- A
The NLB does not support source IP stickiness; use a Classic Load Balancer instead.
Why wrong: NLB does support source IP stickiness for TCP listeners.
- B
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.
NAT changes the source IP, which defeats source IP stickiness.
- C
The NLB has cross-zone load balancing enabled, which distributes traffic across instances in different Availability Zones.
Why wrong: Cross-zone load balancing does not affect stickiness based on source IP.
- D
The idle timeout of the NLB is too low, causing the NLB to close connections prematurely.
Why wrong: Idle timeout would terminate the connection, not route to a different instance.
Quick Answer
The answer is that clients behind a NAT gateway cause source IP changes that break NLB source IP stickiness. This occurs because the NAT gateway translates each client’s private IP to its own public IP, but if the gateway uses multiple public IPs or if clients traverse different gateways, the source IP seen by the NLB can vary between requests, disrupting session persistence. On the AWS Certified SAP on AWS Specialty PAS-C01 exam, this scenario tests your understanding of how Network Load Balancer stickiness interacts with network address translation—a common trap is assuming source IP stickiness works identically to cookie-based persistence. The correct solution is to implement a stitched cookie or migrate to an Application Load Balancer, which maintains session affinity independent of IP changes. Memory tip: “NAT flips the IP, so stickiness slips; stitch a cookie to keep the trip.”
PAS-C01 Technology Practice Question
This PAS-C01 practice question tests your understanding of technology. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Option B is correct because 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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that 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.
Detailed technical explanation
How to think about this question
NLB source IP stickiness uses a hash of the client's source IP address and port to consistently route traffic to the same target. When clients are behind a NAT gateway, the source IP may change if the NAT gateway uses multiple public IPs (e.g., in a pool) or if the client's traffic egresses through different NAT gateways. This causes the hash to change, breaking stickiness. A 'stitched' cookie approach (e.g., using a proxy or ALB) inserts a session cookie that the load balancer can use for persistence, bypassing the dependency on source IP.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PAS-C01 question test?
Technology — This question tests Technology — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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. — Option B is correct because 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.
What should I do if I get this PAS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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