Take a look at this schema :-

    Br-ex is supposed to be OVS BRIDGE with IP belongs 192.168.1.0/24, third Ehernet interface eth2 ( Network Node, not   supporting  mgmt network  and VXLAN (GRE, VLAN data) tunnel is supposed to be OVS PORT of br-ex.

Eth2 should be connected to external router . Br-ex is supposed to belong external network the same on which outside router resides

Follow http://docs.openstack.org/kilo/instal...
To configure the Open vSwitch (OVS) service

# service openvswitch-switch restart
Add the external bridge:
# ovs-vsctl add-br br-ex
Add a port to the external bridge that connects to the physical external network interface:
Replace INTERFACE_NAME with the actual interface name. For example, eth2 or ens256.
# ovs-vsctl add-port br-ex INTERFACE_NAME

View http://docs.openstack.org/kilo/instal...
regarding Eth2 configuration in /etc/network/interfaces
Same manual advises :

External on 203.0.113.0/24 with gateway 203.0.113.1

Then your physical outside router should have IP 203.0.113.1, otherwise , at least via my experience,
it won't work.

 Your gateway and external network created via Neutron CLI or via Dashboard should match real ones. I mean IP addresses been used for gateway and external network address and network mask.

otherwise, physical router 192.168.1.1 won't work for you.

auto br-ex
 iface br-ex inet static
 address 192.168.1.(XX)
 netmask 255.255.255.0
 gateway 192.168.1.1
 dns-nameservers 8.8.8.8

# The external network interface
auto eth2
iface eth2  inet manual
        up ip link set dev $IFACE up
        down ip link set dev $IFACE down

Agree with @dbaxps

Actually, what we need to do is just MOVE eth2's original ip TO br-ex.

Before all the operation, we assume that eth2 has YOUR_ETH2_IP. After moving IP to br-ex, we still need to enable eth2 (but has no IP), add eth2 into br-ex, and correct the route table.

You can manually test by the following steps: (not permanent, lost after reboot)

ifconfig br-ex promisc up
ifconfig eth2 0.0.0.0
ifconfig eth2 promisc 
ifconfig br-ex YOUR_ETH2_IP netmask YOUR_NETMASK

ovs-vsctl add-port br-ex eth2

# Then, re-add the default route if needed
ip route add default via YOUR_GATEWAY_IP

@dbaxps 's answer has the right steps for a permanent network configuration for br-ex & eth2.

Finally, you can follow his configuration.