enetworktutor.com : Cisco Router and Switch IOS Upgrade Tutorial

The following information must be determined before attempting to upgrade the
IOS image on the Cisco network device:

Does the router or switch have the minimum required amount of DRAM? Unsure? Just issue the “show version” command & look for the two numbers (i.e.: 45056K/4096K) followed by the words “bytes of memory.” Combine those two numbers & it will give you the total amount of DRAM. The example would be 48 MB of DRAM.
Does the router or switch have enough flash to support the new image? Unsure? Again, issue the “show version” command & it will tell you the total amount of system flash. If you need to know how much is used & available, the command “Show flash” can be used. This is helpful, since it will let you know if it will be required to delete the existing flash image before upgrading to free up space.
Does the router or switch have IP connectivity to the TFTP server? Typically, pinging the TFTP server’s IP address will prove connectivity, unless of course it is behind a firewall that’s blocking ICMP traffic.
Does the image support the hardware platform? This is very basic, but still very important. You can’t run Cisco 2600 code on a 7200 series router! Cisco’s Software Advisor is a good tool to make sure you use an IOS image supported by the hardware platform.

Once you have verified all the above, you can take the following steps to download and boot a new IOS image.

Once this has been completed, you are ready to change the boot statement so that the correct image is booted.

In this example, we are going to specify the new image first, however, we are then going to specify the original image second, in case the router has a problem booting the new one. This ensures our router boots a working image to avoid having a router stuck in rommon mode.

Following this, you will be prompted by the device to answer a few questions:

Address or name of remote host? Put in the IP address of the TFTP server.
Source Filename? This is the image filename. This is typically a .bin file (Example: c2800nm-ipbase-mz.123-8.T6.bin).
Destination filename? Leave this the same, just hit enter.

Then the router will attempt to access the image via TFTP from the specified TFTP server, then it will ask you if you want to erase the existing flash before copying. I do not recommend deleting the existing flash unless you must in order to free up space to hold the new image. Once you have answered this last question, & hit enter, you should starts seeing a lot of “!’s scrolling across the screen, which indicates the file is being downloaded. If the download fails, you will get an error indicating why it failed. The most common type of problems are file permissions issues, IP connectivity issues, or not enough flash to hold the
new image.

Once completed, the flash will most likely be verified automatically, to make sure there is no corruption with the file. However, some platforms allow you to verify it manually by issuing the “verify flash <image-name>” command. This will also check the flash over to make sure it is OK.

The reason why the image “c2800nm-ipbase-mz.123-8.T6.bin” will be booted & not the other is simply because of the order the commands were put in. To verify the order is correct, issue the show run command and make sure the primary image is listed before the secondary.

Once you have finished setting the boot statements, issue the “reload” command from EXEC mode. This will reload the router & load the new image. If the router fails to load the new image, the router will try the second image that was working previously.

To verify the new image was loaded, issue the “show version” command from EXEC mode. This will display the current IOS.

Is the TFTP file transfer over a slow WAN link? If there is a Cisco router that has the code you need and it is reachable over a higher speed link, you can set that router up as a TFTP server. From the router you want to become a TFTP server, issue the following command in global configuration mode:
tftp-server flash:<flash file name>

So if we wanted Router B to act as a TFTP server for the image mentioned above,
we would issue the following command from global-config mode:

SYNTAX	DESCRIPTION
router
copy tftp flash	The first step to begin the process of downloading a new image

SYNTAX	DESCRIPTION
router
tftp-server flash: c2800nm-ipbase-mz.123-8.T6.bin	Set the router up to serve as a TFTP server for the specified image

SYNTAX	DESCRIPTION
router
boot system flash c2800nm-ipbase-mz.123-8.T6.bin	Set the router to first boot this specified image
boot system flash	Set the router to boot this specified image if the previous image can’t be booted