some quick notes on the MS450 with 10G/40G connections on what "works" & "broken" with the latest Beta 15.4 Firmware...  We can connect MS450 to MX95 via 10G using 3rd party 10G SR modules  and the Cisco CVR-QSFP-SFP10G 40G adapter - no problems... (many 3rd party 40G/10G converter modules failed - thankfully we found a Cisco Refresh version of the adapter module but still expensive)..  MS450 to Mellanox NIC with 40G Direct attach 3rd party also works fine..   What is "broken" is trying to link MS450 to MS390 via 40G module (MA-MOD-2X40G) using a Meraki 40G cable MA-CBL-40G-1M no lights nothing... Direct attached cable should work... Support couldn't resolve... the only workaround is to buy two 40G SR-BD modules and your own OM4 Cable which is crazy /expensive  if you only need 1/2 to 1 Meter uplink.. between two switches..   Regarding the MS390 40G to MX95 10G link above even with Meraki SR Modules if you unplug either module and plug back in the Green light comes back on but you cannot ping the other side unless you reboot the MS390 and MX95 - so I think the MA-MOD-2X40G module has some issues on the MS390 ... View more