NSE7 Advanced Networking and SD-WAN Practice Question
A FortiGate is configured with ECMP load balancing for multiple equal-cost routes. The administrator wants to ensure that all packets belonging to the same session go out the same interface. Which ECMP load balancing method should be used?
⚠ Common exam trap
Test-takers frequently confuse 'source-dest-IP-based' with 'source-IP-based' or assume that any ECMP method inherently preserves session affinity, but only the source-dest-IP-based (or per-flow) method guarantees that all packets of a session use the same interface.
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
✓
Source-dest-IP-based
Source-dest-IP-based ECMP (often called per-flow load balancing) uses a hash of both source and destination IP addresses to consistently map all packets of a session to the same next-hop interface. This ensures session integrity because the hash remains constant for the entire flow, preventing out-of-order delivery or packet drops that would occur if packets from the same session took different paths.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Weighted
Why it's wrong here
Weighted distributes sessions by configured interface ratios, so a new session may still hash to a different path than its predecessor; it controls traffic proportion, not per-session stickiness. It suits unequal-bandwidth links where proportional distribution matters more than session affinity.
- ✓
Source-dest-IP-based
Why this is correct
Source-dest-IP-based hashing derives the egress interface from a hash of the source and destination IP pair, so every packet in a given session maps to the same next hop. This satisfies the stem's requirement that all packets belonging to the same session leave through one interface, preserving session integrity across ECMP paths.
- ✗
Source-IP-based
Why it's wrong here
Source-IP-based hashes only the source address, so multiple sessions from one host, or hosts behind NAT sharing an address, can still be spread across different interfaces, breaking session stickiness. It suits coarse per-host distribution, not guaranteed per-session path consistency.
- ✗
Spillover
Why it's wrong here
Spillover sends traffic through the primary link until a bandwidth threshold is exceeded, then overflows to secondary links, so a session can shift mid-flow and packets arrive out of order. It suits failover and link-utilisation scenarios, not consistent per-session interface selection.
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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.