The answer is to check the route tables in both VPCs for routes pointing to the peering connection for the respective CIDR blocks. This is the correct first step because VPC peering requires symmetric routing: traffic must have a route to the destination VPC, and the destination VPC must have a route back to the source. Without a return route, packets are dropped after the initial connection is established, causing the intermittent drops described. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that VPC peering misconfigurations often manifest as timeouts or drops rather than hard failures, and it’s a common trap to blame security groups or NACLs first—but those are stateful or stateless filters that would block consistently, not intermittently. A key memory tip: “Peering needs pairing—routes must mirror both ways.”
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 network engineer is troubleshooting intermittent connectivity issues between an EC2 instance in a private subnet and an RDS database in another VPC connected via a VPC peering connection. The connection works for a few minutes and then drops. CloudWatch logs show no errors on the peering connection. What should the engineer check first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route tables in both VPCs to ensure they have routes pointing to the peering connection for the respective CIDR blocks.
Option C is correct because the most common cause of intermittent drops over VPC peering is the lack of proper route table entries in both VPCs to route traffic back to the source. Option A is wrong because security groups are stateful and do not cause intermittent drops. Option B is wrong because NACLs are stateless but would block consistently, not intermittently. Option D is wrong because DNS resolution settings affect name resolution, not connectivity drops.
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 DNS resolution settings of the VPC peering connection.
Why it's wrong here
DNS resolution affects hostname resolution, not connectivity drops once connection is established.
✗
The network ACL of the subnet where the EC2 instance resides.
Why it's wrong here
NACLs are stateless but would block consistently if rules are missing, not intermittently.
✗
The security group attached to the RDS database allowing traffic from the EC2 instance.
Why it's wrong here
Security groups are stateful and allow return traffic, so they would not cause intermittent drops.
✓
The route tables in both VPCs to ensure they have routes pointing to the peering connection for the respective CIDR blocks.
Why this is correct
Missing or incorrect routes can cause asymmetric routing and drops.
Clue confirmation
The clue word "first" in the question point toward this answer.
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.
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 route tables in both VPCs to ensure they have routes pointing to the peering connection for the respective CIDR blocks. — Option C is correct because the most common cause of intermittent drops over VPC peering is the lack of proper route table entries in both VPCs to route traffic back to the source. Option A is wrong because security groups are stateful and do not cause intermittent drops. Option B is wrong because NACLs are stateless but would block consistently, not intermittently. Option D is wrong because DNS resolution settings affect name resolution, not connectivity drops.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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 network engineer is troubleshooting intermittent connectivity issues between two VPCs that are peered. The VPC peering connection is in the 'active' state. ICMP ping from an instance in VPC A to an instance in VPC B fails intermittently. What is the most likely cause?
easy
A.The network ACLs are blocking ICMP traffic.
B.The security groups on the instances do not allow inbound ICMP.
C.The VPC peering connection is not in the 'active' state.
✓ D.The route tables in one or both VPCs lack routes to the peer VPC's CIDR via the peering connection.
Why D: Since the VPC peering connection is active, the issue is likely that the route tables in one or both VPCs are not correctly configured to route traffic to the peered VPC's CIDR via the peering connection. Without proper routes, traffic is dropped.
Variation 2. A network engineer is troubleshooting connectivity between two VPCs that are peered. The VPC peering connection is active, and the route tables have appropriate routes. However, instances in VPC A cannot reach instances in VPC B. The security groups in both VPCs allow all traffic. What is the most likely issue?
medium
A.The security groups are not allowing ICMP traffic
✓ B.The route tables in both VPCs do not have routes pointing to the peering connection for the other VPC's CIDR
C.The VPC peering connection is not in the 'active' state
D.The instances are in different availability zones
Why B: VPC peering does not support transitive routing; if there is an intermediate resource (like a VPN or another VPC) involved, it won't work. But the question doesn't mention that. Another common issue is that the VPC peering connection requires that the route tables of both VPCs have routes to each other's CIDR, and that security groups reference each other's CIDR or security group IDs. Since security groups allow all, the issue might be that the security group rules are not allowing traffic from the peer VPC's CIDR. However, since they allow all, the problem is likely that the instances are in different regions and the peering is intra-region? Actually, VPC peering works across regions but requires appropriate route table entries. The most likely issue is that the route tables are missing the necessary routes. Option A is correct.
Last reviewed: Jun 20, 2026
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