Introduction to SD-WAN

Software Defined WAN (SD-WAN) is hot nowadays. Why?

Private WAN connections like MPLS are reliable but also expensive. WAN connections are usually a big chunk of the IT budget, so it’s understandable that organizations are interested in replacing their private WAN connections with regular Internet connections to reduce costs.

To understand SD-WAN, we first have to talk about some “problems” with traditional WAN connections. We can choose between private WAN connections or public Internet connections. Let’s compare these two options:

• Cost: private WAN connections like MPLS are way more expensive than regular Internet connections.
• Time to deploy: it takes longer to deploy a private WAN connection than a regular Internet connection.
• SLA: Service providers offer SLAs for private WAN connections that we don’t have for regular Internet connections. There are providers who offer SLAs for “business” class Internet connections, but these are usually way more expensive than regular (consumer) Internet connections.
• Packet loss: Internet connections have a higher packet loss rate compared to private WAN connections like MPLS.
• QoS: Internet connections don’t offer any QoS. You can prioritize your outgoing traffic but that’s it, the Internet itself is like the wild west. Private WAN connections often support end-to-end QoS.

The way we use our WAN has also changed throughout the years. Most organizations had an HQ, remote users, and perhaps some branch offices. Branch offices were connected to the HQ with private WAN or VPNs over the Internet. Remote users used remote VPN over the Internet to connect.

Nowadays, organizations also run their own applications in the cloud instead of on-premises, and they use applications like Office 365 or Gsuite. Our traffic patterns look different now:

What about network management? Each router has its own control plane, and we use the CLI to manually create our router configurations “box-by-box”. This is time-consuming and prone to errors. We can use network automation tools to make our lives easier, but the control plane remains decentralized.

SD-WAN promises to save money by using a combination of Internet and private WAN connections and make network management much easier.

One problem with SD-WAN is that each vendor has a different idea about what SD-WAN is. I’ll give you a basic overview of what SD-WAN is about. An SD-WAN solution has parts of the control plane centralized and is built with network automation and orchestration in mind. We create network policies globally and push them to all routers from a central location. You could create a QoS policy and push it to all your 500 branch routers with a single mouse click. We don’t use the CLI anymore. Instead, we have a GUI and use APIs to configure and manage our WAN connections. Some vendors still support a CLI if you want to do some troubleshooting.

We use multiple WAN connections and active/active per-application load-balancing. Let’s say we have a site with a fiber, cable, 4G, and DSL connection. SD-WAN monitors all these WAN connections and keeps track of performance metrics like the throughput and delay. It selects the WAN connection with the lowest latency and highest throughput.

When a certain link fails then it can fail over to the next best option. It can also do this on a per-application level. You could use the fiber connection for traffic to the public cloud and the cable connection for low-priority FTP traffic. It protectson traffic over public Internet connections with IPSec.

SD-WAN could be an alternative to an expensive private WAN link with an SLA that promises “five nines” of uptime (99.999%). The idea behind it is that multiple WAN connections are always more reliable than a single WAN connection.

Cisco SD-WAN Solutions

Cisco offers three SD-WAN solutions:

• • Intelligent WAN (IWAN)
  • Meraki SD-WAN
  • Cisco SD-WAN (Viptela)

IWAN is Cisco’s first SD-WAN solution for the ISR platform and intended for hybrid WAN (MPLS and Internet) or Internet-only connections.

Behind the scenes they use some familiar protocols:

• DMVPN with QoS
• Wide Area Application Services (WAAS)
• Application Visibility and Control (AVC)
• Performance Routing (PfR)

Meraki SD-WAN is for existing Meraki customers that are interested in the advantages of SD-WAN. Here are some features that it offers:

• Apply bandwidth, routing, and security policies from a central location to all WAN connections (MPLS, Internet, 4G, etc.)
• Centralized network visibility and control.
• QoS and bandwidth management with Meraki traffic shaping
• Dynamic policy and performance-based path selection with automatic load balancing.
• Secure connectivity with cloud applications, remote offices, or datacenters.

Cisco SD-WAN (Viptela)

Cisco acquired Viptela, a major SD-WAN player, in 2017 and re-branded it to Cisco SD-WAN. This is Cisco’s enterprise SD-WAN solution.

Components

This solution consists of four main components and one optional analytics component:

• vManage (management)
• vSmart (controller)
• vEdge (routers)
• vBond (orchestrator)
• vAnalytics (analytics)

Let me explain these components.

vManage

vManage is the Network Management System (NMS) to configure and manage the entire SD-WAN solution. You can use a GUI or REST API to access it. This is where you create device configurations and network policies. vManage also alerts you when there are events or outages.

vSmart

vSmart is the control plane of the architecture. vSmart controllers advertise routes, security, and policy information. Cisco SD-WAN uses the proprietary Overlay Management Protocol (OMP) for this. vSmart implements the policies that you configure through vManage.

For example, imagine you create a policy through vManage where real-time voice traffic requires a latency of less than 100 ms. The vSmart controller downloads the policy, converts it into a format suitable for the vEdge routers and then implements it on all vEdge routers.

All vEdge routers peer with a vSmart controller, it’s a hub and spoke topology. It’s similar to a BGP route reflector or a DMVPN NHRP server. The vSmart controller only lives in the control plane and is never in the data plane.