How to run OpenStack on a single server - using veth pair

I decided I wanted to implement OpenStack using OpenVSwitch. On one server.

The way I decided to do this, was to spin up a KVM virtual machine (VM) as an OpenStack controller, and have it communicate to the bare metal CentOS7 Linux host (that runs the KVM hypervisor libvirt/qemu).

I did not realize how difficult this would be, until I realized that OpenVSwitch cannot leverage Linux bridges (bridges on the host).

OpenVSwitch allows you to create, delete and otherwise manipulate bridges - but ONLY bridges that are under the control of OpenVSswitch. So, if you happen to have a bridge on the Linux host (we will call it br0), you cannot snap that bridge into OpenVSwitch.

What you would normally do, is to create a new bridge on OpenVSwitch (i.e. br-ex), and migrate your connections from br0, to br-ex.

That's all well and good - and straightforward, most of the time. But, if you want to run a virtual machine (i.e. an OpenStack Controller VM), and have that virtual machine communicate to OpenStack Compute processes running on the bare metal host, abandoning the host bridges becomes a problem.

Virt-Manager, does NOT know anything about OpenVSwitches, nor OpenVSwitch bridges that OpenVSwitch controls. So when you create your VM, if everything is under an OpenVSwitch bridge (i.e. br-ex), Virt-Manager will only provide you a series of macvtap interfaces (macvtap, and  for that matter macvlan, are topics in and of themselves that we won't get into here).

So. I did not want to try and use macvtap interfaces - and jump through hoops to get that to communicate to the underlying host (yes, there are some tricks with macvlan that can do this presumably, but the rabbit hole was getting deeper).

As it turns out, "you can have your cake, and eat it too". You can create a Linux bridge (br0), and plumb that into OpenVSwitch with a veth pair. A veth pair is used just for this very purpose. It is essentially a virtual patch cable between two bridges, since you cannot join bridges (joining bridges is called cascading bridges, and this is not allowed in Linux Networking).

So here is what we wound up doing.