Cisco IOS Cookbook, 2nd Edition

Booting over the Network

Problem

You want to load an IOS image that is too large to store on your router’s local flash.

Solution

You can load an IOS image that is larger than your router’s flash by configuring the router to first use TFTP to download the image and before booting:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#boot system tftp c2500-io-l.122-7a.bin 172.25.1.1
Router1(config)#boot system flash
Router1(config)#end
Router1#

Discussion

We mentioned in Recipe 1.7 that it is possible to load IOS images over the network at boot time. However, booting from remote IOS images presents some unique challenges. Therefore, we decided to dedicate an entire recipe to remote booting.

One of the most important advantages of booting an IOS image over the network is that it allows you to use images that are larger than your router’s flash. Like any other software, each new IOS image tends to be slightly larger than the previous versions. So it is relatively common to discover that you can’t load the latest IOS version because it is too big to fit in an older router’s flash.

Booting over the network also provides a way of loading a backup IOS image if the primary image fails. As we discussed Recipe 1.7, it’s a good idea to configure your router with at least one backup IOS image to load in case the primary fails for any reason. Even if you have a lot of flash storage, you may find that you can’t store two IOS images at once. So booting ...