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TOPIC: BREAK and the configuration register

BREAK and the configuration register 8 years 4 months ago #26140

  • SteveP
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I'm aware that the configuration register determines what happens when a router boots. I've been looking into resetting passwords and understand that I have to go into ROM mon mode by sending a BREAK during the first 60 seconds. That will allow me to set bit 6 of the configuration register to 1 and then reboot. This ensures that NVRAM isn't checked so I can reload startup-configuration and change the password(s).

However, I'm confused about sending the BREAK and bit 8. If bit 8 is set to 1, this disables BREAK ... so how would I go into ROM mon mode to change the configuration register? Doesn't this ensure that sending a BREAK within the first 60 seconds will be ignored?

I realise that the actual procedure (key presses) depends upon the series of the router so I'm interested in the general principle at this stage.
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Re: BREAK and the configuration register 8 years 4 months ago #26164

  • Elohim
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Everything you said in your post does not make any sense. To break into rommon mode, you need to break out of the boot sequence, i.e. send a ctrl-break to the router. Depending on which terminal emulator you use, your key strokes may be different. If you use hyperterm, the key stroke combination is ctrl-break.

Once in the rommon mode, you need to set the configuration register to 0x2142 (bypass the configuration) and reset the router. The router will boot up bypassing the configuration in NVRAM, you may go into initial setup dialog. Exit out of dialog and go to the command line in enable mode. Load the configuration from NVRAM (copy start run). Once it is loaded, change the password with enable password or enable secret. Save your configuration, change the configuration register back to 0x2102 and reload your router again.


I'm aware that the configuration register determines what happens when a router boots. I've been looking into resetting passwords and understand that I have to go into ROM mon mode by sending a BREAK during the first 60 seconds. That will allow me to set bit 6 of the configuration register to 1 and then reboot. This ensures that NVRAM isn't checked so I can reload startup-configuration and change the password(s).

However, I'm confused about sending the BREAK and bit 8. If bit 8 is set to 1, this disables BREAK ... so how would I go into ROM mon mode to change the configuration register? Doesn't this ensure that sending a BREAK within the first 60 seconds will be ignored?

I realise that the actual procedure (key presses) depends upon the series of the router so I'm interested in the general principle at this stage.
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Re: BREAK and the configuration register 8 years 4 months ago #26169

  • SteveP
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Thank you.

The thing that confuses me is that if I set the configuration register to 0x*1**, the 8th bit is set to 1 which disables BREAK (just as 0x**4* sets the 6th bit to 1 and the configuration file in NVRAM isn't examined for <boot system> commands).

If I disable BREAK in this way, how do I get into ROM mon mode? Will the CTRL-Break key combination in HyperTerminal still work? Is this the same BREAK or is this something different?
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Re: BREAK and the configuration register 8 years 4 months ago #26179

  • TheBishop
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When you jump out of the boot using ctrl-break you are doing so at a point before the configuration register gets read. So even if you disable break it won't matter; you'll still be able to get into the monitor
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Re: BREAK and the configuration register 8 years 4 months ago #26180

  • SteveP
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Perfect - that's what I needed to know. Thank you.
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