Warm Upgrade | Router Configuration and File Management

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

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.

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
Proceed with reload? [confirm] 
Decompressing the image : ############################################################################################################################################################ [OK]

Once you issue the warm upgrade command and confirm it, the router will begin to load the image and decompress it. Once the image is loaded into memory, the router will transfer control to the new image. The overall down time is minimized.

If there isn't sufficient memory to load and decompress the new IOS image, then the warm IOS upgrade will terminate. In the next example, we attempt to upgrade to a very large image:

Router1#reload warm file slot0:c3745-entservicesk9-mz.124-8a.bin
Proceed with reload? [confirm] 
Router1#
Sep 2 21:58:40.135 EDT: %SYS-2-NOMEMORY: No memory available for warm upgrade 58197272
Router1

Notice that router aborted the upgrade and created a log message.

We note in passing that it is possible to downgrade the router to an IOS version that does not support the warm upgrade feature. However, it is not possible to upgrade to a new IOS image if the current IOS version does not support the warm upgrade feature.