The answer is that the Transit Gateway attachments are not associated with the same route table, which is the most likely cause of the VPC connectivity failure. For traffic to route between VPC-B and VPC-C through the Transit Gateway, each VPC attachment must be associated with a route table that contains routes for the destination CIDRs—in this case, 10.2.0.0/16 for VPC-B and 10.1.0.0/16 for VPC-C. Even with propagation enabled on the default route table, if the attachments are associated with different route tables, the Transit Gateway cannot perform transitive routing, effectively isolating the VPCs from each other. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the distinction between route table propagation and association—a common trap is assuming propagation alone enables inter-VPC traffic. Remember: propagation populates routes, but association determines which route table the attachment uses for forwarding decisions. A simple memory tip is “associate to communicate, propagate to populate.”
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 company has deployed a multi-tier application across three VPCs (VPC-A, VPC-B, VPC-C) in the us-east-1 region. Each VPC has its own CIDR block (10.0.0.0/16, 10.1.0.0/16, 10.2.0.0/16 respectively). All VPCs are attached to an AWS Transit Gateway. The web tier is in VPC-A, the application tier is in VPC-B, and the database tier is in VPC-C. The application servers in VPC-B need to connect to the database servers in VPC-C on TCP port 3306. The database servers are in a private subnet with a security group (sg-db) that allows inbound traffic from the application server security group (sg-app). The application servers have a security group (sg-app) that allows outbound traffic to the database servers. However, the application servers cannot connect to the database servers. The route tables in VPC-B and VPC-C have routes pointing to the Transit Gateway for the other VPC CIDRs. The Transit Gateway has attachments in all three VPCs and has a default route table with propagation enabled. What is the MOST likely cause of the connectivity 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Transit Gateway attachments are not associated with the same route table
The most likely cause is that the Transit Gateway attachments are not associated with the same route table. In AWS Transit Gateway, for traffic to flow between VPCs, the attachments must be associated with a route table that contains routes for the destination VPC CIDRs. Even though propagation is enabled on the default route table, if the attachments are associated with different route tables (or the default route table is not shared), transitive routing will fail. Without a common route table association, the Transit Gateway cannot forward traffic between VPC-B and VPC-C.
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 Transit Gateway attachments are not associated with the same route table
Why this is correct
Correct; without association, routes are not propagated to the attachments.
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.
✗
The network ACLs in VPC-B or VPC-C are blocking the traffic
Why it's wrong here
Incorrect; default NACLs allow all traffic, and no custom NACLs are mentioned.
✗
The Transit Gateway does not support transitive routing between VPCs
Why it's wrong here
Incorrect; Transit Gateway does support transitive routing.
✗
The security group sg-app does not allow outbound traffic to the database server's security group
Why it's wrong here
Incorrect; the scenario states that sg-app allows outbound to the database servers.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume enabling propagation on the default route table automatically allows all inter-VPC traffic, but they overlook the requirement that all attachments must be associated with the same route table for transitive routing to work.
Trap categories for this question
Scenario analysis trap
Incorrect; the scenario states that sg-app allows outbound to the database servers.
Detailed technical explanation
How to think about this question
AWS Transit Gateway uses route tables to control traffic flow between attachments. Each attachment can be associated with a specific route table, and routes can be propagated from attachments into route tables. For transitive routing to work, all attachments that need to communicate must be associated with the same route table (or a set of route tables that have appropriate routes). In this scenario, even with propagation enabled, if VPC-B and VPC-C attachments are associated with different route tables, the Transit Gateway will not have a route to forward traffic between them. This is a common misconfiguration when using the default route table without verifying association.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Transit Gateway attachments are not associated with the same route table — The most likely cause is that the Transit Gateway attachments are not associated with the same route table. In AWS Transit Gateway, for traffic to flow between VPCs, the attachments must be associated with a route table that contains routes for the destination VPC CIDRs. Even though propagation is enabled on the default route table, if the attachments are associated with different route tables (or the default route table is not shared), transitive routing will fail. Without a common route table association, the Transit Gateway cannot forward traffic between VPC-B and VPC-C.
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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Question Discussion
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