SD-WAN Lowest-Cost Strategy Not Load Balancing
An administrator configured an SD-WAN rule to steer traffic to a specific member interface using the 'lowest-cost' strategy. After applying, the traffic is not being load-balanced as expected. Which configuration element is MOST likely missing?
Quick Answer
The answer is that a performance SLA has not been assigned to the SD-WAN member interfaces. The 'lowest-cost' strategy in FortiGate SD-WAN does not inherently perform load balancing; instead, it selects the single best path based on the lowest calculated cost, and without a performance SLA, the cost remains static and no link quality metrics are measured. This means traffic will always prefer the lowest-cost interface until it fails, rather than distributing traffic across multiple links. On the Fortinet NSE 7 Advanced Security NSE7 exam, this tests your understanding that SD-WAN rules require a performance SLA to dynamically influence cost and enable load balancing behavior—a common trap is confusing the 'lowest-cost' strategy with a load-balancing algorithm. Remember the mnemonic: No SLA, no load balancing—lowest cost picks one, not all.
⚠ Common exam trap
Watch out — candidates often assume the 'lowest-cost' strategy inherently load-balances across multiple interfaces with the same cost, but without a performance SLA, the FortiGate does not dynamically adjust costs or distribute traffic, leading to a single active path and no load balancing.
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
✓
A performance SLA has not been assigned to the SD-WAN member interfaces.
The 'lowest-cost' strategy in Fortinet SD-WAN selects the member interface with the lowest cost value. However, without a performance SLA assigned to the SD-WAN member interfaces, the SD-WAN rule cannot dynamically measure link quality or update interface costs based on real-time conditions. As a result, the rule defaults to static cost values or may not load-balance traffic at all, because the SLA is required to trigger cost-based path selection and failover. Assigning an SLA enables the FortiGate to monitor link performance and adjust the effective cost, allowing proper load balancing across multiple members.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The 'best-quality' strategy was inadvertently selected instead.
Why it's wrong here
The question states 'lowest-cost' was configured, so that is not the issue.
- ✓
A performance SLA has not been assigned to the SD-WAN member interfaces.
Why this is correct
The lowest-cost strategy relies on performance SLA metrics to determine cost. Without an SLA, the cost is not calculated.
- ✗
The SD-WAN member interfaces are not in the same zone.
Why it's wrong here
SD-WAN members do not need to be in the same zone; they are part of the SD-WAN zone.
- ✗
The 'update-static-route' option is disabled on the SD-WAN member.
Why it's wrong here
That option affects route injection, not load balancing.
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Same concept, more angles
1 more way this is tested on NSE7
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An administrator configures an SD-WAN rule to steer traffic from a specific subnet to an SD-WAN member with the lowest cost. Which load balancing algorithm should be selected in the SD-WAN rule to achieve this behavior?
medium- ✓ A.Lowest-cost
- B.Volume
- C.Sessions
- D.Source-dest-IP
Why A: The 'Lowest-cost' algorithm is correct because it directs traffic to the SD-WAN member with the lowest configured cost metric, which directly matches the administrator's requirement to steer traffic from a specific subnet to the member with the lowest cost. In Fortinet SD-WAN, the cost is a static metric assigned per SD-WAN member interface, and the Lowest-cost algorithm selects the member with the smallest cost value for each new session, ensuring traffic follows the least-cost path.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This NSE7 practice question is part of Courseiva's free Fortinet certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the NSE7 exam.