The answer is that misconfigured security group rules in VPC A not allowing inbound traffic from VPC B’s CIDR is a primary reason for connectivity failure. This is correct because VPC peering relies on three distinct layers working together: route tables must contain explicit routes to the peer VPC’s CIDR, security groups must permit the desired traffic direction, and network ACLs must allow both inbound and outbound flows. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of the layered security model in inter-region VPC peering, where overlapping CIDRs are not the issue and the peering connection state defaults to active after acceptance. A common trap is focusing on the peering status rather than verifying that security group rules explicitly reference the peer CIDR or security group ID. Remember the mnemonic “RSA” for VPC peering troubleshooting: Routes, Security groups, and ACLs—check them in that order to isolate the failure.
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 company is troubleshooting connectivity issues between two VPCs that are peered using an inter-region VPC peering connection. The VPCs have the following CIDR blocks: VPC A (10.0.0.0/16) and VPC B (10.1.0.0/16). Which THREE of the following are possible reasons for the connectivity failure?
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 each VPC do not have routes pointing to the peer VPC's CIDR.
For VPC peering, route tables must have routes to the peer VPC, security groups must allow traffic, and network ACLs must allow traffic. Option A is wrong because overlapping CIDRs are not the issue. Option D is wrong because the peering connection is in 'active' state by default after acceptance.
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 VPC CIDR blocks overlap.
Why it's wrong here
They are different: 10.0.0.0/16 and 10.1.0.0/16 do not overlap.
✗
The VPC peering connection is in a 'pending-acceptance' state.
Why it's wrong here
If it were pending, the connection wouldn't be active; but once accepted, it's active.
✓
The route tables in each VPC do not have routes pointing to the peer VPC's CIDR.
Why this is correct
Without routes, traffic will not be forwarded to the peering connection.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route tables in each VPC do not have routes pointing to the peer VPC's CIDR. — For VPC peering, route tables must have routes to the peer VPC, security groups must allow traffic, and network ACLs must allow traffic. Option A is wrong because overlapping CIDRs are not the issue. Option D is wrong because the peering connection is in 'active' state by default after acceptance.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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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 connectivity issues between two VPCs connected via a VPC peering connection. The VPCs are in different AWS accounts and regions. The engineer can ping the private IP of an instance in the peered VPC from one side, but not from the other. What is the most likely cause?
hard
✓ A.Route tables in one VPC do not have a route to the peered VPC CIDR
B.The security group in the peered VPC is blocking ICMP traffic
C.The VPC CIDR blocks overlap
D.The VPC peering connection is in a 'pending-acceptance' state
Why A: Option D is correct because VPC peering connections must have matching route tables in both VPCs to allow bidirectional traffic. The ability to ping from one side but not the other indicates a missing route in one VPC's route table. Options A and B are irrelevant because the peering connection exists and works in one direction. Option C is for security groups, which would affect both directions equally.
Variation 2. A network engineer is troubleshooting connectivity between two VPCs (VPC-A and VPC-B) connected via a VPC peering connection. Both VPCs have CIDR blocks: VPC-A = 10.0.0.0/16, VPC-B = 10.1.0.0/16. An EC2 instance in VPC-A (10.0.1.10) cannot ping an EC2 instance in VPC-B (10.1.1.10). Security groups and NACLs allow all traffic. The route tables are configured as follows: In VPC-A, a route to 10.1.0.0/16 via the peering connection. In VPC-B, a route to 10.0.0.0/16 via the peering connection. What is the most likely cause?
hard
A.The VPCs have overlapping CIDR blocks.
B.Security groups are blocking ICMP traffic.
C.The route tables are missing routes to the peering connection.
✓ D.There is an intermediate VPC or on-premises network that routes traffic incorrectly due to the lack of transitive routing.
Why D: VPC peering connections do not support transitive routing. Even though the route tables in VPC-A and VPC-B correctly point to each other via the peering connection, if there is an intermediate VPC or on-premises network involved in the path, traffic cannot be forwarded through that intermediate point. The ping fails because the peering connection is a direct, one-to-one link and does not allow routing through a third network.
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