Simple definition
An SD-WAN Health Check, also called a Performance SLA on FortiGate, continuously measures the availability and quality of one or more WAN links.
Technical definition
FortiGate sends probes to one or more health-check targets and measures metrics such as latency, jitter, and packet loss. SLA thresholds determine whether an SD-WAN member still meets the required performance. These measurements can influence link selection in SD-WAN rules and provide operational diagnostics.
What is it used for?
Avoid sending critical traffic over a link that is physically up but performing poorly, and prefer a path that still meets the service objectives.
Practical example
Two Internet circuits are SD-WAN members. The first remains connected but experiences 15% packet loss. The Performance SLA fails its configured threshold, so an SD-WAN rule can prefer the second link even though the first interface is still up.
Common issues
- Probe target is unreachable even though Internet access still works
- Latency, jitter, or packet-loss thresholds are too strict
- Probe traffic is allowed on one member but blocked on another
- SD-WAN rule is not using the intended SLA
Key takeaway: A Health Check does more than test whether an interface is up; it verifies whether the path still meets the configured performance SLA.