Fortinet NSE5_SSE_AD-7.6 Exam - Topic 5 Question 3 Discussion

According to the SD-WAN 7.6 Core Administrator study guide and the FortiOS 7.6 Administration Guide, the selection process for the Best Quality (priority) strategy depends on two primary factors: the measured link quality metric and the configured member priority order.

Based on the provided exhibit (image_b40dfc.png), we can determine the following:

Strategy and Metric: The rule is in Mode(priority) (Best Quality) using link-cost-factor(packet loss).

Strict Comparison: The link-cost-threshold is set to 0. This means there is no 'advantage' given to the current preferred link; the FortiGate performs a strict comparison where the link with the objectively best metric is chosen.

Tie-Breaker Logic: When multiple links have the same packet loss, the FortiGate uses the Member Priority Order defined in the rule (set priority-members 6 4 5) as the tie-breaker.

Member 6 (HUB1-VPN3) is the highest priority.

Member 4 (HUB1-VPN1) is the second priority.

Member 5 (HUB1-VPN2) is the lowest priority.

Current State: HUB1-VPN1 is currently selected because its packet loss (2.000%) is lower than HUB1-VPN2 (4.000%) and HUB1-VPN3 (12.000%). Even though HUB1-VPN3 has a higher configuration priority, its significantly higher packet loss prevents it from being chosen.

Evaluation of Options:

Option A (Verified): If all three members have the same packet loss (e.g., they all show 2%), the quality metrics are equal. The SD-WAN engine then refers to the priority-members list. Since HUB1-VPN3 (Seq 6) is the first member in that list, it will immediately become the new preferred member.

Option B: If HUB1-VPN1 reaches 4%, it matches HUB1-VPN2 (4%). HUB1-VPN3 remains at 12%. The system will choose between VPN1 and VPN2. Since VPN1 (Seq 4) is higher in the priority list than VPN2 (Seq 5), HUB1-VPN1 stays preferred.

Option C: If HUB1-VPN1 reaches 12%, it matches HUB1-VPN3. However, HUB1-VPN2 is still better at 4.000%. Therefore, HUB1-VPN2 would become the new preferred member, not HUB1-VPN3.

Option D: If HUB1-VPN3 drops to 4%, it matches HUB1-VPN2. However, HUB1-VPN1 is still the best link at 2.000%, so it remains selected.