Quick Answer
The answer is missing routes in the VPC route tables pointing to the Transit Gateway. For inter-VPC traffic to flow through a Transit Gateway, each VPC’s route table must contain a route that targets the Transit Gateway attachment as the next hop; without this explicit entry, the VPC has no path to forward packets to the gateway, causing them to be dropped. On the AWS Certified SAP on AWS Specialty PAS-C01 exam, this scenario tests your understanding of transitive routing fundamentals and the common pitfall of assuming that attaching a VPC to a Transit Gateway automatically enables routing—it does not, as route tables must be manually updated. A frequent trap is confusing this with security group or NACL misconfigurations, but those are stateful or stateless filters, not routing issues. Memory tip: think of the Transit Gateway as a highway on-ramp—you still need a sign (route) in your VPC telling traffic to take that ramp.
This PAS-C01 practice question tests your understanding of technology. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 SAP on AWS and uses AWS Transit Gateway to connect multiple VPCs. They notice that inter-VPC traffic is being dropped. What is a likely cause?
Security groups not allowing return traffic.
Why wrong: Security groups are stateful.
Missing routes in the VPC route tables pointing to the Transit Gateway.
Without routes, traffic cannot be forwarded to Transit Gateway.
Transit Gateway does not support transitive routing.
Why wrong: Transit Gateway does support transitive routing.
Network ACLs blocking traffic.
Why wrong: NACLs could block, but missing routes is more likely.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Missing routes in the VPC route tables pointing to the Transit Gateway.
Option A is correct because VPC route tables must have routes pointing to the Transit Gateway for traffic to flow. If routes are missing, traffic is dropped. Option B is wrong because NACLs are stateless and inbound/outbound rules must be correct; but missing routes is more common. Option C is wrong because security groups are stateful and allow return traffic. Option D is wrong because Transit Gateway supports transitive routing.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Security groups not allowing return traffic.
Why it's wrong here
Security groups are stateful.
Missing routes in the VPC route tables pointing to the Transit Gateway.
Why this is correct
Without routes, traffic cannot be forwarded to Transit Gateway.
Related concept
Standard ACLs match source addresses.
Transit Gateway does not support transitive routing.
Why it's wrong here
Transit Gateway does support transitive routing.
Network ACLs blocking traffic.
Why it's wrong here
NACLs could block, but missing routes is more likely.
Common exam traps
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
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
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related PAS-C01 ACL questions on filtering logic and placement.
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FAQ
Technology — This question tests Technology — Standard ACLs match source addresses..
The correct answer is: Missing routes in the VPC route tables pointing to the Transit Gateway. — Option A is correct because VPC route tables must have routes pointing to the Transit Gateway for traffic to flow. If routes are missing, traffic is dropped. Option B is wrong because NACLs are stateless and inbound/outbound rules must be correct; but missing routes is more common. Option C is wrong because security groups are stateful and allow return traffic. Option D is wrong because Transit Gateway supports transitive routing.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related PAS-C01 ACL questions on filtering logic and placement.
Standard ACLs match source addresses.
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Last reviewed: Jun 20, 2026
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