The answer is that source IP hashing causes poor load distribution when multiple hosts reside in the same subnet because the hash algorithm uses the source IP address to select an SD-WAN member, and hosts sharing the same first 24 bits produce identical or very similar hash values, mapping them to the same member. This occurs because the hash function lacks sufficient granularity to differentiate hosts within a /24 subnet, especially when the number of available members is small. On the Fortinet NSE 7 Advanced Security NSE7 exam, this concept tests your understanding of SD-WAN load-balancing algorithms and their limitations, often appearing as a trap where engineers overlook subnet-level hashing behavior. A common memory tip is to think of source IP hashing as a “neighborhood effect”—all hosts on the same street (subnet) knock on the same door (member). For better distribution, consider using source-destination IP hashing or a larger hash key.
NSE7 Advanced Networking and SD-WAN Practice Question
This NSE7 practice question tests your understanding of advanced networking and sd-wan. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is troubleshooting an SD-WAN setup where traffic from a specific subnet is not being load-balanced as expected. The SD-WAN rule uses 'source IP' hashing. The engineer notices that the traffic originates from multiple hosts in the same /24 subnet. What is the most likely cause of poor load distribution?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The source IP hashing algorithm causes multiple hosts in the same subnet to map to the same member.
Source IP hashing in SD-WAN uses a hash of the source IP address to select a member for each flow. When multiple hosts reside in the same /24 subnet, their source IPs share the same first 24 bits, which can cause the hash algorithm to map them to the same SD-WAN member if the hash function is not sufficiently granular or if the number of members is small. This results in poor load distribution despite multiple sources.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 SD-WAN rule is not matching the traffic.
Why it's wrong here
If rule not matching, traffic wouldn't be load-balanced at all.
✗
The SD-WAN members have different bandwidths.
Why it's wrong here
Bandwidth differences affect load but not distribution by hash.
✗
Traffic is using a single destination IP and port.
Why it's wrong here
Source IP hashing ignores destination/port; source IP alone determines.
✓
The source IP hashing algorithm causes multiple hosts in the same subnet to map to the same member.
Why this is correct
Source IP hashing can lead to poor distribution for similar IPs.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume multiple hosts in the same subnet automatically distribute traffic evenly, forgetting that source IP hashing can produce identical hash values for IPs sharing the same network prefix, leading to poor load balancing.
Detailed technical explanation
How to think about this question
Source IP hashing typically uses a CRC or XOR-based hash over the source IP address modulo the number of active SD-WAN members. With a /24 subnet, the host portion (last octet) varies, but if the hash algorithm only considers the first 24 bits (e.g., due to a misconfiguration or a simplistic hash), all hosts in that subnet produce the same hash result. In Fortinet SD-WAN, the hash algorithm can be configured to include source IP, destination IP, and ports; using only source IP increases the likelihood of such collisions, especially with small member counts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Advanced Networking and SD-WAN — This question tests Advanced Networking and SD-WAN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The source IP hashing algorithm causes multiple hosts in the same subnet to map to the same member. — Source IP hashing in SD-WAN uses a hash of the source IP address to select a member for each flow. When multiple hosts reside in the same /24 subnet, their source IPs share the same first 24 bits, which can cause the hash algorithm to map them to the same SD-WAN member if the hash function is not sufficiently granular or if the number of members is small. This results in poor load distribution despite multiple sources.
What should I do if I get this NSE7 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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