What is SDN

With SDN, network nodes (switches, routers, bare-metal servers, etc.) are abstracted from their functions, which allows them to be managed globally and coherently. An SDN controller coherently manages the entire system through its control plane (control plane) and data plane (data plane (data plane). “Network programmability” is enabled by Software Defined Controllers. March 2011 saw the founding of the Open Networking Foundation (ONF), a non-profit organization dedicated to promoting and developing OpenFlow. Research centers, such as Stanford University’s ONRC, which produced the first OpenFlow specifications in 2009, were interested in using OpenFlow as a protocol for SDN controllers.

Why do we need it?

IT teams are responsible for building and managing IT infrastructure and applications, but they should also serve key business drivers for their organization, such as these:

1. Affordability
2. Growth
3. Adaptability
4. Ability to scale
5. A secure environment. 

As we know, non-SDN networks in the data center space have many drawbacks and present many operational challenges to modern IT infrastructures. In addition to these challenges, organizations from diverse industries raised new demands for SDN.

SDN Data Centers

In addition to OpenFlow, software-defined networks (SDNs) provide another paradigm shift. In the last few years, the idea of separating the data plane, which runs in hardware ASICs on network switches, from the control plane, which runs on a central controller, has gained traction. This effort aims to develop standardized OpenFlow APIs that expose rich functionality from the hardware to the controller. For the entire data center cluster comprised of different types of switches to be uniformly programmed to enforce a specific policy, SDNs should promote programmatic interfaces that switch vendors should support. At its simplest, the data plane merely programs hardware based on the controller’s directions by serving as a set of “dumb” devices.

Scalability

As server ports increased in density, data centers grew, making it impossible to keep up. A limited number of MAC addresses, inactive links, and multicast streams prevented multicast streams from being transported in this case. Infrastructure growth became more than a “nice to have” as needs evolved. Using SDN controllers and standardized off-the-shelf switches, adding new switches and configuring their configurations quickly became easy.

To maximize downlink throughput, all links on switches must be utilized. Local networks already know about the widespread use of spreading trees (which disable parts of links). As a result of the phenomenal growth of server density, various multipathing scenarios have been addressed using things like Multi-Chassis EtherChannel (MEC) and ECMP (Equal Cost Multi-Path) with CLOS architectures.

Virtualization is one of the abstraction capabilities brought by SDN. Multiple isolated virtual networks were used to compute and store data on servers. There was also a virtualization movement in the network industry. At different layers, SDN has been developed in several variants.

ClOS-based architectures

In recent years, high-speed network switches have made CLOS-based31 architectures extremely popular. The CLOS topology has a simple rule: switches at tier x should only be connected to switches at tier x-1 and x+1 and never to other switches at the same tier. In this topology, redundancy provides high resilience, fault tolerance, and traffic load sharing. Due to the many redundant paths between any two switches, network resources can be utilized efficiently. There is no oversubscription in CLOS-based architectures, which may be advantageous for some applications due to the huge bisection bandwidth. Additionally, the relatively simple topology alleviates the burden of having separate core and aggregation layers inherent in traditional three-tier architectures, which help troubleshoot traffic.

What problems do we have, and what are we doing about them? Ask yourself: Are data centers ready and available for today’s applications and tomorrow’s emerging data center applications? Businesses and applications are putting pressure on networks to change, ushering in a new era of data center design. From 1960 to 1985, we started with mainframes and supported a customer base of about one million users.

Example: ACI Cisco

ACI Cisco, short for Application Centric Infrastructure, is a software-defined networking (SDN) solution developed by Cisco Systems. It provides a holistic approach to managing and automating network infrastructure, allowing organizations to achieve agility, scalability, and security all in one framework.

Cisco ACI is a software-defined networking (SDN) solution that brings automation, scalability, and agility to network infrastructure. It combines physical and virtual elements, creating a unified and programmable network fabric that simplifies operations and accelerates application deployment. By abstracting network policies from the underlying infrastructure, Cisco ACI enables organizations to achieve policy-driven automation and policy-based security across the entire network.

Scalability and Agility:

With the increasing demands of modern business applications, scalability and agility are paramount. Cisco ACI offers a highly scalable architecture that can adapt to changing network requirements. By leveraging a spine-leaf topology and VXLAN overlays, Cisco ACI provides a flexible and scalable foundation that can seamlessly grow to accommodate evolving business needs.

Example: Software-defined data centers

To offer computing and network services to many clients, software-defined data centers (SDDCs) use virtualization technologies to separate hardware infrastructure into virtual machines. All computing, storage, and networking resources can be abstracted and represented as software in a virtualized data center. Anybody could access the data center resources if sold as a service.

Software-defined networking (SDN) and virtual machines are components of SDDCs. Many other open and proprietary software platforms exist for virtualizing computing resources besides Citrix, KVM, OpenDaylight, OpenStack, OpenFlow, Red Hat, and VMware.

The advantage of SDDC is that clients do not have to build their infrastructure. They can meet their computing, networking, and storage needs by renting resources from the cloud. It is advantageous for software companies or service providers to have centralized data centers because they can serve many clients simultaneously. Hardware and storage costs are plummeting, a significant factor driving SDDC and cloud computing. Infrastructure as a Service (IaaS) becomes more economical as these resources become cheaper, making it more advantageous to build large data centers on a large scale.

Example: Open Networking Foundation

We also have the Open Networking Foundation ( ONF ), which leverages SDN principles, employs open-source platforms, and defines standards to build and operate open networking. The ONF’s portfolio includes several areas, such as mobile, broadband, and data centers running on white box hardware.

Related: Before you proceed, you may find the following post helpful:

1. DNS Structure
2. Data Center Network Design
3. Software Defined Perimeter
4. ACI Networks
5. Layer 3 Data Center

The Future of Data Centers 

Exploring Software-Defined Networking (SDN)

In recent years, the rapid advancement of technology has given rise to various innovative solutions transforming how data centers operate. One such revolutionary technology is Software-Defined Networking (SDN), which has garnered significant attention and is set to reshape the landscape of data centers as we know them. In this blog post, we will delve into the fundamentals of SDN and explore its potential to revolutionize data center architecture.

SDN is a networking paradigm that separates the control plane from the data plane, enabling centralized control and programmability of network infrastructure. Unlike traditional network architectures, where network devices make independent decisions, SDN offers a centralized management approach, providing administrators with a holistic view and control over the entire network.

1st Lab Guide: Cisco ACI

The following screenshots show the topology of the Cisco ACI. The design follows the layout of a leaf and spine architecture. The leaf switches connect to the spines and not to each other. All workloads and even WAN networks connect to the leaf layer.

The ACI goes through what is known as Fabric Discovery, where much of this is automated for you, borrowing the main principle of an SDN data center of automation. As you can see below, the fabric has been successfully discovered. There are three registered nodes – Spine, Leaf-a, and Leaf-b. The ACI is based on the Cisco Nexus 9000 Series.

The Benefits of SDN in Data Centers

Enhanced Network Flexibility and Scalability:

SDN allows data center administrators to allocate network resources dynamically based on real-time demands. With SDN, scaling up or down becomes seamless, resulting in improved flexibility and agility. This capability is crucial in today’s data-driven environment, where rapid scalability is essential to meet growing business demands.

Simplified Network Management:

SDN abstracts the complexity of network management by centralizing control and offering a unified view of the network. This simplification enables more efficient troubleshooting, faster service provisioning, and streamlined network management, ultimately reducing operational costs and increasing overall efficiency.

Increased Network Security:

By offering a centralized control plane, SDN enables administrators to implement stringent security policies consistently across the entire data center network. SDN’s programmability allows for dynamic security measures, such as traffic isolation and malware detection, making it easier to respond to emerging threats.

SDN and Network Virtualization:

SDN and network virtualization are closely intertwined, as SDN provides the foundation for implementing network virtualization in data centers. By decoupling network services from physical infrastructure, virtualization enables the creation of virtual networks that can be customized and provisioned on demand. SDN’s programmability further enhances network virtualization by allowing the rapid deployment and management of virtual networks.

Back to Basics: SDN Data Center

From 1985 to 2009, we moved to the personal computer, client/server model, and LAN /Internet model, supporting a customer base of hundreds of millions. From 2009 to 2020+, the industry has completely changed. We have various platforms (mobile, social, big data, and cloud) with billions of users, and it is estimated that the new IT industry will be worth 4.8T. All of these are forcing us to examine the existing data center topology.

SDN data center architecture is a type of architectural model that adds a level of abstraction to the functions of network nodes. These nodes may include switches, routers, bare metal servers, etc.), to manage them globally and coherently. So, with an SDN topology, we have a central place to work a disparate network of various devices and device types.

We will discuss the SDN topology in more detail shortly. At its core, SDN enables the entire network to be centrally controlled, or ‘programmed,’ using a software SDN application layer. The significant advantage of SDN is that it allows operators to manage the whole network consistently, regardless of the underlying network technology.

Statistics don’t lie.

The customer has changed and is making us change our data center topology. Content doubles over the next two years, and emerging markets may overtake mature markets. We expect 5,200 GB of data/per person created in 2020. These new demands and trends are putting a lot of duress on the amount of content that will be made, and how we serve and control this content poses new challenges to data networks.

Knowledge check for other software-defined data center market

The software-defined data center market is considerable. In terms of revenue, it was estimated at $43.178 billion in 2020. However, this has grown significantly; now, the software-defined data center market will grow to $120.3 billion by 2025, representing a CAGR of 22.4%.

Knowledge Check for SDN data center architecture and SDN Topology

Software Defined Networking (SDN) simplifies computer network management and operation. It is an approach to network management and architecture that enables administrators to manage network services centrally using software-defined policies. In addition, the SDN data center architecture enables greater visibility and control over the network by separating the control plane from the data plane. Administrators can control routing, traffic management, and security by centralized managing networks. With global visibility, administrators can control the entire network. They can then quickly apply network policies to all devices by creating and managing them efficiently.

The Value: SDN Topology

An SDN topology separates the control plane from the data plane connected to the physical network devices. This allows for better network management and configuration flexibility, and configuring the control plane can create a more efficient and scalable network.

The SDN topology has three layers: the control plane, the data plane, and the physical network. The control plane controls the data plane, which carries the data packets. It is also responsible for setting up virtual networks, configuring network devices, and managing the overall SDN topology.

A personal network impact assessment report

I recently approved a network impact assessment for various data center network topologies. One of my customers was looking at rate-limiting current data transfer over the WAN ( Wide Area Network ) at 9.5mbps over 10 hours for 34GB of data transfer at an off-prime time window. Due to application and service changes, this particular customer plans to triple that volume over the next 12 months.

They result in a WAN upgrade and a change in the scope of DR ( Disaster Recovery ). Big Data, Applications, Social Media, and Mobility force architects to rethink how they engineer networks. We should concentrate more on scale, agility, analytics, and management.

SDN Data Center Architecture: The 80/20 traffic rule

The data center design was based on the 80/20 traffic pattern rule with Spanning Tree Protocol ( 802.1D ), where we have a root, and all bridges build a loop-free path to that root. This results in half ports forwarding and half in a blocking state—completely wasting your bandwidth even though we can load balance based on a certain number of VLANs forwarding on one uplink and another set of VLANs forwarding on the secondary uplink.

We still face the problems and scalability of having large Layer 2 domains in your data center design. Spanning tree is not a routing protocol; it’s a loop prevention protocol, and as it has many disastrous consequences, it should be limited to small data center segments.

SDN Data Center	Data Center Stability
Layer 2 to the Core layer
STP blocks reduandant links
Manual pruning of VLANs for redudancy design
Rely on STP convergence for topology changes
Efficient and stable design

Data Center Topology: The Shifting Traffic Patterns

The traffic patterns have shifted, and the architecture needs to adapt. Before, we focused on 80% leaving the DC, while now, a lot of traffic is going east to west and staying within the DC. The original traffic pattern made us design a typical data center style with access, core, and distribution based on Layer 2, leading to Layer 3 transport. The route you can approach was adopted as Layer 3, which adds stability to Layer 2 by controlling broadcast and flooding domains.

The most popular data architecture in deployment today is based on very different requirements, and the business is looking for large Layer 2 domains to support functions such as VMotion. We need to meet the challenge of future data center applications, and as new apps come out with unique requirements, it isnt easy to make adequate changes to the network due to the protocol stack used. One way to overcome this is with overlay networking and VXLAN.

The Issues with Spanning Tree

The problem is that we rely on the spanning tree, which was useful before but is past its date. The original author of the spanning tree is now the author of THRILL ( replacement to STP ). STP ( Spanning Tree Protocol ) was never a routing protocol to determine the best path; it was used to provide a loop-free path. STP is also a fail-open protocol ( as opposed to a Layer 3 protocol that fails closed ).

One of the spanning trees’ most significant weaknesses is their failure to open. If I don’t receive a BPDU ( Bridge Protocol Data Unit ), I assume I am not connected to a switch and start forwarding on that port. Combining a fail-open paradigm with a flooding paradigm can be disastrous.

Lab Guide: STP va Routing Blocking Links

Next, let’s address the Spanning Tree Protocol on a network of 3 switches. STP is there to help, but in some cases, it blocks specific ports based on the default configuration or by the administrator forcing traffic to get a certain way. Either way, you can lose bandwidth. It is easy to demonstrate this by looking at three switches in the diagram. You would want all of these links in a forwarding state, but with STP, one of the links is blocked to prevent loops.

Since the spanning tree is enabled, all our switches will send a unique frame to each other called a BPDU (Bridge Protocol Data Unit). The spanning tree requires two pieces of information in this BPDU: the MAC address and Priority. Together, the MAC address and priority make up the bridge ID.

The spanning tree requires the bridge ID for its calculation. Let me explain how it works:

• First, a spanning tree will elect a root bridge; this root bridge will have the best “bridge ID.”
• The switch with the lowest bridge ID is the best one.
• The priority is 32768 by default, but we can change this value.

So, who will become the root bridge? In our example, SW1 will become the root bridge! The bridge ID is made up of priority and MAC address. Since all switches have the same priority, the MAC address will be the tiebreaker. SW1 has the lowest MAC address, thus the best bridge ID, and will become the root bridge. The ports on our root bridge are always designated, which means they are forwarding. 

Above, you see that SW1 has been elected as the root bridge, and the “D” on the interfaces stands for designated.

Now we have agreed on the root bridge, our next step for all our “non-root” bridges (so that’s every switch that is not the root) will be to find the shortest path to our root bridge! The shortest path to the root bridge is called the “root port.” Take a look at my example:

Port States:

 If you have played with some Cisco switches before, you might have noticed that every time you plugged in a cable, the LED above the interface was orange and, after a while, became green. What is happening at this moment is that the spanning tree is determining the state of the interface; this is what happens as soon as you plug in a cable:

• The port is in listening mode for 15 seconds. In this phase, it will receive and send BPDUs but not learn MAC addresses or transmit data.
• The port is in learning mode for 15 seconds.  We are still sending and receiving BPDUs, but now the switch will also learn MAC addresses. There is still no data transmission, though.
• Now we go into forwarding mode, and finally, we can transmit data!

So, how does this compare to routing? With layer 3, we have a TTL, meaning we can stop loops as long as there is no complicated route redistribution at different points in the network topology. So, let’s look at the following example, which uses RIP.

RIP is a distance vector routing protocol and the simplest one. We’ll start by paying attention to the distance vector class. What does the name distance vector mean?

• • Distance: How far away? In the routing world, we use metrics.
  • Vector: Which direction? In the routing world, we care about which interface and the next router’s IP address to send the packet to.

Notice below we are not blocking ports. Instead, we are load balancing.

Analysis:

Load-sharing between packets or destinations (actually source/destination IP address pairs) is supported by Cisco Express Forwarding (CEF) without performance degradation (without CEF, per-packet load-sharing requires process switching). Even though there is no performance impact on the router, per-packet load sharing almost always results in out-of-order packets. As a result of packet reordering, TCP throughput might be reduced in high-speed environments (per-packet load-sharing improves per-flow throughput in low-speed/few-flow scenarios) or applications that cannot survive out-of-order packet delivery, for example, Fast Sequenced Transport for SNA over IP or voice/video streams, may suffer.

Use the ip load-sharing per-packet interface configuration command to configure per-packet load-sharing (the default is per destination). This command must be used to configure all outgoing interfaces where traffic is load-shared.

STP has a bad reputation

STP, in theory, prevents bridging loops. Many reasons contribute to STP’s lousy reputation in practice.

If you prefer plug-and-pray networking over proper routing protocols, you must accept that design choice. There is little we can do in this situation. To use alternate paths, you need an appropriate routing protocol, regardless of whether you’re routing on layer 2 (TRILL, SPB) or layer 3 (IP). Forward-on behavior is one of the main problems with STP. All links forward traffic until BPDUs block some of them.

A forwarding loop is almost certain to occur if a device drops BPDUs or if a switch loses its control plane (for example, due to a memory leak).

Design a Scalable Data Center Topology

To overcome the limitation, some are now trying to route ( Layer 3 ) the entire way to the access layer, which has its problems, too, as some applications require L2 to function, e.g., clustering and stateful devices—however, people still like Layer 3 as we have stability around routing. You have an actual path-based routing protocol managing the network, not a loop-free protocol like STP, and routing also doesn’t fail to open and prevents loops with the TTL ( Time to Live ) fields in the headers.

Convergence routing around a failure is quick and improves stability. We also have ECMP ( Equal Cost Multi-Path) paths to help with scaling and translating to scale-out topologies. This allows the network to grow at a lower cost. Scale-out is better than scale-up.

Whether you are a small or large network, having a routed network over a Layer 2 network has clear advantages. However, how we interface with the network is also cumbersome, and it is estimated that 70% of network failures are due to human errors. The risk of changes to the production network leads to cautious changes, slowing processes to a crawl.

In summary, the problems we have faced so far;

STP-based Layer 2 has stability challenges; it fails to open. Traditional bridging is controlled flooding, not forwarding, so it shouldn’t be considered as stable as a routing protocol. Some applications require Layer 2, but people still prefer Layer 3. The network infrastructure must be flexible enough to adapt to new applications/services, legacy applications/services, and organizational structures.

There is never enough bandwidth, and we cannot predict future application-driven requirements, so a better solution would be to have a flexible network infrastructure. The consequences of inflexibility slow down the deployment of new services and applications and restrict innovation.

The infrastructure needs to be flexible for the data center applications. Not the other way around. It must also be agile enough to be a bottleneck or barrier to deployment and innovation.

What are the new options moving forward?

Layer 2 fabrics ( Open standard THRILL ) change how the network works and enable a large routed Layer 2 network. A Layer 2 Fabric, for example, Cisco FabricPath, is Layer 2; it acts more than Layer 3 as it’s a routing protocol-managed topology. As a result, there is improved stability and faster convergence. It can also support massive ( up to 32 load-balanced forwarding paths versus a single forwarding path with Spanning Tree ) and scale-out capabilities.

2nd Lab Guide: VXLAN Basics

In this lab guide, we have a VXLAN overlay network. The core configuration with VXLAN is the VNI, which needs to match on both sides. Below is a VNI of 6002 tied to the bridge domain. We are creating a layer 2 network for the two desktops to communicate. The layer 2 network traverses the core layer, which consists of the spine layer. The use of the VNI allows VXLAN to scale.

VXLAN overlay networking

What is VXLAN?

Suppose you already have a Layer 3 core and must support Layer 2 end to end. In that case, you could go for an Encapsulated Overlay ( VXLAN, NVGRE, STT, or a design with generic routing encapsulation). You have the stability of a Layer 3 core and the familiarity of a Layer 2 core but can service Layer 2 end to end using UDP port numbers as network entropy. Depending on the design option, it builds an L2 tunnel over an L3 core. 

A use case for this will be if you have two devices that need to exchange state at L2 or require VMotion. VMs cannot migrate across L3 as they need to stay in the same VLAN to keep the TCP sessions intact. Software-defined networking is changing the way we interact with the network.

It provides us with faster deployment and improved control. It changes how we interact with the network and has more direct application and service integration. Having a centralized controller, you can view this as a policy-focused network.

Many prominent vendors will push within the framework of converged infrastructure ( server, storage, networking, centralized management ) all from one vendor and closely linking hardware and software ( HP, Dell, Oracle ). While other vendors will offer a software-defined data center in which physical hardware is virtual, centrally managed, and treated as abstraction resource pools that can be dynamically provisioned and configured ( Microsoft ).