Ensuring Source IP Persistence with ECMP Load Balancing
A FortiGate is configured with ECMP load balancing for equal-cost routes. The administrator wants to ensure that all traffic from a specific source IP uses the same next hop. Which ECMP load balancing method should be selected?
Quick Answer
The answer is Source-IP-based. This ECMP load balancing method is the correct choice because it uses the source IP address as the sole hash input, ensuring that all packets from a specific source IP are consistently forwarded to the same next hop, even when multiple equal-cost routes exist. On the Fortinet NSE 7 Advanced Security NSE7 exam, this concept tests your understanding of how FortiGate’s ECMP load balancing methods differ—specifically, that Source-IP-based provides per-source persistence, while methods like Round Robin or Weighted would distribute traffic across next hops and break session stickiness. A common trap is confusing Source-IP-based with Source-Destination-IP-based, which hashes both source and destination, so a single source talking to different destinations may still switch next hops. Remember: for source IP persistence, think “one source, one path”—the source alone is the key.
⚠ Common exam trap
Many exam-takers confuse 'per-flow' load balancing (which uses source and destination IP) with 'source-IP-based' persistence, assuming that any hash including the source IP will keep all traffic from that source on the same path, but only a hash using exclusively the source IP achieves that guarantee.
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-IP-based
Source-IP-based ECMP load balancing ensures that all packets from a specific source IP address are forwarded to the same next hop by hashing only the source IP field. This maintains session consistency for traffic originating from a single host, which is critical for stateful inspection and applications that require symmetric routing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Destination-IP-based
Why it's wrong here
Destination-IP-based would hash the destination IP, not the source.
- ✓
Source-IP-based
Why this is correct
Source-IP-based ECMP hashes the source IP to select a next hop, ensuring all traffic from the same source uses the same path.
- ✗
Weighted random
Why it's wrong here
Weighted random also does not ensure source-IP persistence.
- ✗
Round-robin
Why it's wrong here
Round-robin does not guarantee persistence per source IP; it cycles through next hops for each packet.
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Same concept, more angles
2 more ways 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. A FortiGate is configured with ECMP load balancing. What is the default behavior when multiple routes have equal cost?
easy- A.The route with the lowest metric is always preferred
- B.The administrator must enable per-packet load balancing
- ✓ C.Traffic is load balanced across the routes using a hash algorithm
- D.All traffic is sent over the first route until it fails
Why C: When ECMP load balancing is configured on a FortiGate, the default behavior is to distribute traffic across multiple equal-cost routes using a hash algorithm. This hash algorithm considers fields such as source/destination IP, protocol, and ports to ensure session consistency, meaning all packets belonging to the same session follow the same path. This is the standard ECMP behavior in FortiOS, as documented in the FortiGate Administration Guide.
Variation 2. 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?
hard- A.Weighted
- ✓ B.Source-dest-IP-based
- C.Source-IP-based
- D.Spillover
Why B: 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.
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.