5ghz outdoor perfect line of sight link speed inexplicably dropped

[RumorConsumer]

Hey all. I usually write about the retreat center I manage but this is about another site I help out with nearby which is a residence.

Its VERY rural. No cell signal on any carrier rural. No other wifi in the area at all. Miles off the highway.

Two buildings. First building has an MR84 with dual omnis (seen in the distance below On the front peak of that building - about 200ft) connected to an MX64 which is in turn connected to a brand new Viasat satellite modem providing 30mbit/3mbit of high latency goodness. It actually has worked very well. There is an MR52 inside that far building supplying indoor signal which is also connected to the MX64. 

The second building (the one I am on top of in this picture) has an MR74 (in frame) with a set of omnis on the 2.4 connectors and a 5ghz sector on the bottom pointed directly at the MR84 as you can see. This MR74 is then connected to a PoE+ switch indoors that has a MR52 on it as well creating a "wired hop" so that the MR52 uses the 74's connection and doesn't try to get its own. It has work flawlessly. The MR74 has gotten a fairly consistent 95mbit mesh link to the M484 even though we aren't using that much bandwidth ever per our satellite max bandwidth. But its been stable and awesome. 

For some reason in the last week or so the mesh speed between the 84 and the 74 has plummeted to 17mbit or so. Its meshing over 5ghz so thats not the problem. Not sure what could be going on. The only thing that changed is I did the most recent firmware update on the MX64 that came out. Not sure how that would change anything.

The MR84 has been using an 80mhz channel for 5ghz while the MR74 has been using a 20mhz wide channel. 

@PhilipDAth had mentioned that the differential may be because we just aren't using much bandwidth and its tuning down for more stability. Fine. But again Ive pulled nearly 100mibt over this link as is for at least a month without hiccup.

Some more data

About 20 minutes ago I told the network to do the update auto channels from the radio settings page. All seems fine but still low link speed. Currently the 74 reports a 16mbit link speed with a 1723 "avg metric" which is damn good. It has a 51db connection to the MR84 over 5ghz with less than 4% packet loss. Thought? 

from MR74

[RumorConsumer]

Ok interesting per channel assignments it does look like they are connecting on 2.4 now which is so weird. It’s possible i missed this. Why would it suddenly change over especially when it still reports the 5ghz connection is so much more powerful?

[RumorConsumer]

Follow up 

the MR74 unit kept using the worst mesh links it’s so weird. So i manually set it to the 5ghz channel of the 84 and it meshed right up back to 100mbit. Why the heck do I need to force this when the connection is literally 10x better?? @MerakiDave ??

[BrechtSchamp]

I wonder if radar is messing with your channel assignment.

Check this link:

https://documentation.meraki.com/MR/Radio_Settings/Dynamic_Frequency_Selection_(DFS) 

Have a look if there are any DFS events in the logs.

[RumorConsumer]

Negative, checked logs. Fantastic suggestion tho. It’s so so weird. Literally every other way it could mesh is horrifically bad except the one it won’t do. 51db, now a solid 98mbit, index is like 1178. It’s ideal yet demands a manual assignment?

[PhilipDAth]

 I understand it, to force it to use the 5Ghz channels you have to use manual channel assignment, like you have done.

https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/Wireless_Mesh_Networking#How_does... 

[RumorConsumer]

Yeah but why for a month plus was it working flawlessly and finding the mesh route that i just had to force it to? Makes no sense,

[RumorConsumer]

I force this MR74 to be on the same channel as the MR84 and it meshes flawlessly. 90-100mibt, perfect 1278 connection index score. I take it back to automatic and it loses its mind. Currently its connecting to the *indoor AP* in that building in the distance instead of the *outdoor AP* it is aimed directly at at a mesh speed of 33mbit instead of the 90-100 I can force it to on the same channel as the MR84. It wont even see the outdoor MR84 on the RF tab under mesh neighbors despite it being *right there*. That looks like a bug. 

@MerakiDave @MerakiDan @MerakiAdam @MerakiCAM @merakigenius 

[RumorConsumer]

I ended up manually configuring all my 5ghz channels and it works fine now. And I dont believe I should have to do that given how, IMHO, obviously my network is set up for it all to work successfully like it did a few months ago with no help at all.