Cisco IOS Cookbook, 2nd Edition

Warm Upgrade

Problem

You want to upgrade the router IOS version with minimal service interruption.

Solution

To perform a warm IOS upgrade, use the following commands:

Router1#configure  terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#warm-reboot
Router1(config)#end
Router1#reload warm file slot0:c3745-ipbasek9-mz.124-7.bin

Tip

In order to perform a warm IOS upgrade, you must enable the warm reboot feature. For more information regarding the warm reboot feature, please see the previous recipe.

Discussion

Beginning with IOS Version 12.3(11)T, Cisco introduced the ability to perform warm IOS upgrades. The warm IOS upgrade feature allows the router to load a new IOS version into memory and decompress it before transferring control to the new image. Since the router continues to operate normally while the new image is loaded and decompressed, it greatly reduces service interruption.

Tip

The warm upgrade also can be used to downgrade an IOS version.

Before you can use the warm IOS upgrade feature, you must first enable the warm reboot feature and perform a cold reboot. For more information on enabling the warm reboot feature, please see the previous recipe, Recipe 1.19.

Once you’ve enable the warm reboot feature, you can perform a warm IOS upgrade by using the reload warm file command. Note that the command can load the new IOS image from a variety of locations including flash, slot0, slot1, a TFTP server, and so on:

Router1#reload warm file slot0:c3745-ipbasek9-mz.124-7.bin ...