Suggested prerequisite reading
»Cisco Forum FAQ »Config Register Value - router lost configuration

The idea is to set the config-register value back to default, which is 0x2102. Note that certain config-register values not just stop the router to boot up normally but also change the baud speed of the CONSOLE port which makes the CONSOLE port output is unreadable or even does not show at all. Therefore to fix the issue, you need to do the following

1. Make sure the CONSOLE port output become readable
2. Issue the 0x2102 config-register value, either via CLI prompt or ROMMON prompt

Following is an illustration.

If you are directly consoled into the router, open a new HyperTerminal session (or any preferred terminal simulator) with these settings:

Bits per second: 1200
Data bits: 8
Parity: None
Stop Bits: 1
Flow control: None

Once this is open, power cycle the router and press the spacebar for roughly 10 seconds (press and release). If you are able to see the rommon prompt, change the register back to normal by typing:

rommon>confreg 0x2102
rommon>reset

if you are not able to see anything on the screen, close the window and open a new one with these settings:

Bits per second: 9600
Data bits: 8
Parity: None
Stop Bits: 1
Flow control: None

And you should now see the rommon> prompt. Change the register from there and the router should be back in normal mode.

Side Notes:

* If you know the current setting of your router's config register value is, you can use the Config Register calculator to find out how the router is configured when it boots. You can download the calculator from this site
* By any chance the router model is 2600 or 3600 series, check out the following FAQ
»Cisco Forum FAQ »Unreadable output from Cisco Router Console

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Derived from this thread.

Cisco documentation
Configuration Register Setting Descriptions

Discussions

»[HELP] Cisco 1800 garbled output in putty

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by Covenant edited by aryoba
last modified: 2012-04-09 09:40:15