Fastest Way to Recover And Upgrade Cisco ISO Using tftpdnld

There are many articles about how to recover or upgrade your Cisco IOS, but personally I find that using tftpdnld is the fastest and easiest way to do this.

tftpdnld is a command that you can use in the ROMmon mode of Cisco devices.

Using tftpdnld you can download files directly to the Cisco routers or switches from ROMmon mode using the console cable (serial connection).

In order to use the tftpdnld command you need a TFTP server running in your computer, any TFTP server will do.
In this example I recover my Cisco IOS image for my 2611 router, but this tutorial also good for 2800 and 3800 series routers.
I need to tell you that you can use tftpdnld to download image file from TFTP server to the router but not from the router to TFTP server.
Also this command requires you to use the first LAN port in the router - in 2611 is ethernet 0/0 port like shown below.


But in 2621 router you can also use the Token Ring or Fast Ethernet port.

WARNING!!!
Use this tutorial only if you understand the effect of the commands provided and you aware the effect of the procedures shown to a production or running network!

 

First thing you need to do to recover or upgrade Cisco IOS is prepare your IOS image file in the TFTP server folder.
Next is to get in to the ROMmon mode of the router.
In the ROMmon prompt, you need to set up some configuration for router to TFTP server connection. In this example I use the following configuration:

Issue the "set" command in the prompt, it shows the current configuration you've made, next is to type in the following commands: 
rommon 1 > set
rommon 2 > IP_ADDRESS=171.68.171.0
rommon 3 > IP_SUBNET_MASK=255.255.254.0
rommon 4 > DEFAULT_GATEWAY=171.68.170.3
rommon 5 > TFTP_SERVER=171.69.1.129
rommon 6 > TFTP_FILE=c2600-is-mz.121-2.T.bin

 

Make sure you got everything right and your TFTP server is connected to the first LAN Port of the router.
Before issuing the tftpdnld, Cisco docummentation tells that in 2600 and 1720 series router, the tftpdnld in ROMmon mode might report a bad checksum comparison when it loads Cisco IOS software images of Cisco IOS Software Release 12.0(2.2)T or later - it's a bug thing.
To overcome this problem you need to issue another command:

rommon 7 > TFTP_CHECKSUM=0

Next you're ready to isssue tftpdnld command, type in the command as shown below then it will show you some output, answer y to the question prompted to you.
The question shows you that the content of the flash memory will be erased and replaced by the downloaded image file.

rommon 8 > tftpdnld
IP_ADDRESS: 10.1.1.1
IP_SUBNET_MASK: 255.255.255.0
DEFAULT_GATEWAY: 10.1.1.1
TFTP_SERVER: 10.1.1.2
TFTP_FILE: c2600-is-mz.121-2.T.bin
Invoke this command for disaster recovery only.
WARNING: all existing data in all partitions on flash will be lost!
Do you wish to continue? y/n: [n]: y
Receiving c2600-is-mz.121-2.T.bin from 10.1.1.2 !!!!!.!!!!!!!!!!!!!!!!!!!.!!
File reception completed.
Copying file c2600-is-mz.121-2.T.bin to flash.
Erasing flash at 0x607c0000
program flash location 0x6044000

rommon 9 > 

 

At this point your new image will be in the flash memory if there is no problem with the connection from TFTP server to the router.
You can verify that your image exist in the flash using the following command:

rommon 9 > dir flash:
File size Checksum File name
4603828 bytes (0x463fb4) 0x9719 c2600-i-mz.121-2.T.bin
rommon 10 >

Last thing to do is to set the boot command to tell the router that you want it to boot using the image file you just downloaded:

rommon 10 > boot flash:c2600-i-mz.121-2.T.bin
program load complete, entry point: 0x80008000, size: 0x51c0dc
Self decompressing the image : #################################################
##################################

 

That's it you just successfully recover your Cisco IOS image, check the new image using the show version from the router prompt:

Router2611>sh version
Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-I-M), Version 12.1(2)T, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2000 by cisco Systems, Inc.
Compiled Tue 16-May-00 15:15 by ccai
Image text-base: 0x80008088, data-base: 0x80865F64

ROM: System Bootstrap, Version 11.3(2)XA4, RELEASE SOFTWARE (fc1)

Router2611 uptime is 1 minute
System returned to ROM by reload
System image file is "flash:c2600-i-mz.121-2.T.bin"
cisco 2611 (MPC860) processor (revision 0x202) with 22528K/2048K bytes of memory.
Processor board ID JAB0317052N (1135645455)
M860 processor: part number 0, mask 49
Bridging software.
X.25 software, Version 3.0.0.
2 Ethernet/IEEE 802.3 interface(s)
1 Serial network interface(s)
32K bytes of non-volatile configuration memory.
8192K bytes of processor board System flash (Read/Write)
Configuration register is 0x2102 
...