This may be an easy one, but I want to make sure I am not missing anything. I am setting up a MX65 that is connected to two MS120 switches. Ports 3 & 4 on the MX will be trunk ports for the uplinks, one to each MS ports 52. I know I can't setup STP configurations on the MX, but is there anything additional that I should be doing to avoid STP issues?
The switches will essentially seem them as connected directly to one another. So, you should plan your STP deployment accordingly. Also, if you trunks are carrying different VLANs be prepared to see VLAN mismatch errors in the dashboard unless you configure both trunks identically.