I have an MX84 that is currently in passthrough mode and behind a Cisco ASA and in front of a Cisco layer 3 switch. When in passthrough mode the MX sources all of its traffic out of the WAN interface which isn't helpful when it needs to reach the AD server that is behind the layer 3 switch. I want to put it in "NAT Mode" so that it becomes a layer 3 device however I don't want it to NAT anything since the ASA is handling all of that. Is it possible to get the MX to be a layer 3 device without having to NAT all traffic to its WAN interface?
There is actually a new "no-nat" mode of operation coming out (I don't think it is released yet). If you ask support they might enable this option for you. This allows you to turn the MX into a router.