/etc/network/interfaces

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto ens32
iface ens32 inet static
    address 20.1.40.109
    netmask 255.255.255.0
    gateaway 20.1.40.1
    dns-nameservers 20.1.40.23
    post-up ip route add 15.17.160.0/20 via 20.1.40.1 dev ens33 
    # up route add -net 15.17.160.0 netmask 255.255.240.0 gw 20.1.40.1   # OLD WAY
    down route del -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.2.1

# The primary network interface
auto ens33
iface ens33 inet dhcp

Promisc Bridge

auto eth4
iface eth4 inet manual
  up ifconfig $IFACE 0.0.0.0 -multicast up
  down ifconfig $IFACE down

auto eth5
iface eth5 inet manual
  up ifconfig $IFACE 0.0.0.0 -multicast up
  down ifconfig $IFACE down

auto Bridge
iface Bridge inet manual
        pre-up ifconfig eth4 0.0.0.0 promisc up
        pre-up ifconfig eth5 0.0.0.0 promisc up
        post-down ifconfig eth4 -promisc up
        post-down ifconfig eth5 -promisc up
        up ifconfig $IFACE 0.0.0.0 -multicast up
        down ifconfig $IFACE down
        bridge_ports eth4 eth5
        bridge_stp off
        bridge_fd 0
        bridge_ageing 0
        bridge_maxwait 0