AP Join Process- Option 43 and DNS Method

[awatson20]

If I have DHCP option 43 configured for my primary WLC discovery method for new out of the box access points, but also have DNS(capwap-cisco-controller) configured, which method will be preferred?  I am being told that if both are setup, the discovery method used depends on which WLC responds first, and also the one that has the least load.  I would think that Option 43 would win since the AP has to go through the DHCP process first.  Can someone clarify this please?  From our testing, we had to remove the DNS entry for cisco-capwap-controller before the AP used option 43.

[Scott Fella]

There is a process in which an ap follows when joining a controller.

https://www.cisco.com/c/en/us/support/docs/wireless/5500-series-wireless-controllers/119286-lap-notjoin-wlc-tshoot.html

[Haydn Andrews]

Option 43 before the DNS

https://www.cisco.com/c/en/us/support/docs/wireless/5500-series-wireless-controllers/119286-lap-notjoin-wlc-tshoot.html#backinfo

https://www.ciscolive.com/c/dam/r/ciscolive/apjc/docs/2016/pdf/BRKEWN-3017.pdf

If you console into the AP when its booting you can watch the steps it takes

[awatson20]

From our testing, that is not exactly the case. If both option 43 and DNS are configured, the AP does not use 43 in all scenarios.

[Scott Fella]

Look at the console output during a join.

[TsvetanVladimirov81607]

Hey there,

If AP is using DHCP to get an IP address, that will be the first thing to check, cause option 43 is sent by default when configured.

Then will be the DNS, broadcast and pre-configured IP addresses.

Therefore if you have static IP, there won't be option 43 immediately available.
If you want to check it, make a span session on the port where the AP is connected and check the order of the messages sent.

[NIRO-7]

Hello there,

I have encountered the same issue here.

To contextualize, I am migrating a WLC AireOS to a 9800, at a big scale, and there will be a period where the 2 controllers will be online together. Also, there are some LAP that will be migrated from the old controller to the new.

Currently, the LAP join controller through DNS. I said that due to the scale, option 43 would be better, but the customer doesn't use DHCP for the APs.

I am currently doing a POC and on a subnet with DHCP, we added option 43 with the IP of the 9800 WLC, and the DNS entry points to the AireOS WLC. I went in console on a 1832 AP, and what happened is that the AP gets her IP from DHCP, with all options, and then during the discovery phase, doesn't care at all about the Option 43 and straightfully send a join request to the AireOS controller and then joins it. 

So, I reseted the AP and booted it on a subnet without DHCP, added the WLC 9800 as primary controller with CLI in console, and then plugged it back on the network. 
The AP Booted, got the IP address from DHCP, did the DNS discovery for the AireOS Controller and then joined the 9800. Still in console, I did a show run and saw the IP address of the AireOS controller under the field "Controller from DNS server".

I then looked at the process for 9115 AP, and during the discovery phase, it collects the IP of the controllers from option 43 and DNS, and then joins the 9800 (because not compatible with AireOS, but in the CLI, it doesn't appears that it tries to join the DNS learned)

So, my question is : Does it depends on the AP to care about the Option 43 ? Because the 1832 clearly doesn't care about it and straightfully resolved CISCO-LWAPP-CONTROLLER.localdomain and then joined it.

Thanks for reading this.

Have a nice day

[mpursell]

I have this same question and scenario. I'm wondering how you made out, or if you had to change anything in the migration process. This setup seems to be working for now, but I'm wondering if I'm going to run into issues once the balance tips and we have more AP's on the 9800 than our old 5520 controller.