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Question 549 of 1,705

Network Management and Operations →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is the VPC peering connection's lack of transitive routing support. This limitation means that a VPC peering connection can only route traffic directly between the two peered VPCs; it cannot forward traffic through an intermediate device, such as a NAT instance, VPN connection, or a third VPC. If traffic from VPC A to VPC B is inadvertently routed through such an intermediary—perhaps due to a more specific route or a stateful device that changes behavior—the peering connection will drop that traffic, causing intermittent failures even when direct routes and security groups appear correct. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of VPC peering’s fundamental design constraint, often appearing as a trick where a seemingly simple connectivity issue is actually caused by an unintended transitive hop. A common trap is assuming that adding routes to a peering connection allows multi-VPC routing, but remember: VPC peering is a one-to-one, non-transitive relationship. Memory tip: “Peering is a direct handshake, not a relay race.”

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 is experiencing intermittent connectivity issues between two VPCs connected via a VPC peering connection. The VPCs are in different AWS regions. VPC A has CIDR 10.0.0.0/16 and VPC B has CIDR 10.1.0.0/16. The route tables in both VPCs have been updated to include routes pointing to the peering connection. Security groups and network ACLs are configured to allow all traffic for testing. However, traffic from VPC A to VPC B fails intermittently. Which of the following is the most likely cause of this intermittent 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 1mediummultiple choice

Study the full ACL explanation →

A
The security group rules in VPC A are not allowing inbound traffic from VPC B's CIDR. The security group must reference the VPC B CIDR explicitly.
Why wrong: The scenario states security groups allow all traffic.
B
The route tables in both VPCs must include explicit routes for each other's CIDR blocks, but they should also include routes to the internet gateway for proper routing.
Why wrong: Internet gateway routes are not required for VPC peering.
C
The VPCs are in different regions, and cross-region VPC peering is not supported. A transit gateway must be used instead.
Why wrong: Cross-region VPC peering is supported.
D
The VPC peering connection does not support transitive routing. If any traffic is being routed through an intermediate device (e.g., a NAT instance or a VPN connection), the peering connection will not forward that traffic.
VPC peering does not support transitive routing, and intermittent failures suggest that some traffic is being sent through an unsupported path.

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 VPC peering connection does not support transitive routing. If any traffic is being routed through an intermediate device (e.g., a NAT instance or a VPN connection), the peering connection will not forward that traffic.

The intermittent failure is most likely due to VPC peering's lack of transitive routing. If traffic from VPC A to VPC B is routed through an intermediate device (e.g., a NAT instance, VPN connection, or another VPC), the VPC peering connection will not forward that traffic because it does not support transitive routing. This can cause intermittent failures when the intermediate device's route or state changes, even though direct routes and security groups are correctly configured.

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.

✗
The security group rules in VPC A are not allowing inbound traffic from VPC B's CIDR. The security group must reference the VPC B CIDR explicitly.
Why it's wrong here
The scenario states security groups allow all traffic.
✗
The route tables in both VPCs must include explicit routes for each other's CIDR blocks, but they should also include routes to the internet gateway for proper routing.
Why it's wrong here
Internet gateway routes are not required for VPC peering.
✗
The VPCs are in different regions, and cross-region VPC peering is not supported. A transit gateway must be used instead.
Why it's wrong here
Cross-region VPC peering is supported.
✓
The VPC peering connection does not support transitive routing. If any traffic is being routed through an intermediate device (e.g., a NAT instance or a VPN connection), the peering connection will not forward that traffic.
Why this is correct
VPC peering does not support transitive routing, and intermittent failures suggest that some traffic is being sent through an unsupported path.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates assume security groups or route table misconfigurations are the cause, but the real issue is the fundamental non-transitive nature of VPC peering, which AWS tests by describing an intermittent failure that points to a transitive routing dependency.

Trap categories for this question

Scenario analysis trap
The scenario states security groups allow all traffic.

Detailed technical explanation

How to think about this question

VPC peering connections are non-transitive, meaning that if traffic must traverse more than two VPCs or pass through a third device (like a NAT instance or VPN gateway), the peering connection will drop the packets. This is enforced at the AWS network layer; the route table must point directly to the peering connection for the destination CIDR, and any intermediate hop breaks the path. In real-world scenarios, this often manifests as intermittent failures when a NAT instance or VPN tunnel flaps, causing temporary route changes that violate the non-transitive rule.

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.

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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 — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The VPC peering connection does not support transitive routing. If any traffic is being routed through an intermediate device (e.g., a NAT instance or a VPN connection), the peering connection will not forward that traffic. — The intermittent failure is most likely due to VPC peering's lack of transitive routing. If traffic from VPC A to VPC B is routed through an intermediate device (e.g., a NAT instance, VPN connection, or another VPC), the VPC peering connection will not forward that traffic because it does not support transitive routing. This can cause intermittent failures when the intermediate device's route or state changes, even though direct routes and security groups are correctly configured.

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.

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?

Read the scenario before looking for a memorised answer.

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