This ANS-C01 practice question tests your understanding of network implementation. 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.
Network Topology
A network engineer examines the route table above. The VPC has a CIDR of 10.0.0.0/16. There is a VPC peering connection (pcx-...) to a VPC with CIDR 192.168.0.0/16. However, instances in this route table's subnet cannot communicate with the peered VPC. What is the most likely cause?
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.
Network Topology
A
The route table is not associated with the subnet.
Why wrong: The subnet uses this route table, as implied.
B
The VPC peering connection is in 'pending-acceptance' state.
Why wrong: The command does not show the state, but likely it is active if the route exists.
C
The security group or network ACL in the source subnet is blocking traffic.
Even with correct routing, security groups/NACLs can block traffic.
D
The route to the peered VPC is missing from the route table.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group or network ACL in the source subnet is blocking traffic.
Option C is correct because the route table shown includes a route for the peered VPC (192.168.0.0/16 via pcx-...), and the route table is associated with the subnet (implied by the question stating 'this route table's subnet'). Since routing is in place, the most likely remaining cause is that a security group (stateful, blocking inbound/outbound traffic) or a network ACL (stateless, blocking inbound/outbound traffic) is filtering the traffic between the subnets. Security groups and network ACLs operate at the instance and subnet level respectively, and misconfigured rules can prevent communication even when routes are correct.
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 route table is not associated with the subnet.
Why it's wrong here
The subnet uses this route table, as implied.
✗
The VPC peering connection is in 'pending-acceptance' state.
Why it's wrong here
The command does not show the state, but likely it is active if the route exists.
✓
The security group or network ACL in the source subnet is blocking traffic.
Why this is correct
Even with correct routing, security groups/NACLs can block traffic.
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 route to the peered VPC is missing from the route table.
Why it's wrong here
The route is present.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that a missing route is the only cause for VPC peering communication failure, but the trap here is that the route is present, so candidates must consider security group or network ACL filtering as the next most likely cause.
Trap categories for this question
Command / output trap
The command does not show the state, but likely it is active if the route exists.
Detailed technical explanation
How to think about this question
Security groups are stateful and evaluate all traffic (inbound and outbound) based on rules; if the outbound rule on the source instance's security group does not allow traffic to the peered VPC CIDR, or the inbound rule on the target instance's security group does not allow traffic from the source CIDR, communication fails. Network ACLs are stateless and require explicit inbound and outbound rules for both directions; a missing ephemeral port range (1024-65535) in the outbound ACL of the source subnet or inbound ACL of the target subnet is a common misconfiguration. In a real-world scenario, an engineer might verify routes are correct but overlook that the security group's outbound rule defaults to 'Allow All' only for the same VPC, not for peered VPCs, unless explicitly configured.
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 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.
Visual reference
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 Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The security group or network ACL in the source subnet is blocking traffic. — Option C is correct because the route table shown includes a route for the peered VPC (192.168.0.0/16 via pcx-...), and the route table is associated with the subnet (implied by the question stating 'this route table's subnet'). Since routing is in place, the most likely remaining cause is that a security group (stateful, blocking inbound/outbound traffic) or a network ACL (stateless, blocking inbound/outbound traffic) is filtering the traffic between the subnets. Security groups and network ACLs operate at the instance and subnet level respectively, and misconfigured rules can prevent communication even when routes are correct.
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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