Would this work? We used to have warnings in Neutron docs indicating that
instances should not be attached to external networks:
"It is important to understand that you should not attach the instance to
Ext-Net directly. Instead, you must use a floating IP to make it accessible
from the external network."

In this particular case and with the OVS plugin, the traffic on the
external network which now hosts tenant VMs (on OpenStack compute nodes)
should get routed from the br-int to the external bridge br-ex using for
example the appropriate vlan id (what if external network does not use
vlan?) and then to the external network without doing the NATing. Would
this traffic go through the veth pair connecting the br-int and br-ex?

Mohammad



From:   Kevin Benton <blak...@gmail.com>
To:     "OpenStack Development Mailing List (not for usage questions)"
            <openstack-dev@lists.openstack.org>
Date:   08/23/2014 01:37 AM
Subject:        Re: [openstack-dev] [Neutron] Use public IP address as instance
            fixed IP



Yes, you should be able to create a shared/external network within Neutron
to accomplish this.


On Fri, Aug 22, 2014 at 7:25 AM, Bao Wang <bywan...@gmail.com> wrote:
  Thank you for your response. Could this be done naturally with Openstack
  neutron or have to be done manually outside neutron ?  As we are
  expecting to orchestrate hundreds of NFV with all similar network
  configuration, programmability is another key element.


  On Thu, Aug 21, 2014 at 3:52 PM, Kevin Benton <blak...@gmail.com> wrote:
   Have you tried making the external network shared as well? Instances
   that need a private IP with NAT attach to an internal network and go
   through the router like normal. Instances that need a public IP without
   NAT would just attach directly to the external network.


   On Thu, Aug 21, 2014 at 7:06 AM, Bao Wang <bywan...@gmail.com> wrote:
     I have a very complex Openstack deployment for NFV. It could not be
     deployed as Flat. It will have a lot of isolated private networks.
     Some interfaces of a group VM instances will need bridged network with
     their fixed IP addresses to communicate with outside world while other
     interfaces from the same set of VM should keep isolated with real
     private/fixed IP addresses. What happen if we use public IP addresses
     directly as fixed IP on those interfaces ? Will this work with
     Openstack neutron networking ? Will Openstack do NAT automatically on
     those ?


     Overall, the requirement is to use the fixed/public IP to communicate
     with outside directly on some interfaces of some VM instances while
     keeping others as private. The floating IP is not an option here

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