When you power on a Cisco IOS router, it will take the following steps:

Perform a POST (Power on self test) to discover the hardware and to check if the hardware is working properly.

Copy a bootstrap program from ROM into the RAM, run the bootstrap program.

The bootstrap program looks for the first IOS image on the flash memory, extracts it into RAM and boots the IOS image. When there is no IOS image, it will boot into ROMMON.

When IOS has loaded, it will look for the startup-config file on the NVRAM. If there is none, it will show you a wizard asking you if you want to configure the router.

ROMMON is a very simple OS with a few features. One of the things it is used for is to copy an IOS image to the flash memory in case you accidentally deleted it. You can also use it for password recovery.

There is nothing we can change in step 1 and 2 but we are able to make some changes to step 3 and 4. It’s possible to configure the router so that it will look for an IOS image in another location, like a TFTP server on your network. You can also configure the router so that it looks for the startup-config somewhere else.

Let’s see how we can configure the router to boot another IOS image.

Which IOS image the router wants to use depends on two options:

The default value of the configuration register is 0x2101:

You can find the meaning of the bits in this Cisco document. The first 4 bits define where the router will look for the IOS image. The default value is 0001 which means that it will use the first IOS image that it finds on the flash memory.

Let’s see what is on our flash memory:

There are two images. Cisco IOS uses a number for each filename and will prefer the lowest file number. We can see which image was booted:

Let’s see how I can select a different image. I can configure the router to boot the other IOS image on my flash memory with the boot system command:

After reloading the router, I can verify that it has booted the newer image:

What if I want to boot an IOS image from the network? This could be useful. Perhaps your flash memory is too small to fit the latest IOS image or maybe you want to make sure all routers run the same IOS image without manually copying the images to the flash memory of each router.

The boot system command has some options:

One of the options is a TFTP server. Let’s see if we can boot from a TFTP server:

This tells the router to look for the specified filename on the TFTP server with IP address 192.168.1.2.

The command above however is only half of the story. The default configuration register will only look for the IOS image on the flash memory. We have to change it to tell the router to look for the IOS image on the network:

This tells the router to look for an IOS image on the network, not on the local flash drive. We also have to make sure the router is able to reach our TFTP server. Let’s enable an interface with an IP address that is in the same subnet as our TFTP server:

Now we can save our configuration and reload the router. Once you do, you will see this:

The router does its POST, initializes ROMMON and then loads the IOS image on the flash memory as usual. Once it is loaded, instead of showing you the command prompt you will see this:

It downloads the IOS image that we specified from the TFTP server. Once the transfer is completed, it extracts and loads the image:

Your router will now run the IOS image from the TFTP server.

What if your flash memory is defective but you still want to run IOS somehow? It is possible with ROMMON to download the image and then extract and run it in RAM, without touching your flash memory. When the router boots, hit the CTRL+BREAK combination to get into ROMMON:

Here we have to configure some variables. If your TFTP server is in the same subnet then you can skip the default gateway:

When you are ready, use the tftpdnld command with the -r parameter. This tells the router not to store the IOS image on the flash memory:

You will see a quick overview with your variables and the router will start downloading:

When it has received the file, it will extract it and load the IOS image in RAM. The router will then boot as normal.

You have now learned how to:

Boot a different IOS image on your flash memory.

How to boot an IOS image from a TFTP server.

How to boot an IOS image from a TFTP server using ROMMON.