From Bridge to OVN: Introduction to OVN

In the past few articles, we introduced OpenStack's use of Linux Bridge and Open vSwitch plug-ins. In this article, we will introduce a relatively new, different architecture Neutron plug-in—OVN.

Table of Contents

Overview

OVN can be described as a distributed SDN controller built on top of OVS. Below are some of the capabilities OVN provides:

Provides an abstraction layer via L2 and L3 overlays for OVS, while also enabling management of virtual and physical network connectivity
Supports stateful ACLs using OVS connection tracking
Supports distributed L3 routing via OVS flows, with simultaneous support for IPv4 and IPv6
Enables NAT and load balancing using OVS connection tracking
Distributed DHCP
Supports all OVS datapaths, such as the default Linux kernel datapath or DPDK
Supports L3 gateway functionality from virtual to physical networks
Supports software-based L2 gateway
Can provide connectivity between VMs and containers running on VMs, in scenarios where a second-layer overlay network is not required

OVN not only offers a plugin implementation for OpenStack Neutron (networking-ovn), but also provides a CNI plugin for Kubernetes.

Components

OVN includes the following components:

OVN Northbound Database
ovn-northd
OVN Southbound Database
ovn-controller
ovs-vswitchd
ovsdb-server

OVN Northbound Database

OVN Northbound Database receives network configurations from the CMS (Cloud Management System), which may be OpenStack, Kubernetes, or any other supported system.

ovn-northd

ovn-northd connects to the OVN Northbound Database at the top and the OVN Southbound Database at the bottom. It translates network abstractions (routers, switches, etc.) from the Northbound Database into logical datapath flows within the database.

OVN Southbound Database

The OVN Southbound Database is the core of the OVN architecture, connecting upward to ovn-northd and downward to ovn-controller on every hypervisor.

The OVN Southbound Database stores three types of data:

Physical Network (PN) tables: store information on how to reach the hypervisor and other components
Logical Network (LN) tables: cache virtual network descriptions provided by northd via logical datapath flows
Binding tables: store the relationships between virtual network elements and physical network components

Hypervisors provide data for the Port Network (PN) and Port Binding tables, while ovn-northd provides data for the Logical Network (LN) tables.

ovn-controller

ovn-controller(8) is the OVN agent and software gateway running on each hypervisor. It connects to the OVN Southbound Database, receives OVN configuration and status, and provides data for the PN and Port Binding tables.

ovs-vswitchd and ovsdb-server are original components of OVS itself—a switch that controls traffic flows and a database server that manages and monitors Open vSwitch configurations.

Architecture

Summary

Above is a general overview of OVN's core concepts and components. In the next article, we will discuss how OVN integrates with OpenStack.

From Bare Metal to Cloud: OVN Introduction