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Cisco SD-WAN: Underlay Network vs Overlay Fabric

July 27, 2023 ,

Cisco SD-WAN: Underlay Network vs Overlay Fabric

Cisco's SD-WAN architecture has been leading the charge, providing businesses with the agility and flexibility they need to thrive in today's ever-changing digital landscape. But how exactly does it work?

If you're looking into SD-WAN, you may have heard people talk about the "underlay network" and the "overlay fabric." But what do these terms actually mean? Let's break it down:

Cisco SD-WAN: Underlay Network vs Overlay Fabric
Fig 1.1- Cisco SD-WAN: Underlay Network vs Overlay Fabric

  • The Underlay Network: This is the hardware infrastructure that connects your WAN transports and local site networks. It's made up of all the network devices, like routers and switches, that you use to connect your sites. The underlay's job is to provide IP reachability between these sites.
  • The Overlay Fabric: This is the part of your SD-WAN that's made up of IPsec tunnels. These tunnels connect your sites together, forming what's called the SD-WAN fabric. The overlay is what allows your traffic to travel securely between your sites.

So why do we need both an underlay and an overlay? Well, the underlay is what provides the foundation for your SD-WAN. It's what ensures that your sites can communicate with each other over the WAN. 

But the overlay is what makes SD-WAN so powerful. It's what allows you to use multiple WAN transports, like broadband and LTE, to create a more resilient and cost-effective network.

Let's take a closer look at how the underlay and overlay work together:

  • The Underlay: The underlay is what connects all your network devices together. It's what allows your devices to communicate over the WAN. To make sure your traffic stays secure, your router interfaces are always configured in VPN 0 (the Transport VPN). This means that your traffic is encrypted between your sites.
  • The Overlay: The overlay is what allows your traffic to travel securely between your sites. It's made up of IPsec tunnels that traverse the underlay network. Each tunnel is formed between two TLOCs. The routing within the overlay is governed by the Overlay Management Protocol (OMP), a control-plane protocol very similar to BGP.

So, what's the bottom line? If you're looking into SD-WAN, it's important to understand the difference between the underlay and overlay. The underlay is what provides the foundation for your network, while the overlay is what allows you to use multiple WAN transports to create a more resilient and cost-effective network. 

Final Thoughts

So, there you have it - a quick overview of Cisco's SD-WAN architecture. From the underlay network to the overlay fabric, every part of the system has been carefully designed to provide businesses with the agility and flexibility they need to thrive.

If you're considering implementing SD-WAN in your business, it's important to choose a provider with a proven track record of success. And with Cisco's SD-WAN architecture, you can rest assured that you're in good hands.

By working together, the underlay and overlay create a powerful SD-WAN solution that can help you improve network performance, reduce costs, and increase security.

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