SOA-C02 Flow-based stickiness Practice Question
A company runs a stateful web application on EC2 instances behind a Network Load Balancer. The application requires that client requests from a particular session are always sent to the same target instance. Which feature should the SysOps administrator configure on the NLB to meet this requirement?
⚠ Common exam trap
Test-takers frequently confuse NLB features with ALB features, assuming path-based routing or HTTP-level cookies apply to NLBs, when in fact NLBs operate at Layer 4 and use flow-based stickiness rather than application-layer session persistence.
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
✓
Configure sticky sessions using flow-based routing (client IP affinity)
Network Load Balancers (NLBs) support session stickiness through flow-based routing based on the client's IP address and port (Layer 4). This ensures that requests from the same client session are consistently routed to the same target instance, meeting the requirement without using application-layer cookies.
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 path-based routing rules
Why it's wrong here
Network Load Balancers operate at Layer 4 (TCP/UDP) and do not inspect HTTP headers or URL paths; path-based routing is exclusively an Application Load Balancer Layer 7 feature. Even if an ALB were used, configuring path-based routing rules only directs requests to different target groups based on URL patterns, without any session affinity—each request within a given path can still go to any healthy target. Consequently, this option neither provides nor influences sticky sessions.
- ✗
Enable health checks on the target group
Why it's wrong here
Health checks on an NLB target group are responsible for determining whether a target is available to receive traffic; unhealthy targets are automatically deregistered and healthy ones remain in rotation. However, health checks do not influence the load balancer's routing decision for individual flows—without a stickiness policy, each new connection can still be sent to any healthy target. Therefore, enabling health checks promotes availability but cannot provide session stickiness, which is a separate routing attribute.
- ✓
Configure sticky sessions using flow-based routing (client IP affinity)
Why this is correct
Sticky sessions on an NLB are implemented through flow-based routing, also known as client IP affinity, which uses a consistent hash of the connection's protocol, source IP, source port, destination IP, and destination port to route all flows from a client to the same target. Unlike ALBs, NLBs cannot rely on cookie-based stickiness because they operate at Layer 4, so this affinity mechanism is the correct way to ensure a stateful web application's user requests consistently hit the same EC2 instance, preserving session state.
- ✗
Enable cross-zone load balancing
Why it's wrong here
Cross-zone load balancing enables an NLB to evenly distribute incoming traffic across all registered targets in every enabled Availability Zone, rather than restricting traffic to targets in the same zone as the client. It improves fault tolerance and utilization but does not create any pinning of a client to a specific target; each new connection can be independently load-balanced to a different instance. Since there is no per-client routing state, enabling it does not maintain session stickiness.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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