/ip firewall mangle
add action=mark-connection chain=prerouting connection-mark=no-mark in-interface=l2tp-out new-connection-mark=l2tp-mark passthrough=yes
add action=mark-routing chain=prerouting connection-mark=l2tp-mark in-interface=bridge new-routing-mark=l2tp-route passthrough=yes
/ip route
add distance=1 gateway=l2tp-out routing-mark=l2tp-route