Warm Upgrade and Reload in Cisco IOS

In this How To we will explain the procedure for Warm Upgrade and Reload.

The examples are shown on a Cisco 2811 Router. The warm upgrade feature of the Cisco IOS was offered in Cisco IOS version 12.3(11)T. The warm reload feature of the Cisco IOS was offered in IOS version 12.3(2)T.

What are the warm upgrade and warm reload features?

Routers are one of the most critical features of your network infrastructure. As network administrators, it is our job to minimize network downtime. Both of these Cisco IOS features — warm upgrade and warm reload — are there to minimize router downtime anytime you do a router upgrade or reboot.

Without further delay, let’s see how to use these powerful Cisco IOS features.

How to perform a warm upgrade in the Cisco IOS

Begin by downloading the new IOS image using your Cisco license from Cisco and placing it either on your PC or the TFTP server of choice.

Next, use the reload warm file command, like this:

Cell_Router# reload warm file tftp://

This command actually has a number of options such as scheduling and creating a comment:

reload [/verify | /noverify] [warm [file url]] [in [hh:]mm | at hh:mm [month day | day month]] [cancel] [text]

The file that you are upgrading from could be any valid source that the router can access.

To reiterate, the reload warm file command allows the router to read and decompress the image while the router continues to process packets. You will be able to ping the router the whole time the image was being downloaded and decompressed. The router is only down for the very short time while the IOS image is overwritten with the new image. From our experience that downtime will be less than 60 seconds. It is amazing to be able to transfer a 45MB file to the router, have it installed, decompressed, and have the router reload, with only 45 seconds of downtime!

How do you use warm reload in the Cisco IOS?

The job of the warm reload feature is to allow you to reload your routers without having to read the IOS image from flash. This allows the router to skip the ROMmon phase and copying the IOS image from flash and decompressing it. In other words, with warm reload, the router is able to reboot much faster than ever before. Regular reloads can take up to one minute where a warm reload takes only 25 seconds.

Using warm reload is simple. You can configure the warm reboot functionality with the warm reboot global command, like this:

Cell_Router(config)# warm-reboot 10 uptime 10

This enables the router to reboot a maximum number of 10 times using the warm reboot function and ensures that the router will sit for no more than 10 minutes after an attempted warm reload that doesn’t result in a successful boot.

Next, you must do one clean, cold reload of the router. After that, you can use the reload warm command to quickly reboot your router.

You can also use show warm-reboot to find the statistics concerning how many warm reloads have happened and how much space is taken up by warm reload storage. Here is what it looks like:

Cell_Router# show warm-reboot
Warm Reboot is enabled
Maximum warm reboot count is 10
Uptime after which warm reboot is safe in case of a crash is 10 (min)

0 warm reboots due to crashes and 0 warm reboots due to requests have taken
place since the last cold reboot
1140 KB taken up by warm reboot storage

Other related commands are show warm-reboot and debug warm-reboot.

A warm reboot looks similar to a normal reboot, but there is no initializing of the memory, no ROMmon initialization, and no decompressing of the IOS image. Again, don’t forget to use the reload warm command or else it is just a regular reload.

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