The answer is to enable stickiness using the source IP address on the Network Load Balancer. This feature, known as source IP persistence, works by hashing the client’s source IP—and optionally the port and protocol—to consistently route all packets from a given session to the same target instance, ensuring session continuity without relying on application-layer cookies. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of Layer 4 load balancing mechanics versus Layer 7 cookie-based stickiness; a common trap is confusing NLB stickiness with ALB stickiness, which uses cookies. Remember that NLB operates at the transport layer, so it cannot inspect HTTP cookies—only IP-based hashing works here. A useful memory tip: “NLB sticks by IP, ALB sticks by cookie.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 runs a critical application on Amazon EC2 instances in an Auto Scaling group behind a Network Load Balancer (NLB). The application requires that all packets from a given client session are sent to the same target instance for the duration of the session. Which feature should be enabled on the NLB to meet this requirement?
Why wrong: Proxy protocol adds client IP information to the packet, but does not enforce stickiness.
B
Enable cross-zone load balancing
Why wrong: Cross-zone load balancing distributes traffic across all targets, but does not guarantee that packets from a client go to the same target.
C
Enable stickiness using the source IP address
NLB uses a consistent hash based on source IP, protocol, and port to route packets from the same client to the same target, ensuring session persistence.
D
Configure health checks to mark targets as healthy
Why wrong: Health checks determine target availability, not session stickiness.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable stickiness using the source IP address
Option C is correct because enabling stickiness using the source IP address on a Network Load Balancer (NLB) ensures that all packets from a given client IP are routed to the same target instance for the duration of the session. This is achieved by the NLB hashing the source IP address (and optionally port and protocol) to consistently select the same target, which meets the requirement for session persistence without relying on application-layer cookies.
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.
✗
Enable proxy protocol v2
Why it's wrong here
Proxy protocol adds client IP information to the packet, but does not enforce stickiness.
✗
Enable cross-zone load balancing
Why it's wrong here
Cross-zone load balancing distributes traffic across all targets, but does not guarantee that packets from a client go to the same target.
✓
Enable stickiness using the source IP address
Why this is correct
NLB uses a consistent hash based on source IP, protocol, and port to route packets from the same client to the same target, ensuring session persistence.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure health checks to mark targets as healthy
Why it's wrong here
Health checks determine target availability, not session stickiness.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse Proxy Protocol v2 with session persistence, thinking that passing client IP information to the backend automatically ensures stickiness, when in fact Proxy Protocol only provides metadata and does not influence load balancer routing decisions.
Detailed technical explanation
How to think about this question
Under the hood, NLB stickiness using source IP works by computing a hash of the source IP address (and optionally the source port and protocol) and mapping that hash to a specific target in the target group. This hash remains consistent as long as the target group membership does not change; if a target is added or removed, the hash mapping may shift, potentially breaking persistence for some clients. In real-world scenarios, this is critical for stateful applications like gaming or financial trading where session state is stored locally on the instance.
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
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable stickiness using the source IP address — Option C is correct because enabling stickiness using the source IP address on a Network Load Balancer (NLB) ensures that all packets from a given client IP are routed to the same target instance for the duration of the session. This is achieved by the NLB hashing the source IP address (and optionally port and protocol) to consistently select the same target, which meets the requirement for session persistence without relying on application-layer cookies.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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