• Can OpenStack Run Over a VXLAN Fabric Without an Overlay Controller?

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At the OpenStack Summit in Hong Kong at the end of 2013, I gave a talk (video, slides) on the requirements, tradeoffs, and potential designs for deploying OpenStack over a VXLAN fabric. It’s been long enough that it feels like it’s time to revisit the topic. More specifically, I want to focus on the question of whether you can now build such a fabric with a mix of both hardware and software networking elements while only running standalone Neutron, which wasn’t really possible back when I originally gave the talk. Using an external overlay controller was considered the only way to make it work, but in two years a lot has changed.

The need for cooperation between physical and virtual networking elements is even more important now that Ironic and Neutron work together to provide automated orchestration of tenant networks for both virtual machines and bare metal servers. But I won’t rehash the reasons for using VXLAN, the requirements for production deployments, and the key design decisions and associated tradeoffs here, as they largely remain the same – plus you can just watch my original talk :)

One of the proposed designs that meets all the requirements is to run Neutron without an external overlay controller (named “Standalone Neutron”) managing a mix of software and hardware VTEPs (Virtual Tunnel End Points). The vswitches on each compute node are the software VTEPs and front the virtual machines. The top of rack leaf-switches can be hardware VTEPs for any physical infrastructure that needs to be brought into the tenant networks (ie for physical servers, existing infrastructure not managed by OpenStack, non-virtualized firewalls and load balancers, etc). Here’s the basic picture from my original presentation:

Screen Shot 2016-01-27 at 12.11.32 AM

Back in 2013, this sort of design wasn’t really possible in practice as Neutron lacked both:

  • a way for exchanging VXLAN reachability information between physical and virtual VTEPs, and
  • a general model and API for VXLAN gateway nodes, to enable mapping physical infrastructure into tenant networks

Let’s first recap why both of these are necessary.

When building a VXLAN fabric, all of the different VTEPs must learn about every other VTEP in the network and which VXLAN networks they’re participating in. This is required so that each VTEP knows where to flood all BUM (Broadcast, Unknown unicast, and Multicast) traffic. In a standalone Neutron world, Neutron is itself responsible for configuring each software VTEP (ie vswitch) with the location of every other software VTEP. In order to bring other VTEPs into the picture, there needs to be a way to similarly exchange this information with other entities.

A related issue is the need to model VXLAN gateway nodes in OpenStack. Ultimately to bridge physical infrastructure into tenant networks, Neutron needs to provide an API to the end user for mapping which parts of the physical network should map into which tenant network. Without such an API, the only alternative is to do this manually, which isn’t really practical.

So where do things stand now, two years later? Is this deployment now possible?

The short answer is – yes! Neutron now has a model for managing VXLAN gateway nodes as of the Kilo release via the L2 Gateway project, an effort chaired by Arista’s DE Sukhdev Kapur. There’s now also an L2 Gateway plugin written by our friends at HP which implements the standard OVSDB VTEP schema to manage hardware VXLAN gateways. Together this means that a standalone Neutron can now in fact manage a mix of hardware and software VTEPs without the use of a separate overlay controller. In Arista’s case, the L2 Gateway plugin integrates with our CloudVision eXchange via OVSDB, thus bringing the physical infrastructure under Neutron’s control.

The introduction of OVN, an open-source overlay controller for Open vSwitch, provides another interesting alternative to deploying standalone Neutron. Back in 2013, the only options ready for production deployments were commercial overlay controllers. This created the need for an open-source alternative, where having Neutron itself play the role of the overlay controller managing a mix of software and hardware VTEPs was one solution. But OVN also supports the OVSDB standard for managing hardware VXLAN gateways, thus enabling a VXLAN fabric with a mix of hardware and software VTEPs. All that’s missing is for it to expose the L2 Gateway API by implementing an L2 Gateway Plugin for OVN. So while relatively new, OVN is a compelling option when looking for an open source alternative to commercial overlay controllers.

Another new development in this context is the emergence of EVPN as an open control plane for exchanging VXLAN reachability information. EVPN was in use by a few vendors back in 2013, but largely as a mechanism for coordinating between their own controllers, or between their controllers and their hardware devices. The big change over the past two years is that there’s now broader acceptance across all the major vendors on this direction. While this doesn’t solve the question of how Neutron can configure physical infrastructure to map into tenant networks, it does make the problem of how to share reachability information between VTEPs from multiple vendors more achievable. In the context of OpenStack and virtual machines, running EVPN to each compute node doesn’t seem that tractable, so some intermediary will continue to be necessary. There are various proposals to bring EVPN in open-source form to Neutron, including this one, but it still remains to be seen how and when this will happen.

So while there is always plenty more work to do, it’s gratifying to look back and see the progress that’s been made in pushing broader deployment options for running OpenStack on top of a VXLAN network fabric!


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