9130 APs on 9300 WLC : Monitor mode

[Jim Blake]

I'm investigating an issue with a newish installation: 9130APs in RTLS-density deployment running on an HA pair of 9300 controllers.17.3.5a software.

The 5GHz looks fine, but on performing an Ekahau survey it became apparent that the 2.5GHz was not. In fact, the majority of the APs were found, on closer examination to be running with the 2.4GHz radio in Monitor mode, and the 5GHz and the XOR radios both running 5GHz. That would be reasonable if the 2.4 GHz strength around the affected APs was high (from other APs), but it's not.

When I first looked at the Ekahau report, I identified one AP that was leaving a big hole in the 2.4GHz coverage, but on wider investigation, found it was happening in many areas: in fact on one small floor, I found that all but one AP had gone into Monitor/5GHz/5GHz mode, leaving the 2.4GHz very poortly supported.

The WLC was configured by the client, who by his own admission, left the config as basic as it could be, satisfying the "Best Practices" requirements after getting the APs on board and working, but not doing much more than this. I can see no reason for so many APs to have gone to monitor.

BTW, the building is multistory reinforced concrete. I'm not seing much, if any, inter-floor RF bleed, and there are not a lot of rogues in either 2.4 or 5GHz bands; its a relatively clean environment.

Can anyone suggest why the APs should have gone across to Monitor in such great numbers, leaving the 2.4GHz inadequately served?

Thanks

Jim

[Rasika Nayanajith]

It seems like FRA (Flexible Radio Assignment) has been enabled. Maybe due to high co-channel interference (CCI) in 2.4GHz, WLC go ahead & changed those radios to monitor mode. If FRA is not needed you can disable & test it out (that will make all 2.4GHz radios in client serve mode and that can increase CCI)

Note: Always go with proper RF design & make sure your 2.4GHz & 5GHz channels got a proper plan to minimize CCI (RRM normally does that job, However in 2.4GHz you only got 3 channels and many radios need to be disable if you want to minimize CCI in that band)

HTH

Rasika

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[Jim Blake]

Thanks for your comments Rasika. However, it seems very odd that FRA, with which I am very familiar, would chose to shut down nearly ALL 2.4GHz radios and leave areas unserved. The areas affected are too far away from operational 2.4GHz radios to have a useable service, and there are no other sources (rogues) in those areas so the areas are complete black spots.

I originally detected the issue from the output of the Ekahau survey I did. One particular AP was working fine on 5GHz, but there was a 2.4GHz coverage hole around it. Initially, I suspected a failed 2.4GHz radio, but on examination of the radios on that specific AP, I found that the configuration was  Radio 0 ; Monitor, Radio 1:5GHz, Radio 2: 5GHz.

The 2.4GHz radio in Monitor Mode should have meant the WLC detected the AP was in a coverage hole and switched it back on, but it did not, and on wider examination, many of the APs were working similarly.

The APs are configured with "Role Assignment" set to "auto". I could change that to "Client serving", but this would probably force all APs to run a 2.4GHz radio, and result in significant CCI. I could disable FRA, but that would necessitate manual configuration and the inflexibility of static configurations.

This seems to be either a misbehaviour of FRA or a misconfiguration, but the latter seems unlikely because the configuration is so basic

[Rasika Nayanajith]

Hi Jim,

If you get Cisco TAC support in this environment, I would always recommend going with that & see if TAC can find out the reason for this FRA behavior.

As a general note, disabling FRA does not mean everything has to configure manually. But in 2.4GHz disable certain 2.4GHz based on a proper RF design you do bring some predictable outcome as you exactly know how you minimize CCI in that band. Still, you can let RRM handle power & channel (again you have to set some TPC min/max with a certain offset, where a client will not pick 2.4GHz as attractive at a given location compared to 5GHz on the same location).

If you have done this deployment based on the Ekahau RF design, I would go & validate the current setup against that design and identify the gaps. That may bring some improved experience to this environment.

HTH

Rasika

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[Jim Blake]

It's getting stranger....we disabled FRA and the 2.4GHz APs stayed in monitor mode, even after resets. We are going to raise a TAC. Will advise of the outcome

[Arshad Safrulla]

I know that when you enable and disable FRA, radios will retain last known status before disabling as confirmed by one TAC engineer during a tshoot session in one of my TAC cases. But I cannot find the documentation for this.