RT-AX92U 5GHz-2 can only use DFS channels?

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Thw0rted

Regular Contributor
I have an RT-AX92U mesh and configured the 5GHz-2 band for backhaul. In the "Wireless - General" tab, I'm able to select this band, and look at the Control Channel dropdown. It offers every multiple of 4 from 100 - 140. There's also a checkbox for "Auto select including DFS channels".

My problem is, deselecting that checkbox does nothing. If I click Apply, it's checked again when the page reloads. Under System Log -> Wireless Log, the details for the 5GHz-2 look like

Code:
DFS status: state In-Service Monitoring(ISM) time elapsed 19246950ms radar channel cleared by DFS channel 100/80 (0xE06A)

Channel Information                    
----------------------------------------
Channel 100    A Band, RADAR Sensitive
Channel 104    A Band, RADAR Sensitive
Channel 108    A Band, RADAR Sensitive
Channel 112    A Band, RADAR Sensitive
Channel 116    A Band, RADAR Sensitive, Passive
Channel 120    A Band, RADAR Sensitive, Passive
Channel 124    A Band, RADAR Sensitive, Passive
Channel 128    A Band, RADAR Sensitive, Passive
Channel 132    A Band, RADAR Sensitive, Passive
Channel 136    A Band, RADAR Sensitive, Passive
Channel 140    A Band, RADAR Sensitive, Passive

They're all "RADAR Sensitive", which I think means DFS, right? Does this mean that the device is incapable of using a backhaul that isn't subject to radar disruption? Sometimes I have a node get knocked offline for a bit, and I'm worried it's because of DFS issues. (Is there a way to check in the logs when this happens? I don't see any entries under "DFS status", per the paste above.)

I don't have this problem with the 5GHz-1 band, which allows channels from 36-64. The last 4 of those are also "RADAR Sensitive" but the first 4 aren't, so I have it set to channel 36.


ETA: these are devices purchased in Germany, used in Germany; maybe there are non-DFS channels available on this band in other countries?
 

ColinTaylor

Part of the Furniture
They're all "RADAR Sensitive", which I think means DFS, right? Does this mean that the device is incapable of using a backhaul that isn't subject to radar disruption?
Correct.
 

Thw0rted

Regular Contributor
I guess I should have asked explicitly, is that... OK? Normal? Common?

It seems to me like you'd want your backhaul to be as stable as possible, so if all else fails maybe like have the radio that's only capable of DFS channels make a Smart Connect bond with the 2.4 band, and let the 5-1 band act as backhaul, or *something*. Is that possible with stock firmware? I don't think it asks which 5GHz radio is used for Smart Connect, at least by default. (I'd strongly prefer to keep Smart Connect if at all possible.)

Of course it might not be a DFS issue at all -- I've also seen some worrying other log messages so it could be a bigger problem, unrelated to radar.
 

drabisan

Very Senior Member
In Europe DFS channels are allowed to use up to 1W. And that's easy to see on coverage.
There's a price though: any radar or weather station will force that radio to stop working for at least 1 minute. Although such events are easy to find in the logs. If you suspect that's the reason, read carefully the logs after an interruption.
To work around, as someone already advised, use Smart Connect and your backhaul will switch to other radio.
A nasty aspect is a dual radio node will choose the same DFS channel and very likely your backhaul will fall all the way to 2.4. so bad!
 

Thw0rted

Regular Contributor
I do use Smart Connect, which on my AX92U is only possible with 2.4 + 5-1. 5-1 only supports channels 36-64, and 5-2 only supports 100-140, so it's not possible for both 5GHz bands to choose the same channel. I don't think I've ever seen it using 5-1 for backhaul, but then I don't think I've actually seen anything about getting kicked off a DFS channel in the logs so maybe that has never actually been a problem for me in practice.

I'm still losing nodes sometimes, but I'm leaning towards firmware or hardware issues, rather than (my poor understanding of) DFS.
 

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