NSE7 Advanced Networking and SD-WAN Practice Question
A FortiGate is configured with two ISPs in an SD-WAN. The administrator wants to use the link with the highest bandwidth for bulk downloads, but if that link fails, all traffic should automatically use the backup link. Which load balancing algorithm should be used?
⚠ Common exam trap
Candidates often confuse 'Spillover' with 'Volume' or 'Sessions' because they think bandwidth-based algorithms automatically handle failover, but only Spillover explicitly supports a threshold-based failover to a backup link when the primary link fails or is overloaded.
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
✓
Spillover
The Spillover algorithm is designed to use a preferred link until its bandwidth capacity is reached, then 'spill over' traffic to a backup link. In this scenario, the administrator wants to use the highest-bandwidth link for bulk downloads and fail over to the backup link only when the primary link fails. Spillover matches this requirement because it can be configured with a bandwidth threshold; once the primary link's bandwidth is exhausted or the link fails, traffic automatically shifts to the backup link.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Sessions
Why it's wrong here
Sessions balances based on number of sessions, not bandwidth.
- ✗
Volume
Why it's wrong here
Volume balances based on bytes transferred, but does not provide failover.
- ✓
Spillover
Why this is correct
Spillover uses a configured bandwidth threshold; traffic is sent to the primary link until it reaches the threshold, then spills over to the backup. If the primary fails, all traffic goes to backup.
- ✗
Source-destination IP
Why it's wrong here
This algorithm hashes based on IP pairs, and does not consider bandwidth or failover in the described manner.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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