SD-WAN Traffic Engineering: Active, Backup, and Standby Links
Traffic engineering is how WiLine SD-WAN decides, moment to moment, which of your WAN links carries which traffic. Every SD-WAN service runs over two or more WAN links (for example WAN0 and WAN1), and each link is assigned a WAN Link Mode — Active, Backup, or Standby. Those modes, combined with the real-time quality of each circuit, are what let traffic keep flowing when a link gets congested or fails.
This page explains the three link modes, the two designs they produce — Active-Active and Active-Backup — and how the SD-WAN steers individual traffic flows between links.
The Three WAN Link Modes
How each link is used
Active links define your everyday capacity. Backup and Standby links define your protection: the difference is when they step in — Backup on failure, Standby as soon as an Active link degrades.
How the Modes Combine
Two designs, built from the same modes
Active-Active
When both links are set to Active, the SD-WAN uses them at the same time:
- Traffic is shared across both links according to their Weight (the relative share each link receives — equal weights split traffic evenly).
- The SD-WAN continuously grades each circuit on packet loss, latency, and jitter (circuit levels CL1–CL8 — see Monitor Service Quality).
- Routing is per flow: when a link's quality crosses a quality threshold (by default around circuit level CL4), new and affected flows are steered onto the healthier link, while the degraded link keeps serving the traffic that is still doing fine.
Active-Active gives you the most usable bandwidth day to day, because no link sits idle. This is a common setup when both circuits are reliable and you want to use all of your capacity.
Active-Backup
When one link is Active and the other is Backup (or Standby), you get an Active-Backup design — this is what customers usually mean by "active backup."
- The Active link carries all production traffic under normal conditions.
- The reserve link takes over automatically when the Active link can no longer deliver acceptable service:
- A Backup link activates on failover — when the Active link goes down.
- A Standby link is kept warm and can step in as soon as the Active link's quality degrades past the threshold, giving a faster, smoother handover.
- When the Active link recovers, traffic returns to it.
Active-Backup favors predictability: your traffic normally rides one preferred circuit, with a second circuit guaranteeing continuity if the first has trouble.
Choose Active-Active to use all your bandwidth at once and spread risk across both links. Choose Active-Backup when you want traffic to stay on one preferred link and only move when that link has a problem.
How SD-WAN Steers Traffic Between Links
Regardless of the mode, the SD-WAN is always measuring each circuit's health and moving traffic toward the best path:
- Every circuit is graded in real time on packet loss, latency, and jitter, producing a circuit level (CL1–CL8) — lower is better.
- Each application can have a quality threshold. When a circuit's level rises past that threshold (by default around CL4), the SD-WAN treats the circuit as no longer good enough for that traffic.
- Affected flows are redirected onto a healthier link — an Active link with headroom, or a Standby link held ready for exactly this moment.
- Standby links are kept in the forwarding path and weighted so they can absorb redirected traffic immediately, rather than being spun up from cold.
Because steering happens per flow rather than per link, the SD-WAN can move just the traffic that is affected — keeping your network stable through everyday fluctuations, not only outright failures. You can watch this behavior in the portal: the Service Quality graph shows each circuit's level over time and how the SD-WAN shifts traffic to the best-performing WAN link.
Where This Is Configured
Your link modes and weights are part of your service configuration. In the SD-WAN Portal they are visible under Services > Service Profile Bindings, where each WAN link in a service shows its WAN Link Mode and Weight, along with related settings:
| Setting | What it controls |
|---|---|
| WAN Link Mode | How the link is used: Active, Backup, or Standby. |
| Weight | The share of traffic a link receives relative to the others when more than one link is active. |
| Flow Mode | How traffic is distributed across active links — for example, by Weights. |
| Asymmetric Weights | Lets upstream and downstream directions use different weights when your two links have different capacities. |
| No of WANs | How many WAN links this service uses. |
Service bindings are provisioned together with your service. To change a link mode or weight, contact WiLine — see Need Help? — so the change is validated end to end.
Common Questions
Good to know
Active backup, answered
Traffic engineering is what turns two independent circuits into one resilient connection — all links pulling at once in Active-Active, or one leading with another ready to step in for Active-Backup — so your network stays stable through everyday fluctuations and outright failures alike.