PAS-C01 Technology Practice Question
A company runs a critical web application on EC2 instances behind an Application Load Balancer (ALB). The application experiences intermittent high latency, and the operations team suspects that the load balancer is not distributing traffic evenly. Which configuration should the team check to confirm or rule out uneven traffic distribution?
⚠ Common exam trap
Many candidates confuse uneven distribution with session affinity (sticky sessions) or connection draining, but the root cause is typically the cross-zone load balancing setting, which directly controls whether traffic is balanced across all targets or confined to each Availability Zone.
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
✓
Verify that cross-zone load balancing is enabled on the ALB
Uneven traffic distribution across EC2 instances behind an ALB is most commonly caused by cross-zone load balancing being disabled. By default, ALBs have cross-zone load balancing enabled, which distributes traffic evenly across all registered instances in all enabled Availability Zones. If it is disabled, each ALB node distributes traffic only to instances in its own Availability Zone, leading to imbalances when instance counts or capacities differ across zones. Verifying this setting directly addresses the suspected uneven distribution.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure slow start on the target group
Why it's wrong here
Slow start ramps up traffic to new instances gradually, but does not affect the distribution of existing traffic across instances.
- ✗
Increase the deregistration delay on the target group
Why it's wrong here
Deregistration delay controls how long in-flight requests are allowed to complete before an instance is deregistered. It does not affect traffic distribution.
- ✗
Enable sticky sessions (session affinity) on the target group
Why it's wrong here
Stickiness ties a user's session to a specific instance, which can actually cause uneven load if one instance gets more sticky sessions. It does not help balance distribution.
- ✓
Verify that cross-zone load balancing is enabled on the ALB
Why this is correct
Cross-zone load balancing distributes traffic evenly across all instances in all enabled AZs. If disabled, each ALB node only sends traffic to instances in its own AZ, causing uneven distribution.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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