Resetting a Cisco 1801 router

This guide explains how to reset password-protected Cisco 1801 routers. To do this we first have to boot into ROM monitor mode. This can be done in two ways:

  • Sending a BREAK command just after boot
  • Removing the flash memory from the device. After two failed boot attempts the router will boot in ROM monitor mode. (this post)

But first, we need a serial to RJ-45 cable, whose RJ-45 end should be connected to the router’s console port. The standard Cisco cable is depicted below:

In case your computer does not have a serial port (like most) you will also need a serial to USB converter, like the one below:

Connect the cable on both ends and proceed to download a terminal emulator. I chose to use minicom, which, on Ubuntu, can be installed with sudo apt-get install minicom. After installing minicom we have to create a profile for connecting to the router. To do so type:

minicom -s
  • Select “Serial port setup”, then press “A” and type the port the router is connected to
    • Assuming you are using a USB-to-Serial converter you can find the port name using dmesg | grep tty. The port name might be something like ttyUSB* (where * is a number)
  • Type “E” and select:
    • Speed: 9600 Baud
    • Parity: None
    • Data: 8
    • Stopbits: 1
  • Then press enter and select “Save setup as..”. Exit minicom
  • Now start minicom with the newly created profile using the following command
minicom <profile-name>

Since I could not send a BREAK command to the router I decided to remove the flash memory card, boot the router and wait until it enters ROM monitor mode (ROMMON), which should take two reboots. After entering ROMMON mode insert back the flash memory and type:

confreg 0x2142

The first command bypasses the password verification on boot and the second one restarts the router. The router will thus reboot and prompt us to perform the initial setup. You can press CTRL + C to skip the setup or answer no to all questions. After that type:

write erase # You will be warned that all configuration files will be gone. Press return

After that, will be restored to its factory settings! Wait until the commands are executed and a CLI prompt appears.


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