Hi @SAM-Al Between the MXs and the MS425 you can use any VLAN number I would suggest not to tag traffic, just configure the MX devices with all LAN ports as Access VLAN X (Enable the option below then configure them), and the MX425 with Access VLAN X (choose X to be unique VLAN to keep a clean design). I would not recommend to keep a different VLAN between the MXs only, as this will be a confusing design, and it is preferred to have different paths between the MX devices and the switches, you are protected from loops by RSTP, bottom line, after all the MX LAN is actually a switch. On your second point, the MX devices will only have single VLAN configured and there is no overload, however you need to design your next hop carefully (the one which gets you to the end Access VLANs) just print the diagram and start simulating what if the one of the MXs is down, what if one of the switches is down, etc...
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