Nov 26, 2009

Download the IOS to the nonworking Router

The best way to accomplish this is via Ethernet. The alternative—using XMODEM through the console—will be very slow, even if you increase the serial port baud rate to the maximum. So, instead, connect the PC's reverse Ethernet cable to the nonworking router (or use two normal/straight-through Ethernet cables and a switch).

Without an IOS, the nonworking router will boot up in ROMmon mode. The prompt will look like this:
rommon>

Verify the necessary settings

On the console of the nonworking router while in ROMmon mode, use the setcommand to display the current settings, which the program will use to download the IOS via TFTP. (For a Cisco example of these steps, check out this Cisco documentation.)
For example, here are the settings you must set for the tftpdnld command to work:
rommon> set
IP_ADDRESS=10.253.100.126
IP_SUBNET_MASK=255.255.0.0
TFTP_SERVER=10.253.15.72
DEFAULT_GATEWAY=10.253.1.1
TFTP_FILE={the name of the IOS that is saved on the PC}
However, your settings may not contain any of the required variables for the tftpdnldcommand. By default, it may look something like this:
rommon 9 > set
PS1=rommon ! > 
BSI=0
RET_2_RUTC=0
RET_2_RTS=
?=1
If this is the case, set the required variables. While it might seem obvious to use theset command to set the variables, you can actually set the variables simply by entering the name of the variable, the equal sign [=], and the value.
For example, to set the IP address of the broken router, you would use the following:
IP_ADDRESS=10.253.100.126

Final steps

After setting your values, run the tftpdnld command, which will replace the IOS.

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