I would like to setup my MX84 in passthrough mode behind my Cisco ASA and into a layer 3 switch. I have it configured as a passthrough from the dashboard but it the physical connections to the ASA and the layer 3 switch are where it isn't working as expected. The MX need an IP from the WAN 1 connection but it also need two other connections to the ASA and layer 3 switch respectively. The current documentation doesn't clearly explain.
As a test, if I plug in a laptop to one of the ports it hands out a DHCP address in the same VLAN as the MX but I don't want it to hand out anything to the L3 switch I just want traffic to go through it. I though about plugging the uplink of the L3 switch (currently goes to the ASA) to the MX only, is that what you are suggesting?