Cisco IOS Cookbook, 2nd Edition

Booting a Different IOS Image

Problem

You want to boot using an alternate IOS image.

Solution

To specify which IOS image the router should load next time it reboots, use the boot system command:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#boot system flash:c3620-jk9o3s-mz.122-7a.bin
Router1(config)#boot system flash:c3620-jos56i-l.120-11.bin
Router1(config)#boot system slot0:c3620-ik9s-mz.122-13.bin
Router1(config)#boot system rom
Router1(config)#end

Tip

The sequence of the boot system commands is extremely important, as the router will attempt to load the IOS images in the order that they appear in the configuration file.

Discussion

The router can store as many IOS images in its flash memory as there is space to hold. If there is only one file, it can safely assume that this must be the IOS image to load. However, if the router has several images in its flash storage, you need to specify which one it should load, or the router will simply select one. This is particularly true on routers that have additional flash memory in the form of PCMCIA cards, which can hold many files, not all of which are even necessarily IOS images.

With the default configuration register settings, the router will attempt to load the first accessible IOS image it finds in its flash storage. However, loading the first available image might not be appropriate. For instance, in our last recipe we showed that, if you have space, you can download a new IOS ...