Cisco WLC9800 - backup image

[dolahler]

Hi Experts,

I have WLC9800-L running on 17.9.5 code with C9120AXI access-points. These APs have primary image on 17.9.5, and backup image 17.3.6.76

I believe AP backup image should also be on same family as primary image. This is especially critical since in image 17.9.x there are features no longer compatible with 17.3.x.

My questions are:-

1. What is/are the purpose of 'backup image'?
2. Can/should I set the backup image to be identical as primary?
3. How do I set the AP to use a particular 'backup image'

Any ideas/suggestions are greatly appreciated. Feel free to correct me if any of my understanding above is incorrect.

Thanks

[marce1000]

   - In standard single controller environments the backup image does not play any role , as the AP (running) version is dictated by the 9800 software version in use. Applications where it could be used are N+1 High Availability setups where either controller runs an image that is available as backup image on the AP. or during rolling AP upgrades :
                 https://www.cisco.com/c/dam/en/us/td/docs/wireless/controller/9800/17-1/deployment-guide/c9800-ha-rau-apsp-apdp-issu-rel-17-1.pdf
                        

 M.

[jagan.chowdam]

1. The WLC, by default, maintains two images. These images are the primary image and the backup image. The primary image is the active image used by the WLC while the backup image is used as a backup for the active image. The controller bootloader stores a copy of the active primary image and the backup image. If the primary image becomes corrupted, you can use the bootloader to boot with the backup image.

2. You can't have same primary & backup images on the controller. 

3. AP gets its image based on the WLC Primary image. You can't pick and choose primary or secondary image on APs. It'll always be based on WLC Primary image.

The other use of backup image is during the code upgrade. We can do AP image pre-download without rebooting WLC and load them with correct image. When you do WLC Code upgrade, it'll be stored as backup image first and then we switch it to primary before rebooting your WLC. 

Jagan Chowdam

/**Pls rate useful responses**/

[marce1000]

                               >...1. The WLC, by default, maintains two images.
 @jagan.chowdam Isn't that from the aireos days ? I would presume that on 9800 platforms it can hold as many images as the bootflash space will allow for , where the active image is determined by the boot variable 

 M.

[jagan.chowdam]

@marce1000 Thank you for pointing that out. I appreciate the clarification regarding the 9800 platform vs AirOS.

[Rich R]

In practice the 9800 bootflash: tends to run out of space quite quickly with more than 1 IOS-XE version and it only needs 1.
AP on the other hand always tries to keep 2 images.

[dolahler]

Hi marce1000 - thanks for your reply. 

These AP will also be setup with backup controller which has identical hardware & image as primary WLC. With that in mind,would it be useful for the APs to have identical image? sorry for the blunt question.

[marce1000]

                    >...,would it be useful for the APs to have identical image? 
  - I don't think it is ; because APs don't have support for failing partitions and image switchovers for such cases

 M.

[Rich R]

No - that will not offer any advantage as @jagan.chowdam explained above.  It is 100% normal for the backup partition to have a different version on it, including much older version like you see.

If you were planning to migrate the AP to another WLC running a different version then you could install the other WLC software version in the backup partition in preparation for the migration.  On joining the new WLC the AP checks to see whether it has matching software version in either partition already. If it already has a matching version in the backup partition it simply changes that to the active partition and reboots saving it from having to download the software from the WLC.

[Leo Laohoo]

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@dolahler wrote:
What is/are the purpose of 'backup image'?

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TECHNICALLY, in case the primary image gets corrupted (it happens), the AP can boot the secondary image.  I emphasise "technically" because current reality is that when the primary image gets corrupted, the software to "switch" to the secondary image will be guaranteed to hit a bug and render the AP useless until someone consoles into the AP and manually forces the AP to boot the secondary image.  

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@dolahler wrote:
Can/should I set the backup image to be identical as primary?

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There is no such need.

[Rich R]

For info the bug Leo is referring to is the subject of a field notice:
https://www.cisco.com/c/en/us/support/docs/field-notices/741/fn74109.html
The bug which causes the AP to try to boot an image which the AP already knows is corrupted is https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvx32806 which is fixed in all the latest releases.  Also more info in
https://www.cisco.com/c/en/us/support/docs/wireless/wireless-lan-controller-software/221869-safely-upgrade-access-points-avoiding-i.html and 
https://www.cisco.com/c/en/us/support/docs/wireless/catalyst-9800-series-wireless-controllers/220443-how-to-avoid-boot-loop-due-to-corrupted.html

[Gaurav Kansal]

Dear Dolahler,

You can use command on access point to boot your AP with required image:
#configure boot path 1 or 2
1 is for primary image
2 is for backup image
after this issue a reload command and your access point will boot with selected (1 or 2) image.

GoodLuck