@Kasking at the moment the MX LAN ports aren't treated like switch ports for monitoring, this is why all the devices attached to the MX LAN ports appear to be connected to the MS port that links to the MX.
There is work underway to improve on this so that the device plugged into the MX LAN port will show which port it is connected to on the MX, and so that you can see what devices are connected to each MX port, rather than just seeing that they are connected to the MX somewhere.
I don't have an exact timeline, but it is actively being worked on so I'd hope within a few months at most.
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