Introduction to SDN – Transformation from legacy to SDN

23 min read

In this article, by Reza Toghraee, the author of the book, Learning OpenDayLight, we will:

  • What is and what is not SDN
  • Components of a SDN
  • Difference between SDN and overlay
  • The SDN controllers

(For more resources related to this topic, see here.)

You might have heard about Software-Defined Networking (SDN). If you are in networking industry this is a topic which probably you have studied initially when first time you heard about the SDN.To understand the importance of SDN and SDN controller, let’s look at Google. Google silently built its own networking switches and controller called Jupiter. A home grown project which is mostly software driven and supports such massive scale of Google.

The SDN base is There is a controller who knows the whole network.

OpenDaylight (ODL), is a SDN controller. In other words, it’s the central brain of the network.

Why we are going towards SDN

Everyone who is hearing about SDN, should ask this question that why are we talking about SDN. What problem is it trying to solve?

If we look at traditional networking (layer 2, layer 3 with routing protocols such as BGP, OSPF) we are completely dominated by what is so called protocols.

These protocols in fact have been very helpful to the industry. They are mostly standard. Different vendor and products can communicate using standard protocols with each other. A Cisco router can establish a BGP session with a Huawei switch or an open source Quagga router can exchange OSPF routes with a Juniper firewall.

Routing protocol is a constant standard with solid bases. If you need to override something in your network routing, you have to find a trick to use protocols, even by using a static route.

SDN can help us to come out of routing protocol cage, look at different ways to forward traffic. SDN can directly program each switch or even override a route which is installed by routing protocol.

There are high-level benefits of using SDN which we explain few of them as follows:

  • An integrated network: We used to have a standalone concept in traditional network. Each switch was managed separately, each switch was running its own routing protocol instance and was processing routing information messages from other neighbors. In SDN, we are migrating to a centralized model, where the SDN controller becomes the single point of configuration of the network, where you will apply the policies and configuration.
  • Scalable layer 2 across layer 3:Having a layer 2 network across multiple layer 3 network is something which all network architects are interested and till date we have been using proprietary methods such as OTV or by using a service provider VPLS service. With SDN, we can create layer 2 networks across multiple switches or layer 3 domains (using VXLAN) and expand the layer 2 networks. In many cloud environment, where the virtual machines are distributed across different hosts in different datacenters, this is a major requirement.
  • Third-party application programmability: This is a very generic term, isn’t it? But what I’m referring to is to let other applications communicate with your network. For example,In many new distributed IP storage systems, the IP storage controller has ability to talk to network to provide the best, shortest path to the storage node. With SDN we are letting other applications to control the network. Of course this control has limitation and SDN doesn’t allow an application to scrap the whole network.
  • Flexible application based network:In SDN, everything is an application. L2/L3, BGP, VMware Integration, and so on all are applications running in SDN controller.
  • Service chaining:On the fly you add a firewall in the path or a load balancer. This is service insertion.
  • Unified wired and wireless: This is an ideal benefit, to have a controller which supports both wired and wireless network. OpenDaylight is the only controller which supports CAPWAP protocols which allows integration with wireless access points.

Components of a SDN

A software defined network infrastructure has two main key components:

  • The SDN Controller (only one, could be deployed in a highly available cluster)
  • The SDN enabled switches(multiple switches, mostly in a Clos topology in a datacenter):

 Learning OpenDayLight

SDN controller is the single brain of the SDN domain. In fact, an SDN domain is very similar to a chassis based switch. You can imagine supervisor or management module of a chassis based switch as a SDN controller and rest of the line card and I/O cards as SDN switches. The main difference between a SDN network and a chassis based switch is that you can scale out the SDN with multiple switches, where in a chassis based switch you are limited to the number of slots in that chassis:

Learning OpenDayLight

Controlling the fabric

It is very important that you understand the main technologies involved in SDN. These methods are used by SDN controllers to manage and control the SDN network.

In general, there are two methods available for controlling the fabric:

  • Direct fabric programming: In this method, SDN controller directly communicates with SDN enabled switches via southbound protocols such as OpenFlow, NETCONF and OVSDB. SDN controller programs each switch member with related information about fabric, and how to forward the traffic. Direct fabric programming is the method used by OpenDaylight.
  • Overlay:In overlay method, SDN controller doesn’t rely on programing the network switches and routers. Instead it builds an virtual overlay network on top of existing underlay network. The underlay network can be a L2 or L3 network with traditional network switches and router, just providing IP connectivity. SDN controller uses this platform to build the overlay using encapsulation protocols such as VXLAN and NVGRE. VMware NSX uses overlay technology to build and control the virtual fabric.

SDN controllers

One of the key fundamentals of SDN is disaggregation. Disaggregation of software and hardware in a network and also disaggregation of control and forwarding planes.

SDN controller is the main brain and controller of an SDN environment, it’s a strategic control point within the network and responsible for communicating information to:

  • Routers and switches and other network devices behind them. SDN controllers uses APIs or protocols (such as OpenFlow or NETCONF) to communicate with these devices. This communication is known as southbound
  • Upstream switches, routers or applications and the aforementioned business logic (via APIs or protocols). This communication is known as northbound. An example for a northbound communication is a BGP session between a legacy router and SDN controller.

If you are familiar with chassis based switches like Cisco Catalyst 6500 or Nexus 7k chassis, you can imagine a SDN network as a chassis, with switches and routers as its I/O line cards and SDN controller as its supervisor or management module. Infact SDN is similar to a very scalable chassis where you don’t have any limitation on number of physical slots.

SDN controller is similar to role of management module of a chassis based switch and it controls all switches via its southbound protocols and APIs.

The following table compares the SDN controller and a chassis based switch: 

SDN Controller

Chassis based switch

Supports any switch hardware

Supports only specific switch line cards

Can scale out, unlimited number of switches

Limited to number of physical slots in the chassis

Supports high redundancy by multiple controllers in a cluster

Supports dual management redundancy, active standby

Communicates with switches via southbound protocols such as OpenFlow, NETCONF, BGP PCEP

Use proprietary protocols between management module and line cards

Communicates with routers, switches and applications outside of SDN via northbound protocols such as BGP, OSPF and direct API

Communicates with other routers and switches outside of chassis via standard protocols such as BGP, OSPF or APIs.

The first protocol that popularized the concept behind SDN was OpenFlow. When conceptualized by networking researchers at Stanford back in 2008, it was meant to manipulate the data plane to optimize traffic flows and make adjustments, so the network could quickly adapt to changing requirements. Version 1.0 of the OpenFlow specification was released in December of 2009; it continues to be enhanced under the management of the Open Networking Foundation, which is a user-led organization focused on advancing the development of open standards and the adoption of SDN technologies.

OpenFlow protocol was the first protocol that helped in popularizing SDN. OpenFlow is a protocol designed to update the flow tables in a switch. Allowing the SDN controller to access the forwarding table of each member switch or in other words to connect control plane and data plane in SDN world. Back in 2008, OpenFlow conceptualized by networking researchers at Stanford University, the initial use of OpenFlow was to alter the switch forwarding tables to optimize traffic flows and make adjustments, so the network could quickly adapt to changing requirements.

After introduction of OpenFlow, NOX introduced as original OpenFlow controller (still there wasn’t concept of SDN controller). NOX was providing a high-level API capable of managing and also developing network control applications. Separate applications were required to run on top of NOX to manage the network.NOX was initially developed by Nicira networks (which acquired by VMware, and finally became part of VMware NSX). NOX introduced along with OpenFlow in 2009. NOX was a closed source product but ultimately it was donated to SDN community which led to multiple forks and sub projects out of original NOX. For example, POX is a sub project of NOX which provides Python support. Both NOX and POX were early controllers. NOX appears an inactive development, however POX is still in use by the research community as it is a Python based project and can be easily deployed.

POX is hosted at

NOX apart from being the first OpenFlow or SDN controller also established a programing model which inherited by other subsequent controllers. The model was based on processing of OpenFlow messages, with each incoming OpenFlow message trigger an event that had to be processed individually. This model was simple to implement but not efficient and robust and couldn’t scale.

Nicira along with NTT and Google started developing ONIX, which was meant to be a more abstract and scalable for large deployments. ONIX became the base for Nicira (the core of VMware NSX or network virtualization platform) also there are rumors that it is also the base for Google WAN controller. ONIX was planned to become open source and donated to community but for some reasons the main contributors decided to not to do it which forced the SDN community to focus on developing other platforms.

Started in 2010, a new controller introduced,the Beacon controller and it became one of the most popular controllers. It born with contribution of developers from Stanford University. Beacon is a Java-based open source OpenFlow controller created in 2010. It has been widely used for teaching, research, and as the basis of Floodlight. Beacon had the first built-in web user-interface which was a huge step forward in the market of SDN controllers. Also it provided a easier method to deploy and run compared to NOX. Beacon was an influence for design of later controllers after it, however it was only supporting star topologies which was one of the limitations on this controller.

Floodlight was a successful SDN controller which was built as a fork of Beacon. BigSwitch networks is developing Floodlight along with other developers. In 2013, Beacon popularity started to shrink down and Floodlight started to gain popularity. Floodlight had fixed many issues of Beacon and added lots of additional features which made it one of the most feature rich controllers available. It also had a web interface, a Java-based GUI and also could get integrated with OpenStack using quantum plugin. Integration with OpenStack was a big step forward as it could be used to provide networking to a large pool of virtual machines, compute and storage. Floodlight adoption increased by evolution of OpenStack and OpenStack adopters. This gave Floodlight greater popularity and applicability than other controllers that came before. Most of controllers came after Floodlight also supported OpenStack integration.

Floodlight is still supported and developed by community and BigSwitch networks, and is a base for BigCloud Fabric (the BigSwitch’s commercial SDN controller).

There are other open source SDN controllers which introduced such as Trema (ruby-based from NEC), Ryu (supported by NTT), FlowER, LOOM and the recent OpenMUL.

The following table shows the current open source SDN controllers: 

Active open source SDNcontroller

Non-active open source SDN controllers





















OpenDaylight started in early 2013, and was originally led by IBM and Cisco. It was a new collaborative open source project. OpenDaylight hosted under Linux Foundation and draw support and interest from many developers and adopters. OpenDaylight is a platform to provide common foundations and a robust array of services for SDN environments. OpenDaylight uses a controller model which supports OpenFlow as well as other southbound protocols. It is the first open source controller capable of employing non-OpenFlow proprietary control protocols which eventually lets OpenDaylight to integrate with modern and multi-vendor networks.

The first release of OpenDaylight in February 2014 with code name of Hydrogen, followed by Helium in September 2014. The Helium release was significant because it marked a change in direction for the platform that has influenced the way subsequent controllers have been architected. The main change was in the service abstraction layer, which is the part of the controller platform that resides just above the southbound protocols, such as OpenFlow, isolating them from the northbound side and where the applications reside.

Hydrogen used an API-driven Service Abstraction Layer (AD-SAL), which had limitations specifically, it meant the controller needed to know about every type of device in the network AND have an inventory of drivers to support them.

Helium introduced a Model-driven service abstraction layer (MD-SAL), which meant the controller didn’t have to account for all the types of equipment installed in the network, allowing it to manage a wide range of hardware and southbound protocols.

Helium release made the framework much more agile and adaptable to changes in the applications; an application could now request changes to the model, which would be received by the abstraction layer and forwarded to the network devices.

The OpenDaylight platform built on this advancement in its third release, Lithium, which was introduced in June of 2015. This release focused on broadening the programmability of the network, enabling organizations to create their own service architectures to deliver dynamic network services in a cloud environment and craft intent-based policies.

Lithium release was worked on by more than 400 individuals, and contributions from Big Switch Networks, Cisco, Ericsson, HP, NEC, and so on, making it one of the fastest growing open source projects ever. The fourth release, Beryllium come out in February of 2016 and the most recent fifth release, Boron released in September 2016.

Many vendors have built and developed commercial SDN controller solutions based on OpenDaylight. Each product has enhanced or added features to OpenDaylight to have some differentiating factor. The use of OpenDaylight in different vendor products are:

  • A base, but sell a commercial version with additional proprietary functionality—for example: Brocade, Ericsson, Ciena, and so on.
  • Part of their infrastructure in their Network as a Service (or XaaS) offerings—for example: Telstra, IBM, and so on.
  • Elements for use in their solution—for example: ConteXtream (now part of HP)

Open Networking Operating System (ONOS), which was open sourced in December 2014 is focused on serving the needs of service providers. It is not as widely adopted as OpenDaylight, ONOS has been finding success and gaining momentum around WAN use cases. ONOS is backed by numerous organizations including AT&T, Cisco, Fujitsu, Ericsson, Ciena, Huawei, NTT, SK Telecom, NEC, and Intel, many of whom are also participants in and supporters of OpenDaylight.

Apart from open source SDN controllers, there are many commercial, proprietary controllers available in the market. Products such as VMware NSX, Cisco APIC, BigSwitch Big Cloud Fabric, HP VAN and NEC ProgrammableFlow are example commercial and proprietary products.

The following table lists the commercially available controllers and their relationship to OpenDaylight: 



Non-ODL based


Cyan (acquired by Ciena)








ConteXtream (HP)










Huawei (also ships non-ODL controller)

VMware NSX

Core features of SDN

Regardless of an open source or a proprietary SDN platform, there are core features and capabilities which requires the SDN platform to support. These capabilities include:

  • Fabric programmability:Providing the ability to redirect traffic, apply filters to packets (dynamically), and leverage templates to streamline the creation of custom applications. Ensuring northbound APIs allow the control information centralized in the controller available to be changed by SDN applications. This will ensure the controller can dynamically adjust the underlying network to optimize traffic flows to use the least expensive path, take into consideration varying bandwidth constraints, meet quality of service (QoS) requirements.
  • Southbound protocol support:Enabling the controller to communicate to switches and routers and manipulate and optimize how they manage the flow of traffic. Currently OpenFlow is the most standard protocol used between different networking vendors, while there are other southbound protocols that can be used. A SDN platform should support different versions of OpenFlow in order to provide compatibility with different switching equipments.
  • External API support:Ensuring the controller can be used within the varied orchestration and cloud environments such as VMware vSphere, OpenStack, and so on. Using APIs the orchestration platform can communicate with SDN platform in order to publish network policies. For example VMware vSphere shall talk to SDN platform to extend the virtual distributed switches(VDS) from virtual environment to the physical underlay network without any requirement form an network engineer to configure the network.
  • Centralized monitoring and visualization:Since SDN controller has a full visibility over the network, it can offer end-to-end visibility of the network and centralized management to improve overall performance, simplify the identification of issues and accelerate troubleshooting. The SDN controller will be able to discover and present a logical abstraction of all the physical links in the network, also it can discover and present a map of connected devices (MAC addresses) which are related to virtual or physical devices connected to the network. The SDN controller support monitoring protocols, such as syslog, snmp and APIs in order to integrate with third-party management and monitoring systems.
  • Performance: Performance in a SDN environment mainly depends on how fast SDN controller fills the flow tables of SDN enabled switches. Most of SDN controllers pre-populate the flow tables on switches to minimize the delay. When a SDN enabled switch receives a packet which doesn’t find a matching entry in its flow table, it sends the packet to the SDN controller in order to find where the packet needs to get forwarded to. A robust SDN solution should ensure that the number of requests form switches are minimum and SDN controller doesn’t become a bottleneck in the network.
  • High availability and scalability: Controllers must support high availability clusters to ensure reliability and service continuity in case of failure of a controller. Clustering in SDN controller expands to scalability. A modern SDN platform should support scalability in order to add more controller nodes with load balancing in order to increase the performance and availability. Modern SDN controllers support clustering across multiple different geographical locations.
  • Security:Since all switches communicate with SDN controller, the communication channel needs to be secured to ensure unauthorized devices doesn’t compromise the network. SDN controller should secure the southbound channels, use encrypted messaging and mutual authentication to provide access control. Apart from that the SDN controller must implement preventive mechanisms to prevent from denial of services attacks. Also deployment of authorization levels and level controls for multi-tenant SDN platforms is a key requirement.

Apart from the aforementioned features SDN controllers are likely to expand their function in future. They may become a network operating system and change the way we used to build networks with hardware, switches, SFPs and gigs of bandwidth. The future will look more software defined, as the silicon and hardware industry has already delivered their promises for high performance networking chips of 40G, 100G. Industry needs more time to digest the new hardware and silicons and refresh the equipment with new gears supporting 10 times the current performance.

Current SDN controllers

In this section, I’m putting the different SDN controllers in a table. This will help you to understand the current market players in SDN and how OpenDaylight relates to them: 


Based on OpenDaylight?

Commercial/open source


Brocade SDN controller



It’s a commercial version of OpenDaylight, fully supported and with extra reliable modules.

Cisco APIC



Cisco Application Policy Infrastructure Controller (APIC) is the unifying automation and management point for the Application Centric Infrastructure (ACI) data center fabric.

Cisco uses APIC controller and Nexus 9k switches to build the fabric.

Cisco uses OpFlex as main southbound protocol.

Erricson SDN controller



Ericsson’s SDN controller is a commercial (hardened) version OpenDaylight SDN controller.

Domain specific control applications that use the SDN controller as platform form the basis of the three commercial products in our SDN controller portfolio.

Juniper OpenContrial




OpenContrail is opensource, and Contrail itself is a commercial product.

Juniper Contrail Networking is an open SDN solution that consists of Contrail controller, Contrail vRouter, an analytics engine, and published northbound APIs for cloud and NFV.

OpenContrail is also available for free from Juniper.

Contrail promotes and use MPLS in datacenter.

NEC Programmable Flow



NEC provides its own SDN controller and switches. NEC SDN platform is one of choices of enterprises and has lots of traction and some case studies.


Avaya SDN Fx controller



Based on OpenDaylight, bundled as a solution package.


Big Cloud Fabric



BigSwitch networks solution is based on Floodlight opensource project. BigCloud Fabric is a robust, clean SDN controller and works with bare metal whitebox switches.

BigCloud Fabric includes SwitchLightOS which is a switch operating system can be loaded on whitebox switches with Broadcom Trident 2 or Tomahawk silicons. The benefit of BigCloud Fabric is that you are not bound to any hardware and you can use baremetal switches from any vendor.


Ciena’s Agility



Ciena’s Agility multilayer WAN controller is built atop the open-source baseline of the OpenDaylight Project—an open, modular framework created by a vendor-neutral ecosystem (rather than a vendor-centric ego-system) that will enable network operators to source network services and applications from both Ciena’s Agility and others.

HP VAN (Virtual Application Network)



The building block of the HP open SDN ecosystem, the controller allows third-party developers to deliver innovative SDN solutions.

Huawei Agile controller

Yes and No (based on editions)


Huawei’s SDN controller which integrates as a solution with Huawei enterprise switches




Nuage Networks VSP provides SDN capabilities for clouds of all sizes. It is implemented as a non-disruptive overlay for all existing virtualized and non-virtualized server and network resources.

Pluribus Netvisor



Netvisor Premium and Open Netvisor Linux are distributed network operating systems. Open Netvisor integrates atraditional, interoperable networking stack (L2/L3/VXLAN) with an SDN distributed controller that runs in everyswitch of the fabric.

VMware NSX



VMware NSX is an Overlay type of SDN, which currently works with VMware vSphere. The plan is to support OpenStack in future. VMware NSX also has built-in firewall, router and L4 load balancers allowing micro segmentation.

OpenDaylight as an SDN controller

Previously, we went through the role of SDN controller, and a brief history of ODL.ODL is a modular open SDN platform which allows developers to build any network or business application on top of it to drive the network in the way they want.

Currently OpenDaylight has reached to its fifth release (Boron, which is the fifth element in periodic table). ODL releases are named based on periodic table elements, started from first release the Hydrogen. ODL has a 6 month release period, with many developers working on expanding the ODL, 2 releases per year is expected from community.

For technical readers to understand it more clearly, the following diagram will help:

Learning OpenDayLight

ODL platform has a broad set of use cases for multivendor, brown field, green fields, service providers and enterprises. ODL is a foundation for networks of the future.

Service providers are using ODL to migrate their services to a software enabled level with automatic service delivery and coming out of circuit-based mindset of service delivery.

Also they work on providing a virtualized CPE with NFV support in order to provide flexible offerings.

Enterprises use ODL for many use cases, from datacenter networking, Cloud and NFS, network automation and resource optimization, visibility, control to deploying a fully SDN campus network.

ODL uses a MD-SAL which makes it very scalable and lets it incorporate new applications and protocols faster.

ODL supports multiple standard and proprietary southbound protocols, for example with full support of OpenFlow and OVSDB, ODL can communicate with any standard hardware (or even the virtual switches such as Open vSwitch(OVS) supporting such protocols). With such support, ODL can be deployed and used in multivendor environments and control hardware from different vendors from a single console no matter what vendor and what device it is, as long as they support standard southbound protocols.

ODL uses a micro service architecture model which allows users to control applications, protocols and plugins while deploying SDN applications. Also ODL is able to manage the connection between external consumers and providers.

The followingdiagram explains the ODL footprint and different components and projects within the ODL:

Learning OpenDayLight

Micro servicesarchitecture

ODL stores its YANG data structure in a common data store and uses messaging infrastructure between different components to enable a model-driven approach to describe the network and functions.

In ODL MD-SAL, any SDN application can be integrated as a service and then loaded into the SDN controller. These services (apps) can be chained together in any number and ways to match the application needs.

This concept allows users to only install and enable the protocols and services they need which makes the system light and efficient.

Also services and applications created by users can be shared among others in the ecosystem since the SDN application deployment for ODL follows a modular design.

ODL supports multiple southbound protocols. OpenFlow and OpenFlow extension such as Table Type Patterns (TTP), as well as other protocols including NETCONF, BGP/PCEP, CAPWAP and OVSDB. Also ODL supports Cisco OpFlex protocol:

Learning OpenDayLight

ODL platform provides a framework for authentication, authorization and accounting (AAA), as well as automatic discovery and securing of network devices and controllers.

Another key area in security is to use encrypted and authenticated communication trough southbound protocols with switches and routers within the SDN domain. Most of southbound protocols support security encryption mechanisms.


In this article we learned about SDN, and why it is important. We reviewed the SDN controller products, the ODL history as well as core features of SDN controllers and market leader controllers. We managed to dive in some details about SDN .

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