password recovery.

Also notice that the digits are hex digits so the 4 here is not in the less meaning 4 bits.

So there is no contradiction values among 2-F are meant in the least significant digits (=the last hex digit).

A very important point is the 1 in the second position:

if you put a zero there the router can dump to rommon by simply removing the console cable !

It happened to me recently in a 2620XM that I was prepearing for an installation.

this bit should be the one that inhibits this behaviour not desirable in a production router.

Hope to help

Giuseppe

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