← SOA-C02·Amazon Web Services

Question 669 of 1,546

Reliability and Business Continuity →mediumMultiple ChoiceObjective-mapped

Auto Scaling Single AZ Limitations — Why Scaling Fails | AWS SysOps Associate Explained

This SOA-C02 practice question tests your understanding of reliability and business continuity. 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 runs a web application on EC2 instances in an Auto Scaling group behind an Application Load Balancer. The application is deployed in a single Availability Zone. The SysOps administrator notices that during peak hours, the application becomes slow and some requests fail. CloudWatch metrics show that CPU utilization on the instances reaches 90%, but the Auto Scaling group does not scale out. The administrator has configured a target tracking scaling policy based on average CPU utilization with a target value of 75%. The Auto Scaling group has a minimum of 2, maximum of 10, and desired capacity of 2. What is the MOST likely reason the Auto Scaling group is not scaling out?

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.

A
The Auto Scaling group is configured with a single Availability Zone, and the target tracking policy cannot scale out beyond the capacity of that single AZ.
Auto Scaling can scale out within a single AZ, but may be limited by instance types or quotas.
B
The target tracking policy uses a target value of 75%, but the average CPU is above that, so it should scale out.
Why wrong: The policy should scale out when CPU exceeds 75%.
C
The target tracking policy requires detailed monitoring to be enabled on the instances.
Why wrong: Target tracking works with basic monitoring.
D
The Auto Scaling group has reached its maximum capacity of 10 instances.
Why wrong: The group has only 2 instances, far from max.

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 Auto Scaling group is configured with a single Availability Zone, and the target tracking policy cannot scale out beyond the capacity of that single AZ.

The most likely reason is that the Auto Scaling group is configured with a single Availability Zone. Target tracking scaling policies operate within the constraints of the configured subnets. If the group is only in one AZ, the subnet may have insufficient IP addresses or the AZ may have reached its instance limit, preventing the group from scaling out beyond that AZ's capacity. Since the group has a maximum of 10, but the AZ capacity is limited, the scaling policy cannot add more instances. This is a common issue: AWS recommends using multiple Availability Zones for Auto Scaling groups to allow scaling across AZs and avoid single points of failure.

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 Auto Scaling group is configured with a single Availability Zone, and the target tracking policy cannot scale out beyond the capacity of that single AZ.
Why this is correct
Auto Scaling can scale out within a single AZ, but may be limited by instance types or quotas.
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 target tracking policy uses a target value of 75%, but the average CPU is above that, so it should scale out.
Why it's wrong here
The policy should scale out when CPU exceeds 75%.
✗
The target tracking policy requires detailed monitoring to be enabled on the instances.
Why it's wrong here
Target tracking works with basic monitoring.
✗
The Auto Scaling group has reached its maximum capacity of 10 instances.
Why it's wrong here
The group has only 2 instances, far from max.

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.

Visual reference

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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this SOA-C02 question test?

Reliability and Business Continuity — This question tests Reliability and Business Continuity — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The Auto Scaling group is configured with a single Availability Zone, and the target tracking policy cannot scale out beyond the capacity of that single AZ. — The most likely reason is that the Auto Scaling group is configured with a single Availability Zone. Target tracking scaling policies operate within the constraints of the configured subnets. If the group is only in one AZ, the subnet may have insufficient IP addresses or the AZ may have reached its instance limit, preventing the group from scaling out beyond that AZ's capacity. Since the group has a maximum of 10, but the AZ capacity is limited, the scaling policy cannot add more instances. This is a common issue: AWS recommends using multiple Availability Zones for Auto Scaling groups to allow scaling across AZs and avoid single points of failure.

What should I do if I get this SOA-C02 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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