By 2021, 94 percent of workloads and compute instances will be processed by cloud data centers; 6 percent will be processed by traditional data centers. (Cisco Cloud Index Study)
The applications for SDN have been primarily cloud and data center oriented. There are a variety of cloud solutions providers that have now developed SD-WAN controller applications. The purpose of SD-WAN is to enable the same features in the context of internet and private WAN connectivity. The expense of WAN connectivity and management issues has made SD-WAN an attractive option with programmability and automation.
Figure 1 SD-WAN Characteristics
The branch office by some estimates is the location for 30% – 90% of employees. There is the trend to an internet-based model where branch offices are connected to cloud and enterprise data centers across cheaper and faster broadband services. The popular broadband services include DSL, cable and 4G LTE where lower cost make it viable to deploy multiple circuits per location as well. SD-WAN supports any commercial off the shelf router hardware and VM-based server solutions at branch offices.
The control plane software is decoupled from the router hardware (data plane) and migrated to an SD-WAN controller. There is additional orchestration software and APIs that comprise the SD-WAN architecture. The popular APIs used with SD-WAN include REST and NETCONF. The two deployment models for SD-WAN are customer-premise and cloud-based managed service. There are advantages and disadvantages to each model. The cloud-based model provides SD-WAN controller services from within the cloud. There is software installed at each branch office (vCPE) to communicate with the controller.
Figure 2 SD-WAN Architecture
SD-WAN applications are network-aware with features that enable analysis of operational state from management and reporting software. The bandwidth capacity of branch office links can be increased, decreased or reassigned based on network conditions and requirements. The rapid deployment of bandwidth to meet increased workloads and traffic bursts is a key advantage. That was not previously available with the traditional static WAN deployment model. The older static model required expensive proprietary router hardware and 30-60 day lead time on new or upgraded bandwidth and services.
The on-demand pay-as-you-go provisioning is very cost effective where customers are no longer paying for bandwidth that is essentially idle for extended periods of time. The services insertion feature can add WAN optimization, VPN and firewalling as well to branch office links.
The controller analyzes real-time performance and routing state for each endpoint. Traffic can be load balanced, rerouted across under-utilized lower cost links and optimized for minimizing latency across the internet. The decoupling of control plane from hardware allows for any router hardware and as mentioned cost effective broadband services. The routing policies for a lot of branch offices is often based on a default route. That decreases the dependence on routing protocols and enables centralized best path selection from the SD-WAN controller.