Problem:
This document briefly explains how to perform disaster recovery on a Cisco router in a situation where the router is unable to boot because the image is incompatible with the router hardware, there is a CRC error or the system flash memory has been replaced. It may not apply to all router variants and versions, please remember that this information is presented with no warranties expressed or implied.
 

Solution:
First of all, you need the following equipment ready:

1. A running a TFTP server connected to the router via the Ethernet interface
2. A copy of a working system image
3. A Cisco serial console cable
4. A computer with terminal software and at least one available serial port

Note that the disaster recovery can also be performed without using TFTP, but that's so primitive I won't even go into it. TFTP is the most primitive form of network based file transfer in existance, and it's available at no charge for most any operating system. If you want to experiment with X/Ymodem instead, be my guest.

To invoke the disaster recovery:

1. Within 60 seconds after powering up the router, press Ctrl+Break
2. Type "tftpdnld" for a help screen describing how to set up the mandatory environment variables:

IP_ADDRESS

IP_SUBNET_MASK

DEFAULT_GATEWAY

TFTP_SERVER

TFTP_FILE

3. Type "tftpdnld" again to perform the disaster recovery, you will be asked for several confirmations before download will start.