You need to configure bridge as you cannot use the physical interface /ethernet interface to communicate with openstack, Either use the linux bridge or openvswitch to achive this , You can follow this post for more information.

I have liberty installation with openvswitch http://docs.openstack.org/liberty/net...

br-ex is ovs bridge with external ip address

physical interface eth0 has no ip address

root@Dell-Precision:~# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master ovs-system state UP group default qlen 1000
    link/ether 90:b1:1c:7b:58:ab brd ff:ff:ff:ff:ff:ff
    inet6 fe80::92b1:1cff:fe7b:58ab/64 scope link
       valid_lft forever preferred_lft forever
3: ovs-system: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default
    link/ether ee:8d:c3:9a:5a:ee brd ff:ff:ff:ff:ff:ff
4: br-ex: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default
    link/ether 90:b1:1c:7b:58:ab brd ff:ff:ff:ff:ff:ff
    inet 10.40.96.199/24 brd 10.40.96.255 scope global br-ex
       valid_lft forever preferred_lft forever
    inet6 fe80::92b1:1cff:fe7b:58ab/64 scope link
       valid_lft forever preferred_lft forever
5: br-int: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default
    link/ether ce:2e:27:2c:dd:49 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::cc2e:27ff:fe2c:dd49/64 scope link


root@Dell-Precision:~# ovs-vsctl show
.
.
.
    Bridge br-ex
        Port "tape53e9f55-f7"
            Interface "tape53e9f55-f7"
        Port br-ex
            Interface br-ex
                type: internal
        Port "tap50b878a9-3a"
            Interface "tap50b878a9-3a"
        Port phy-br-ex
            Interface phy-br-ex
                type: patch
                options: {peer=int-br-ex}
        Port "eth0"
            Interface "eth0"
.
.
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