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Question 1,184 of 1,705

Network Design →hardMultiple ChoiceObjective-mapped

ANS-C01 Network Design Practice Question

This ANS-C01 practice question tests your understanding of network design. 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 large e-commerce company is redesigning its global network architecture. They have three VPCs in us-east-1: production (10.0.0.0/16), staging (10.1.0.0/16), and development (10.2.0.0/16). They also have two VPCs in eu-west-1: production (10.10.0.0/16) and staging (10.11.0.0/16). All VPCs are connected via a Transit Gateway with inter-region peering. The company wants to allow the staging VPCs in both regions to communicate with each other for data replication, but no other cross-region traffic should be allowed. Additionally, the production VPC in us-east-1 must be able to send traffic to the production VPC in eu-west-1 for a disaster recovery pilot. The security team has configured Network ACLs and security groups appropriately. However, after implementation, the staging VPCs can communicate, but the production VPCs cannot. A network engineer checks the Transit Gateway route tables and finds that both production VPC attachments are associated with the same route table, which has a static route for the 10.0.0.0/16 and 10.10.0.0/16 prefixes. What is the MOST likely reason for the failure?

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.

Question 1hardmultiple choice

Study the full ACL explanation →

A
The security groups in the production VPCs are blocking the traffic
Why wrong: The security team configured them appropriately, and staging works, so security groups are not the issue.
B
The production VPCs are attached to the same Transit Gateway route table, causing asymmetric routing
If both production VPCs are in the same route table, the route for the remote production CIDR might be incorrectly propagated or cause a loop. They should be in separate route tables to control routing direction.
C
The production VPCs have overlapping CIDR ranges, causing a routing conflict
Why wrong: The CIDRs are different (10.0.0.0/16 and 10.10.0.0/16), so no overlap.
D
The Transit Gateway route table for the production VPCs does not have a route for the remote production CIDR
Why wrong: The engineer confirmed static routes are present.

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 production VPCs are attached to the same Transit Gateway route table, causing asymmetric routing

Transit Gateway route tables propagate routes from VPC attachments. If both production VPCs are in the same route table, the routes for their CIDRs will be propagated. However, if there is a propagated route for the local VPC, it may override the static route or cause a conflict. The issue is likely that the static routes are being ignored because a more specific or conflicting propagated route exists, or the route table lacks a route for the remote production CIDR if the attachment is not associated correctly. Actually, the most common issue is that the production VPC attachments are in the same route table, but the static route for the remote production CIDR might be pointing to the VPC attachment itself, causing a loop. But the key point: Transit Gateway route tables do not allow overlapping CIDRs. Since both production VPCs have overlapping CIDRs (10.0.0.0/16 and 10.10.0.0/16 are different), that's not the issue. The issue is likely that the route table has a static route for the remote production CIDR, but the propagation from the local production VPC attachment might be creating a route that is not correct. The best answer is that the production VPC attachments need to be in separate route tables to avoid conflicting routes.

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 groups in the production VPCs are blocking the traffic
Why it's wrong here
The security team configured them appropriately, and staging works, so security groups are not the issue.
✓
The production VPCs are attached to the same Transit Gateway route table, causing asymmetric routing
Why this is correct
If both production VPCs are in the same route table, the route for the remote production CIDR might be incorrectly propagated or cause a loop. They should be in separate route tables to control routing direction.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The production VPCs have overlapping CIDR ranges, causing a routing conflict
Why it's wrong here
The CIDRs are different (10.0.0.0/16 and 10.10.0.0/16), so no overlap.
✗
The Transit Gateway route table for the production VPCs does not have a route for the remote production CIDR
Why it's wrong here
The engineer confirmed static routes are present.

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

A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.

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 Design — This question tests Network Design — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The production VPCs are attached to the same Transit Gateway route table, causing asymmetric routing — Transit Gateway route tables propagate routes from VPC attachments. If both production VPCs are in the same route table, the routes for their CIDRs will be propagated. However, if there is a propagated route for the local VPC, it may override the static route or cause a conflict. The issue is likely that the static routes are being ignored because a more specific or conflicting propagated route exists, or the route table lacks a route for the remote production CIDR if the attachment is not associated correctly. Actually, the most common issue is that the production VPC attachments are in the same route table, but the static route for the remote production CIDR might be pointing to the VPC attachment itself, causing a loop. But the key point: Transit Gateway route tables do not allow overlapping CIDRs. Since both production VPCs have overlapping CIDRs (10.0.0.0/16 and 10.10.0.0/16 are different), that's not the issue. The issue is likely that the route table has a static route for the remote production CIDR, but the propagation from the local production VPC attachment might be creating a route that is not correct. The best answer is that the production VPC attachments need to be in separate route tables to avoid conflicting routes.

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". 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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