<cfg_dir>: /etc/sysconfig/network-scripts/<if>
<gw>: of the <net>
<if>: interface file: e.g. ifcfg-eth0
<ip>: ip address of the host
<net>: <ip>'s network
<nm>: netmask of <net>

NetworkManager-dispatcher-routing-rules

cat <<END >/etc/sysctl.d/99-multinic.conf
net.ipv4.ip_forward = 1
net.ipv4.conf.all.rp_filter = 0
net.ipv4.conf.default.rp_filter = 0
END

# for each net | x elem N and x increasing
echo '20X    netX' >> /etc/iproute2/rt_tables

# for each interface (incl. vlans etc.) not used as
# default route
sed -i 's/(defroute|peerroutes)=yes/\1=no/i' \
    <cfg_dir>/<if>

cat <<END > <cfg_dir>/route-<if>
default via <gw> dev <if> tab netX
<net>/<nm> dev <if> tab netX
END

cat <<END > <cfg_dir>/rule-<if>
from <ip>/32 tab netX
to <ip>/32 tab netX
END