Cisco Software-Defined WAN (SD-WAN) leverages a transport-independent fabric technology that is used to connect remote locations together. This is accomplished by using an overlay technology. The overlay works by tunneling traffic over any kind of transport between any destination within the WAN environment. This is the VPN concept that was mentioned earlier in this chapter—for example, being able to connect remote branches that use MPLS to remote branches that use broadband Internet circuits. This gives true flexibility to routing applications across any portion of the network regardless of what type of circuit or transport is in use. This is the definition of transport independence. By having a fabric overlay network, it means that every remote site, regardless of physical or logical separation, is always a single hop away from another. This is of great benefit in terms of application latency and dynamic communication scenarios such as voice or interactive video. This not only provides increased simplicity in terms of network operations, but also provides seamless mobility from a user experience perspective. Transport independence is also one of the primary aspects of Cisco SD-WAN that allows for the use of flexible, lower-cost commodity circuits versus high-cost, inflexible static bandwidth. Although service providers can upgrade the bandwidth of a circuit, cost is usually a barrier. In addition, there are many times that, based on the type of circuit the bandwidth is riding on, an entire physical circuit upgrade or swap may be more likely. An example of this is having a 100Mbps MPLS handoff wherein the physical circuit it is delivered on is also only 100Mbps. In cases like this, another higher-speed port on the provider side is required, such as gigabit or 10-gigabit Ethernet ports. Many times, the circuit may ride over a different type of medium, and the entire circuit and delivery mechanism must be changed—for example, trying to go from a 45Mbps DS3 to a 1-gigabit Ethernet link. All of this takes time, and that is one of the things SD-WAN was created to address. Businesses can typically order a high-speed commodity Internet circuit and have it delivered within weeks. This new Internet circuit can be immediately added to the environment and taken advantage of by using SD-WAN. There are situations where multiple branch locations need to act as a single large branch across the WAN. This means having a virtual fabric over disparate transports such as MPLS and Internet. Given everything that has been covered thus far, it is important to show what an example of a Cisco SD-WAN diagram would look like. Figure 1-6 illustrates the high-level overview of a Cisco SD-WAN environment and how users, devices, and applications fit into the overall design.
FIGURE 1.6 High-Level SD-WAN Overview
Moving from a network-centric WAN to an application- and services-focused WAN requires a different view of the wide area network. Figure 1-7 illustrates the new view of a business intent–based network, its components, and how they fit within the new model.
FIGURE 1.7 Business Intent–Based Network Components