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Question 9 of 1,705

Network Management and Operations →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the health check timeout being set equal to the interval is the most likely cause of the flapping. When the timeout and interval are both 5 seconds, any transient network delay or brief latency spike can cause the health check request to time out, and because the timeout consumes the entire interval window, the next check is immediately triggered—meaning two consecutive timeouts can occur rapidly, marking the instance unhealthy for roughly two minutes until the next successful check cycle. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how Network Load Balancer health check timing parameters interact under real-world network conditions; a common trap is to blame instance CPU or network bandwidth when the root cause is a misconfigured health check timing that leaves no buffer for jitter. Remember the memory tip: “Timeout eats the interval” — always set your timeout shorter than your interval to allow room for retries and prevent flapping.

ANS-C01 Network Management and Operations Practice Question

This ANS-C01 practice question tests your understanding of network management and operations. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 has a production VPC with a public and private subnet across two Availability Zones. The public subnet hosts a Network Load Balancer (NLB) that distributes traffic to EC2 instances in the private subnet. The application experiences periodic failures where the NLB marks all targets as unhealthy for about 2 minutes, then they recover. The health checks are HTTP on port 80 with a 5-second interval, 2 consecutive successes to be healthy, and 2 consecutive failures to be unhealthy. The target group health check timeout is 5 seconds. The EC2 instances are behind an Auto Scaling group with a minimum of 2 instances per AZ. CPU utilization on the instances is stable at 40%. The NLB's CloudWatch metrics show HealthyHostCount drops to zero suddenly. The network engineer suspects a network issue. What is the most likely cause?

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.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.

Question 1hardmultiple choice

Review the full subnetting walkthrough →

A
The security group for the private subnet is blocking inbound traffic from the NLB.
Why wrong: If blocked, health checks would always fail, not periodically.
B
The health check timeout is equal to the interval, causing timeouts under transient network delays.
If timeout equals interval, any delay in response results in consecutive failures, marking the instance unhealthy. Increasing the interval or decreasing the timeout would help.
C
The NLB's cross-zone load balancing is disabled, causing all traffic to go to one AZ.
Why wrong: This would cause uneven load, but not sudden simultaneous failures across both AZs.
D
The route tables for the private subnets are missing a route to the NLB's subnet.
Why wrong: Health checks are initiated by the NLB to the instances; return traffic is handled by the instances' route tables, which should have a default route via NAT or IGW. Missing routes would cause consistent failures.

Full breakdown with real-world context →

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

✓

The health check timeout is equal to the interval, causing timeouts under transient network delays.

If the health check timeout equals the interval, a single delayed response can cause consecutive failures. Under load, if the health check request is delayed, the NLB might timeout, count a failure, and after two such timeouts, mark the instance unhealthy. The instances are not overloaded (CPU 40%), but network contention or latency could cause timeouts. However, the most common cause is that the health check timeout is equal to the interval, meaning a missed response leads to immediate failure. The recommended practice is to set timeout less than interval to allow for retries. Option D addresses this by increasing the interval or decreasing the timeout.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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 security group for the private subnet is blocking inbound traffic from the NLB.
Why it's wrong here
If blocked, health checks would always fail, not periodically.
✓
The health check timeout is equal to the interval, causing timeouts under transient network delays.
Why this is correct
If timeout equals interval, any delay in response results in consecutive failures, marking the instance unhealthy. Increasing the interval or decreasing the timeout would help.
Clue confirmation
The clue words "most likely", "minimum / minimize" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The NLB's cross-zone load balancing is disabled, causing all traffic to go to one AZ.
Why it's wrong here
This would cause uneven load, but not sudden simultaneous failures across both AZs.
✗
The route tables for the private subnets are missing a route to the NLB's subnet.
Why it's wrong here
Health checks are initiated by the NLB to the instances; return traffic is handled by the instances' route tables, which should have a default route via NAT or IGW. Missing routes would cause consistent failures.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The health check timeout is equal to the interval, causing timeouts under transient network delays. — If the health check timeout equals the interval, a single delayed response can cause consecutive failures. Under load, if the health check request is delayed, the NLB might timeout, count a failure, and after two such timeouts, mark the instance unhealthy. The instances are not overloaded (CPU 40%), but network contention or latency could cause timeouts. However, the most common cause is that the health check timeout is equal to the interval, meaning a missed response leads to immediate failure. The recommended practice is to set timeout less than interval to allow for retries. Option D addresses this by increasing the interval or decreasing the timeout.

What should I do if I get this ANS-C01 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely", "minimum / minimize". 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?

CIDR notation defines the prefix length.

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