How and why to apply Cisco’s WLC Field Software Upgrade (FUS)

Boring weekend and my -you gotta do something old man- feeling drag me to dust off an old Cisco WLC 2504 I bought years back on eBay. As expected it was still working pretty well so I connected a 1602i AP to play for a while but to my surprise, the AP wasn’t registering to the controller. I could see the AP in the “AP join” section but it seems the discovery messages were being rejected.

The unpopular debug commands had something for me and  debug capwap events enable  was enough for me to realize there was a mismatch between the AP and the controller.

I have a strong vendor neutral background but one of Cisco’s strongest points (and the reason why I’m still with them for routing and switching) is its well documented library and its user base; a quick google search for “Discarding discovery request in LWAPP from AP supporting CAPWAP” got me to a Cisco Community post from a user having the same issue, which turned out to be the AP not being supported in the current software version. 

I was running AireOS and the 1602 APs are supported from and up. “Easy, it’s just a software upgrade” -exclaimed the fool- until finding the following note when downloading at Cisco’s software central:

So, I had to upgrade the FUS first but, what on earth was a FUS!? for my soul I didn’t remember that acronym at all. The Field Upgrade Software is an AES package that allows the controller to update various system-related components such as the bootloader, the recovery image, and base hardware firmware, in summary, it’s yet-another auxiliary piece of software that you rarely touch but that you need to upgrade under certain circumstances because it’s important, so better do it right.

Warning – From here on the FUS upgrade process takes about 30 minutes with the WLC rebooting several times. Be patient and have the WLC connected to a UPS circuit if necessary.

Upgrading the FUS is a straight forward process, I downloaded “AIR-CT2500-K9-2-0-0-0-FUS.aes” from Cisco’s software central and installed it using the following CLI commands, plus the ol-reliable tftp64 for Windows:

transfer download datatype code
transfer download serverip #YourServerIp
transfer download mode tftp
transfer download filename AIR-CT2500-K9-2-0-0-0-FUS.aes
transfer download path /
transfer download start

Once you enter the command  transfer download start  you will get a confirmation prompt with all the configured parameters; be sure to crosscheck the parameters before continuing. If you want to keep track of the process connect to the WLC by console cable instead, only the adventurous trigger this over SSH.

Once the process is over you will see the software version still being your current ( in my case) but the bootloader having upgraded, as well as the field recovery image to the baseline version required by the installed FUS image. Another thing to pay attention is last reset reason, “software reset” is a good indicator but if you see anything else (especially “power loss” or blank) then scroll up the upgrade logs (if you were connected by console cable) and look for failure logs.

Now you can continue with the AireOS software upgrade to 8.3 if that’s your intention. I will have a follow up post with that procedure as well.

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