MT7531 LAN Switch issues

Hi,

I am using an Asus AX 4200 (mediatek FiLogic with MT7531 Switch) and have
some issues with LAN to LAN connectivity.

I use one of the Switch Ports (LAN4) as an uplink to the Network, while I am using
LAN Ports 1 to 3 for an WIIM Media Streamer, an VU+ Settop Box and an FireTV Cube.
In the upstream Network there are two more AX4200s with Openwrt, all AX4200 are
running as dumb access points.

The issue is with LAN to LAN Port connectivity. ARP Packets coming from the upstream Network don't get a reply (tough with tcpdump I have seen them going
out to the device, and also the ARP Reply comes back to the device but does
not get forwarded to the upstream network.)

In practice it causes the iPhone App not to discover the WIIM Media streamer when the iPhone ist associated to another AX4200 (traffic to the device is then LAN to LAN
on the AX4200 in Question), while it works fine when the iPhone is accociated to the same AX4200 where the WIIM is connected to. Means in summary: WLAN to LAN traffic is
ok, LAN to LAN traffic is faulty.

Using Openwrt 23.05-Snaphot with Kernel 5.15 is working as expected, also LAN to LAN
traffic is working good. Using an master build with Kernel 6.6 (and earlier also with 6.1)
shows this failure.

A workaround is to include Ethernet eth1 into the br-lan bridge and use that as the uplink
port, forcing the traffic though the software bridge bypassing the hardware switch.

So I am using the 23.05-Snaphot now but I'd like to go to master because of some
newer features and because WED with Bridger works as expected with master while it
doesn't with 23.05-Snaphot.

I'd assume it is related to the kernel and the DSA subsystem.

Does anyone else made the same observation?
By intent I used the thread title "MT7531 LAN Switch issue" because the error may or may not occour on other devices using the same chipset, which would be interesting to know.

If you google for MT7531 arp lan etc you'll find some possibly related issue, but none of
them explicitely is about LAN to LAN traffic.

Thanks for reading,
best regards,
Stephan

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... seems to be related to