If using neutron + OVSNeutronPlugin or ML2Plugin, you can set up as many external networks, as many you want.

$> neutron net-create  --provider:network_type=flat \
--provider:physical-network=br-ex --shared ext_net -- --router:external=True

Then create a floating IP from this subnet

$> neutron floatingip-create  ext_net
| id                  | 547d4156-078c-426f-8534-027169d51026 |

Then you can assign this floating IP to one of you servers

 $> nova interface-list 6c79552c-7006-4b74-a037-ebe9707cc9ce
 | Port State | Port ID | Net ID | IP addresses | MAC Addr          |
 |  ACTIVE     | 392d179d-7d30-4250-ac27-f166d8b62f9d | ed83138a-7d3a-4365-897a-e742d33c65aa | 10.6.0.1     |       fa:16:3e:ff:5 |
 $>  neutron floatingip-associate  547d4156-078c-426f-8534-027169d51026 \
 392d179d-7d30-4250-ac27-f166d8b62f9d

Note, that bridge 'br-ex' should exists before the first command, so on RedHat you should have a config like

ifcfg-br-ex
DEVICE=br-ex
IPADDR=192.168.100.52 
NETMASK=255.255.255.0
ONBOOT=yes
DEVICETYPE=ovs
TYPE="OVSBridge"

An plumb your 'public interface' into br-ex.

ifcfg-eth2
DEVICE=eth2
ONBOOT=yes
OVS_BRIDGE=br-ex
DEVICETYPE=ovs
TYPE="OVSPort"

If you public network is defined as a vlan over eth2, use

$> neutron net-create  --provider:network_type=vlan --provider:physical-network=br-ex \
--provider:segmentation_id=<vlan id> --shared ext_net -- --router:external=True

You might also check http://docs.openstack.org/admin-guide-cloud/content/l3_workflow.html (l3_workflow)