SD-WAN: Link-State Monitoring & Dynamic Path Selection

Software-Defined Wide Area Networking (SD-WAN) is an innovative new approach to the deployment, management and operation of WANs. In this 5-part series, the key benefits of this exciting new technology will be explored.

There are two areas of WAN connectivity that often bring frustration – availability and performance. Link-state monitoring enables a routing device to operate multiple WAN connections simultaneously, while monitoring the status of those links in real-time. Not only does it check the ‘up-state’, but latency, RTT (Round-Trip Time), jitter and link utilization are other metrics that can be used to classify a link ‘state’. On the one hand, this provides a foundation for link redundancy and failover, on the other, it can be used for dynamic path selection and traffic optimization.

Link Redundancy & Failover

The ability to perform real-time failover in the event of a WAN link becoming unavailable is crucial to those needing an always-on WAN or VPN. An example of this could be a router which has three uplinks; two broadband lines and a backup LTE. The primary broadband link is typically the preferred option. In the event it should fail or something were to interrupt the connection, an SD-WAN router can adapt. Using link-state monitoring, it would instantly become aware of the situation and re-route traffic across the other available link(s). This ensures that no matter what happens to the primary connection, there is always a secondary or even tertiary backup.

Dynamic Path Selection & Traffic Optimization

In the real world it is not always a case of a link being ‘up’ or ‘down’. Often, a path is available but operating with reduced bandwidth or intermittent latency fluctuations.

This is where dynamic path selection comes into play. With this technology the router can forward traffic on a packet-by-packet basis across the most optimal path at any given time, based on link-state information. This results in a WAN and VPN connection that is always performing as desired, downtime is virtually eliminated, user experience improved, and less time is spent troubleshooting and resolving problems.

Another aspect of dynamic path selection is the ability to route traffic across separate links based on application or identity-driven policies (more on this in part 4!)


Rather than reacting to WAN and VPN issues, a proactive approach using SD-WAN with link-state monitoring and dynamic path selection ultimately improves on every aspect of WAN availability, adaptability, flexibility and performance. 

The combination of link-state monitoring and dynamic path selection with SD-WAN is also having a dramatic effect on the desire for MPLS services. Organizations will commonly invest in an MPLS environment because it offers consistent, reliable performance and site-to-site connectivity. SD-WAN offers a modern and cost-effective alternative that is able to achieve a similar outcome utilizing standard broadband lines and advanced VPN technology (more on Auto-VPN in part 5!)

Link-state monitoring and dynamic path selection is only one piece of the puzzle that makes SD-WAN such an exciting new development. In this series, the following is explored:

SD-WAN Series – Part 1: Centralized Orchestration

SD-WAN Series – Part 2: Policy Unification

SD-WAN Series – Part 3: Link-State Monitoring & Dynamic Path Selection

SD-WAN Series – Part 4: Application & Identity-Driven Policies

SD-WAN Series – Part 5: Zero-Touch Provisioning & Auto-VPN


Nathaniel Moore (CWNE #222) is a Product Marketing Manager for Aerohive Networks with experience as a Systems Engineer in computer networking and wireless systems across multi-vendor solutions.

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