- A
The inspection VPC route table does not have a route to the transit gateway for the spoke VPC CIDRs.
Correct: Without a route to the transit gateway, the firewall cannot send traffic back to the spoke VPCs.
- B
The inspection VPC route table does not have a route to the spoke VPC CIDRs via the transit gateway.
Why wrong: Incorrect: The inspection VPC needs a route to the transit gateway for return traffic, not to the spoke VPCs.
- C
The inspection VPC route table has a blackhole route for the spoke VPC CIDRs.
Why wrong: Incorrect: A blackhole route would drop traffic, but the issue is missing route.
- D
The transit gateway route table for the spoke VPCs does not have a route to the egress VPC.
Why wrong: Incorrect: The spoke VPCs send traffic to the inspection VPC; the inspection VPC handles egress.
Quick Answer
The answer is that the inspection VPC route table does not have a route to the transit gateway for the spoke VPC CIDRs. This is correct because in a Transit Gateway centralized inspection routing architecture, the inspection VPC must be able to route return traffic back to the transit gateway for the spoke VPCs; without a specific route pointing to the transit gateway attachment for those CIDRs, the firewall appliances will drop the response packets, breaking the traffic flow even though the forward path is configured. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of asymmetric routing and the necessity of bidirectional route propagation in centralized inspection designs—a common trap is assuming the inspection VPC only needs a default route for egress, when it actually requires explicit spoke CIDR routes back to the transit gateway. Remember the memory tip: “Spokes send to inspection, inspection must send back to spokes via TGW—no blackhole, no missing route.”
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 multi-account AWS environment using AWS Organizations. Each account contains a VPC with a private subnet and a public subnet. The company uses a centralized inspection VPC in the network account with third-party firewall appliances. All internet-bound traffic from the VPCs must be routed through the inspection VPC via an AWS Transit Gateway. The network team has configured the transit gateway with separate route tables: one for the inspection VPC and one for the spoke VPCs. The spoke VPCs have a default route (0.0.0.0/0) pointing to the transit gateway. The inspection VPC has a default route pointing to an egress VPC that has an internet gateway. However, traffic from a spoke VPC is not reaching the internet. The network engineer has verified that the firewall appliances are running and that the security groups and NACLs allow traffic. What is the most likely cause of the issue?
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 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 inspection VPC route table does not have a route to the transit gateway for the spoke VPC CIDRs.
Option B is correct because in a centralized inspection architecture, spoke VPCs must send traffic to the inspection VPC, and the inspection VPC must have a route to send traffic back to the transit gateway for egress. If the inspection VPC's route table does not have a route pointing to the transit gateway for the spoke VPCs' CIDRs, the return traffic will be dropped. Option A is wrong because the inspection VPC does not need a route to the spoke VPCs via the transit gateway; it needs a route to the transit gateway for return traffic. Option C is wrong because the transit gateway route table for spoke VPCs needs a route to the inspection VPC, not the egress VPC. Option D is wrong because the inspection VPC route table should not have a blackhole route for spoke VPCs.
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 inspection VPC route table does not have a route to the transit gateway for the spoke VPC CIDRs.
- ✗
The inspection VPC route table does not have a route to the spoke VPC CIDRs via the transit gateway.
Why it's wrong here
Incorrect: The inspection VPC needs a route to the transit gateway for return traffic, not to the spoke VPCs.
- ✗
The inspection VPC route table has a blackhole route for the spoke VPC CIDRs.
Why it's wrong here
Incorrect: A blackhole route would drop traffic, but the issue is missing route.
- ✗
The transit gateway route table for the spoke VPCs does not have a route to the egress VPC.
Why it's wrong here
Incorrect: The spoke VPCs send traffic to the inspection VPC; the inspection VPC handles egress.
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 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.
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 inspection VPC route table does not have a route to the transit gateway for the spoke VPC CIDRs. — Option B is correct because in a centralized inspection architecture, spoke VPCs must send traffic to the inspection VPC, and the inspection VPC must have a route to send traffic back to the transit gateway for egress. If the inspection VPC's route table does not have a route pointing to the transit gateway for the spoke VPCs' CIDRs, the return traffic will be dropped. Option A is wrong because the inspection VPC does not need a route to the spoke VPCs via the transit gateway; it needs a route to the transit gateway for return traffic. Option C is wrong because the transit gateway route table for spoke VPCs needs a route to the inspection VPC, not the egress VPC. Option D is wrong because the inspection VPC route table should not have a blackhole route for spoke VPCs.
What should I do if I get this ANS-C01 question wrong?
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.
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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Same concept, more angles
3 more ways this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A financial services company is deploying a multi-account environment using AWS Organizations. The security team requires that all network traffic to and from the internet must flow through a centralized inspection VPC that hosts third-party firewall appliances. The architecture uses a single AWS Transit Gateway with a centralized inspection VPC attached. Which THREE steps are necessary to enforce this architecture? (Choose THREE.)
hard- ✓ A.Configure each VPC's route table with a default route (0.0.0.0/0) pointing to the transit gateway attachment.
- B.Use AWS Organizations service control policies to prevent direct internet access from spoke VPCs.
- C.Deploy VPC Gateway Endpoints for S3 and DynamoDB in each spoke VPC.
- ✓ D.In the inspection VPC, route traffic from the transit gateway to the firewall appliances, then back to the transit gateway for egress.
- ✓ E.Create separate transit gateway route tables for the inspection VPC and spoke VPCs, and propagate routes appropriately.
Why A: Option A is correct because all VPCs need default routes pointing to the transit gateway for internet-bound traffic. Option C is correct because the inspection VPC must have routes to send inspected traffic to an egress VPC or Direct Connect for internet access. Option D is correct because the transit gateway route tables must be configured with blackhole routes or specific routes to force traffic through the inspection VPC. Option B is wrong because VPC endpoints do not route internet traffic. Option E is wrong because service control policies do not enforce routing.
Variation 2. A financial company has a multi-account AWS environment using AWS Organizations. They have deployed a centralized inspection VPC with a third-party firewall appliance. All VPCs are attached to a Transit Gateway. The security team wants to ensure that all traffic between VPCs is inspected by the firewall. The firewall is deployed in an Auto Scaling group behind a Network Load Balancer (NLB). What is the BEST way to route traffic to the firewall?
hard- A.Use a Gateway Load Balancer (GWLB) endpoint in each VPC to route traffic to the firewall.
- B.Use VPC peering between each VPC and the inspection VPC.
- C.Deploy a firewall appliance in each VPC and route traffic locally.
- ✓ D.Create a Transit Gateway attachment in the inspection VPC and point the NLB as the target. Route traffic through the Transit Gateway route tables to the inspection VPC.
Why D: Option A is correct because Transit Gateway supports equal-cost multi-path (ECMP) routing, allowing traffic to be distributed across multiple firewall instances via the NLB. Option B is incorrect because VPC peering bypasses the Transit Gateway. Option C is incorrect because the firewall should be in the inspection VPC, not in each VPC. Option D is incorrect because the GWLB is not used here; the NLB is the correct choice.
Variation 3. A company has a multi-account AWS environment using AWS Transit Gateway with a centralized network account. The network team wants to ensure that only specific VPCs can communicate with each other. What is the best practice to achieve this?
medium- A.Configure security groups in each VPC to allow traffic only from specific VPC CIDRs.
- B.Use VPC peering connections between the VPCs that need to communicate.
- ✓ C.Create multiple Transit Gateway route tables and associate each VPC attachment with the appropriate route table.
- D.Use AWS Network Manager to create routing policies.
Why C: Option C is correct because Transit Gateway route tables with route propagation and static routes allow fine-grained control over which VPCs can communicate. Option A is wrong because Network Manager is for monitoring, not routing control. Option B is wrong because VPC peering is point-to-point and does not scale. Option D is wrong because security groups in each VPC add complexity and do not control Transit Gateway routing.
Last reviewed: Jun 20, 2026
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