- A
Add a route in VPC-A's route table pointing to the VPC peering connection for the CIDR of VPC-B.
Why wrong: The stem says route tables already have appropriate routes, so this is already in place.
- B
Enable DNS resolution for the VPC peering connection.
Why wrong: DNS resolution is not required for ICMP ping; it is for hostname resolution.
- C
Verify that the network ACLs in VPC-B allow inbound ICMP traffic from VPC-A.
Network ACLs are stateless and must allow both inbound and outbound traffic; if ICMP is denied, pings will fail.
- D
Verify that the security groups of the instances in VPC-B allow inbound ICMP traffic from the CIDR of VPC-A.
Security groups are stateful but default to deny inbound traffic; if ICMP is not allowed, pings will fail.
- E
Configure a transit gateway to route traffic between the two VPCs.
Why wrong: VPC peering is direct; a transit gateway is not required and would change the architecture.
Quick Answer
The correct answer is to verify that the security groups of the instances in VPC-B allow inbound ICMP traffic from the CIDR of VPC-A and to check that the network ACLs in VPC-B permit inbound ICMP traffic from VPC-A’s CIDR. This is because when troubleshooting VPC peering connectivity with correct routes but ICMP blocked by NACL or security group, the issue lies in the stateless nature of network ACLs and the stateful nature of security groups: a NACL must explicitly allow both inbound and outbound ICMP traffic, while a security group only needs an inbound rule for ICMP since return traffic is automatically permitted. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the layered security model in VPC peering, where route tables are often verified first but ACLs and security groups are the common traps. A key memory tip is “routes get you there, rules let you in”—always check both the NACL (stateless, bidirectional) and the security group (stateful, inbound only) when ICMP fails despite correct routes.
ANS-C01 Network Implementation Practice Question
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.
A network engineer is troubleshooting a connectivity issue between two VPCs (VPC-A and VPC-B) that are connected via a VPC peering connection. The engineer has verified that the route tables in both VPCs have the appropriate routes. However, instances in VPC-A cannot ping instances in VPC-B. Which TWO actions should the engineer take to resolve this issue? (Choose two.)
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
Verify that the network ACLs in VPC-B allow inbound ICMP traffic from VPC-A.
Option C is correct because network ACLs are stateless firewalls that control inbound and outbound traffic at the subnet level. Even if the route tables are correctly configured, a network ACL in VPC-B that denies inbound ICMP traffic from VPC-A's CIDR will block ping requests. Option D is correct because security groups are stateful and must explicitly allow inbound ICMP traffic from VPC-A's CIDR; without this rule, the instances in VPC-B will drop the ping requests.
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.
- ✗
Add a route in VPC-A's route table pointing to the VPC peering connection for the CIDR of VPC-B.
Why it's wrong here
The stem says route tables already have appropriate routes, so this is already in place.
- ✗
Enable DNS resolution for the VPC peering connection.
- ✓
Verify that the network ACLs in VPC-B allow inbound ICMP traffic from VPC-A.
Why this is correct
Network ACLs are stateless and must allow both inbound and outbound traffic; if ICMP is denied, pings will fail.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Verify that the security groups of the instances in VPC-B allow inbound ICMP traffic from the CIDR of VPC-A.
Why this is correct
Security groups are stateful but default to deny inbound traffic; if ICMP is not allowed, pings will fail.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Configure a transit gateway to route traffic between the two VPCs.
Why it's wrong here
VPC peering is direct; a transit gateway is not required and would change the architecture.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the distinction between stateless network ACLs and stateful security groups, and candidates mistakenly assume that correct route tables alone guarantee connectivity, overlooking the need to verify both firewall layers for the specific protocol (ICMP).
Detailed technical explanation
How to think about this question
ICMP Echo Requests (ping) are subject to both network ACL rules (stateless, requiring both inbound and outbound allow rules) and security group rules (stateful, requiring only inbound allow rules). In a VPC peering scenario, the route tables must have a route for the peer VPC's CIDR pointing to the peering connection, but the actual packet delivery also depends on the subnet-level network ACLs and instance-level security groups. A common real-world scenario is when a network ACL's default deny all inbound rule blocks ICMP, while the security group might be correctly configured, leading to confusion during troubleshooting.
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
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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: Verify that the network ACLs in VPC-B allow inbound ICMP traffic from VPC-A. — Option C is correct because network ACLs are stateless firewalls that control inbound and outbound traffic at the subnet level. Even if the route tables are correctly configured, a network ACL in VPC-B that denies inbound ICMP traffic from VPC-A's CIDR will block ping requests. Option D is correct because security groups are stateful and must explicitly allow inbound ICMP traffic from VPC-A's CIDR; without this rule, the instances in VPC-B will drop the ping requests.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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