MR74 and Wireless Meshing

[SM9]

I have a pretty common scenario but haven't found a simple solution with Meraki equipment and hoping the community can help out with some ideas and feedback.

We have a 2-building scenario with a length of <50 feet between them but because of outside factors we cannot run cabling between them. Each building is basically a Faraday cage for RF because of corrugated steel walls.

I would like to use two MR76 devices (acting as bridged stations) with directional ANT-25 or ANT-27 antennas for each side of the wireless bridge between buildings and then a nameless MR within the remote building to serve up a very low bandwidth client (overall throughput is not a concern, it could be 

Two things:

First:

I am told by Cisco that the only fully-supported config that can be done on the remote side of the bridge is by putting a L3 switch...otherwise they all have to run in mesh mode. Getting a L3 switch is both time, space and cost prohibitive right now.

I was also told by Cisco that when in mesh mode, you can run a L2 switch on the remote side but it's only good for supplying PoE and connecting wired clients to the segmented meshed SSID LAN. All wireless traffic will have to bounce through the mesh. I can bust a hole in the wall to wire the exterior bridge but according to them, the interior and exterior APs will still only mesh through the wall still with this design. That's a headscratcher but understandable maybe...but still frustrating that this configuration isn't available according to the engineer.

This doesn't seem to make sense. Why can't the interior AP serving up wireless to clients connect wired through the L2 switch to get to the exterior bridge as if it was a wired client on the remote LAN?

Second:

Frequently Asked Questions for Cisco Meraki Access Point Antennas - Cisco Meraki

According to this on an MR76, the ANT-25 or 27 antennas need to be used in pairs to handle both a vertical and horizontal polarization for each (2.4 & 5Ghz) band. My question is: If I am only interested in horizontal polarization, purely from a connectivity perspective, it seems to me that you could run dual band in two different directions with another antenna configuration--from one top (2.4 Ghz) & one bottom (5 Ghz) connector for one antenna and one top and one bottom connector for the other? I could then punch a hole through the wall for the the antenna cables and put one ANT-25 antenna outside and use the omnis inside for dual band in both directions. With the MR76 being dual-stream instead of quad-stream I don't see why this wouldn't provide the needed coverage.

Am I missing another possible solution?

[GreenMan]

A few things are going on here:

Your post title mentions MR74 (which is End of Sale), whereas the body mentions MR76.   As it happens these work very similarly (see below), but it's probably worth clarifying the detail.
I think much of the first part of your question is covered by the following documentation:
https://documentation.meraki.com/MR/Deployment_Guides/Mesh_Deployment_Guide     and more specifically:

https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/Extending_the_LAN_with_a_Wireless...

One key thing to remember here is that there is no specific 'bridge mode' operation for Meraki MR APs;   the functionality is based around Meraki meshing, which is more general than that;   it looks to use a wireless uplink for a powered up AP who's wired uplink isn't working.

You haven't said very much about how you want to logically treat clients connecting at the far side of the mesh link, which likely affects your options and matching functionality (see the docs for more) .
For your question about antennas, I think you may be mixing up the guidance for different AP models, between those with dedicated radios for each frequency (MR74/6) and those supporting > 2 spatial streams, with both 2.4 and 5 GHz served by the same antenna posts (MR84/6)

For the latter, the expectation wold be that a single client should be able to communicate via all four antenna posts simultaneously - in order to make use of the extra spatial streams - hence all the connected antennas need to provide essentially the same coverage.   This is also why such APs require dual-band antennas.

For the former you could indeed have entirely different coverage areas for the 2.4 GHz versus the 5 GHz - e.g. 5 GHz directional antennas, on the outside of the buildings, pointing at each other, with the expectaion that the mesh link will form across that path - with a 2.4 GHz (maybe omni-directional?   or broad patch?) coverage area inside the building, for clients.   You'd obviously have to choose an appropriate antenna for each and route the cables for one of them through the wall (depending on whether you have the AP indoors or out).   You could not use this separate coverage areas concept with MR84/6

[SM9]

@GreenMan , thanks for taking the time to reply and thank you for the reference links.

The network is very flat and no segmentation is needed for the clients. I had seen the topology examples on the second link but all of the scenarios that have wireless on the remote side require a L3 switch that I was trying to avoid for the reasons described earlier. 

I apologize for using the terms bridge, etc, it is strictly to try and explain the topology.

I see your point about the meshing functionality is strictly designed to be a failover to the wired link. Couldn't the remote AP act as if the wired uplink is up then if connected to the wired L2 switch in the first solution I am proposing? It should only have to act as a mesh member if not, right?

I am glad that my thought process for the MR74/6 is somewhat sound on the antenna side...but you mention separating the bands for each direction. My question is with the dual spatial stream MR74/6 why exactly (other than polarity which shouldn't matter as long as they are on the same azimuth) I cannot have a dual band pair outside going to the outside mesh link and a dual band pair inside to support the clients that way there is less mesh incompatibility and client incompatibility. Am I wrong? I am just trying to understand this and identify why this pretty standard scenario requires an MS250 at minimum!

[GreenMan]

Even when (legacy) clients aren't using multiple spatial streams, they are using antenna diversity to avoid multi-path distortion.   Thus the antennas associated with any one frequency band need to be covering the same area.