PAS-C01 Technology Practice Question
A company is migrating a legacy application to AWS. The application uses a custom TCP protocol and requires session persistence. The application runs on a fleet of EC2 instances behind a Network Load Balancer (NLB). The current configuration uses a TCP listener with 'source IP' stickiness. However, some clients are being routed to different instances mid-session, causing application errors. What is the most likely cause and solution?
⚠ Common exam trap
Many candidates assume source IP stickiness works identically for all clients, failing to consider that NAT gateways or proxy servers can cause the source IP to change between requests, which is a common scenario in hybrid or multi-VPC architectures.
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
✓
Clients are behind a NAT gateway, so the source IP changes between requests, breaking stickiness. Use a 'stitched' cookie or switch to an Application Load Balancer.
Clients behind a NAT gateway will have their source IP address translated to the NAT gateway's public IP, which remains consistent for all requests from that gateway. However, if multiple clients share the same NAT gateway, the source IP is the same, but the NLB's source IP stickiness is based on the client's source IP as seen by the NLB. If the NAT gateway uses multiple public IPs or if clients are behind different NAT gateways, the source IP can change between requests, breaking stickiness. The solution is to use a 'stitched' cookie or switch to an Application Load Balancer, which supports cookie-based session persistence independent of source IP.
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 NLB does not support source IP stickiness; use a Classic Load Balancer instead.
Why it's wrong here
NLB does support source IP stickiness for TCP listeners.
- ✓
Clients are behind a NAT gateway, so the source IP changes between requests, breaking stickiness. Use a 'stitched' cookie or switch to an Application Load Balancer.
Why this is correct
NAT changes the source IP, which defeats source IP stickiness.
- ✗
The NLB has cross-zone load balancing enabled, which distributes traffic across instances in different Availability Zones.
Why it's wrong here
Cross-zone load balancing does not affect stickiness based on source IP.
- ✗
The idle timeout of the NLB is too low, causing the NLB to close connections prematurely.
Why it's wrong here
Idle timeout would terminate the connection, not route to a different instance.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PAS-C01 practice question is part of Courseiva's free Amazon Web Services 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 PAS-C01 exam.