I need to force routing over a specific interfaces - as in the following (simplified) picture. Assume that between ether2 and ether3 there's just a cable, and no addresses have been assigned so far. The goal is to have a UTM in bridge mode plugged in with WAN interface connected to ether2 and LAN to ether3.
I cannot just daisy-chain these devices, because real configuration is quite complicated with multiple VLANs (e.g. both Internet and LAN are tagged VLANs on the same wire). I can cope with that, it's just I have no idea how to configure the router so that it push packets using non-quite-optimal path

Any ideas? Thanks in advance!