[Openstack-operators] DVR and public IP consumption

Robert Starmer robert at kumul.us
Thu Jan 28 04:34:53 UTC 2016

I think I've created a bit of confusion, because I forgot that DVR still
does SNAT (generic non Floating IP tied NAT) on a central network node just
like in the non-DVR model.  The extra address that is consumed is allocated
to a FIP specific namespace when a DVR is made responsible for supporting a
tenant's floating IP, and the question then is: Why do I need this _extra_
external address from the floating IP pool for the FIP namespace, since
it's the allocation of a tenant requested floating IP to a tenant VM that
triggers the DVR to implement the FIP namespace function in the first

In both the Paris and Vancouver DVR presentations "We add distributed FIP
support at the expense of an _extra_ external address per device, but the
FIP namespace is then shared across all tenants". Given that there is no
"external" interface for the DVR interface for floating IPs until at least
one tenant allocates one, a new namespace needs to be created to act as the
termination for the tenant's floating IP.  A normal tenant router would
have an address allocated already, because it has a port allocated onto the
external network (this is the address that SNAT overloads for those
non-floating associated machines that lets them communicate with the
Internet at large), but in this case, no such interface exists until the
namespace is created and attached to the external network, so when the
floating IP port is created, an address is simply allocated from the
External (e.g. floating) pool for the interface.  And _then_ the floating
IP is allocated to the namespace as well. The fact that this extra address
is used is a part of the normal port allocation process (and default
port-security anti-spoofing processes) that exist already, and simplifies
the process of moving tenant allocated floating addresses around (the port
state for the floating namespace doesn't change, it keeps it's special mac
and address regardless of what ever else goes on). So don't think of it as
a Floating IP allocated to the DVR, it's just the DVR's local
representative for it's port on the external network.  Tenant addresses are
then "on top" of this setup.

So, in-efficient, yes.  Part of DVR history, yes.  Confusing to us mere
network mortals, yes.  But that's how I see it. And sorry for the SNAT
reference, just adding my own additional layer of "this is how it should
be"  on top.


On Wed, Jan 27, 2016 at 3:33 PM, Fox, Kevin M <Kevin.Fox at pnnl.gov> wrote:

> But there already is a second external address, the fip address that's
> nating. Is there a double nat? I'm a little confused.
> Thanks,
> Kevin
> ------------------------------
> *From:* Robert Starmer [robert at kumul.us]
> *Sent:* Wednesday, January 27, 2016 3:20 PM
> *To:* Carl Baldwin
> *Cc:* OpenStack Operators; Tomas Vondra
> *Subject:* Re: [Openstack-operators] DVR and public IP consumption
> You can't get rid of the "External" address as it's used to direct return
> traffic to the right router node.  DVR as implemented is really just a
> local NAT gateway per physical compute node.  The outside of your NAT needs
> to be publicly unique, so it needs it's own address.  Some SDN solutions
> can provide a truly distributed router model, because they globally know
> the inside state of the NAT environment, and can forward packets back to
> the internal source properly, regardless of which distributed forwarder
> receives the incoming "external" packets.
> If the number of external addresses consumed is an issue, you may consider
> the dual gateway HA model instead of DVR.  This uses classic multi-router
> models where one router takes on the task of forwading packets, and the
> other device just acts as a backup.  You do still have a software
> bottleneck at your router, unless you then also use one of the plugins that
> supports hardware L3 (last I checked, Juniper, Arista, Cisco, etc. all
> provide an L3 plugin that is HA capable), but you only burn 3 External
> addresses for the router (and 3 internal network addresses per tenant side
> interface if that matters).
> Hope that clarifies a bit,
> Robert
> On Tue, Jan 26, 2016 at 4:14 AM, Carl Baldwin <carl at ecbaldwin.net> wrote:
>> On Thu, Jan 14, 2016 at 2:45 AM, Tomas Vondra <vondra at czech-itc.cz>
>> wrote:
>> > Hi!
>> > I have just deployed an OpenStack Kilo installation with DVR and
>> expected
>> > that it will consume one Public IP per network node as per
>> >
>> http://assafmuller.com/2015/04/15/distributed-virtual-routing-floating-ips/
>> ,
>> > but it still eats one per virtual Router.
>> > What is the correct behavior?
>> Regardless of DVR, a Neutron router burns one IP per virtual router
>> which it uses to SNAT traffic from instances that do not have floating
>> IPs.
>> When you use DVR, an additional IP is consumed for each compute host
>> running an L3 agent in DVR mode.  There has been some discussion about
>> how this can be eliminated but no action has been taken to do this.
>> > Otherwise, it works as a DVR should according to documentation. There
>> are
>> > router namespaces at both compute and network nodes, snat namespaces at
>> the
>> > network nodes and fip namespaces at the compute nodes. Every router has
>> a
>> > router_interface_distributed and a router_centralized_snat with private
>> IPs,
>> > however the router_gateway has a public IP, which I would like to getr
>> id of
>> > to increase density.
>> I'm not sure if it is possible to avoid burning these IPs at this
>> time.  Maybe someone else can chime in with more detail.
>> Carl
>> _______________________________________________
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>> OpenStack-operators at lists.openstack.org
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