/etc/network/interfaces

interfaces

# The loopback network interface
auto lo
iface lo inet loopback

# vdsl modem / wan port
auto eth0
iface eth0 inet static
        address 192.168.10.2
        netmask 255.255.255.0
        # to support jumbo frames on ppp (1500 mtu + 8 byte PPPoE overhead)
        mtu 1508
        post-up /sbin/ethtool -K eth0 gro off gso off tso off

# PPPoE connection
auto provider
iface provider inet ppp
        provider provider

# Bridged interfaces
iface eth1 inet manual
iface eth2 inet manual
iface eth3 inet manual

# LAN Bridge
auto br0
iface br0 inet static
    bridge_ports eth1 eth2 eth3
        address 192.168.1.1
        netmask 255.255.255.0

        # Disable any sort of offloading to avoid buffers/latency
        post-up /sbin/ethtool -K br0 gro off gso off tso off
        post-up /sbin/ethtool -K eth1 gro off gso off tso off
        post-up /sbin/ethtool -K eth2 gro off gso off tso off
        post-up /sbin/ethtool -K eth3 gro off gso off tso off