Why SD-WAN Failback Fails After Link Restoration
A FortiGate is configured with two WAN members in an SD-WAN zone. The performance SLA monitors latency to a probe server. The rule uses 'best quality' strategy. After some time, one member fails the SLA. Which action does the FortiGate take for existing sessions that were using that member?
⚠ Common exam trap
Candidates often assume SD-WAN 'best quality' strategy dynamically re-routes all traffic, including existing sessions, when an SLA fails, but FortiGate only applies path selection changes to new sessions to maintain session stability.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Existing sessions continue on the failed member until they timeout
When a WAN member fails the performance SLA in an SD-WAN 'best quality' strategy, FortiGate does not disrupt existing sessions that were already using that member. Instead, those sessions continue on the failed member until they naturally timeout or are torn down, because the SD-WAN rule only influences the path selection for new sessions. This behavior is by design to avoid breaking active connections due to transient SLA fluctuations.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
All sessions are dropped and the member is removed from the zone
Why it's wrong here
Existing sessions are not dropped; the FortiGate re-evaluates and moves them to a member passing the SLA, preserving the session. The option misstates SD-WAN behaviour. It tempts because failed members are removed from new-session selection, but that applies to new flows, not established ones.
- ✗
Existing sessions are re-evaluated and may be moved based on policy
Why it's wrong here
The 'best quality' strategy selects members per new session; it does not re-evaluate established sessions when a member later fails its SLA. It is tempting because re-evaluation sounds like dynamic steering, and would be correct under a strategy explicitly configured to reassign existing traffic, not best quality.
- ✗
Existing sessions are immediately moved to another member
Why it's wrong here
FortiGate does not migrate established sessions merely because a member fails the SLA; existing flows persist until they end or are explicitly cleared. It is tempting because immediate failover matches intuition about redundancy, and would be correct for new sessions or when the member interface itself goes down.
- ✓
Existing sessions continue on the failed member until they timeout
Why this is correct
SD-WAN's best quality strategy affects only member selection for new sessions. Sessions already pinned to a member that later fails its SLA remain there until they close or time out, since FortiGate does not forcibly rebalance established flows.
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