The answer is that the route tables in VPC A do not have a route to VPC B’s CIDR pointing to the peering connection. This is the most likely cause because VPC peering does not support transitive routing—each VPC must have an explicit route in its route table for the peered VPC’s CIDR block, with the peering connection as the target. Even if security groups and network ACLs allow ICMP, missing or misdirected routes will silently drop traffic, making ping fail. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of VPC peering connectivity troubleshooting, a core topic where the common trap is assuming that adding a route in one VPC automatically propagates to the other. Remember: VPC peering is a direct, one-to-one link; routes must be manually added in both directions. A helpful memory tip is “peer the path, not the packet”—each VPC must know the exact path to the other’s CIDR, or the packet never leaves the source subnet.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 has a VPC peering connection between VPC A (10.0.0.0/16) and VPC B (10.1.0.0/16). They have added routes in both route tables. However, instances in VPC A cannot ping instances in VPC B. The security groups and network ACLs allow ICMP. 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.
The security groups in VPC B do not allow inbound ICMP from VPC A CIDR
Why wrong: The question states security groups allow ICMP, so this is not the cause.
B
The route tables in VPC A do not have a route to VPC B CIDR pointing to the peering connection
Without a specific route to the peered VPC CIDR, traffic will not be sent over the peering connection.
C
The VPC peering connection is not in the 'active' state
Why wrong: If the peering connection is not active, the route tables would show the target as 'blackhole', but the question implies routes are added, so likely active.
D
The instances are in different Availability Zones
Why wrong: Availability Zones do not affect VPC peering; traffic between VPCs is independent of AZ.
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 VPC A do not have a route to VPC B CIDR pointing to the peering connection
VPC peering does not support transitive routing. If VPC A tries to reach VPC B via a CIDR that is not directly peered, traffic will not be forwarded. The route tables must have specific routes for the peered VPC CIDR. The issue likely is that the route tables are missing the route to the peer, or the route is pointing to an incorrect target.
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 security groups in VPC B do not allow inbound ICMP from VPC A CIDR
Why it's wrong here
The question states security groups allow ICMP, so this is not the cause.
✓
The route tables in VPC A do not have a route to VPC B CIDR pointing to the peering connection
Why this is correct
Without a specific route to the peered VPC CIDR, traffic will not be sent over the peering connection.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The VPC peering connection is not in the 'active' state
Why it's wrong here
If the peering connection is not active, the route tables would show the target as 'blackhole', but the question implies routes are added, so likely active.
✗
The instances are in different Availability Zones
Why it's wrong here
Availability Zones do not affect VPC peering; traffic between VPCs is independent of AZ.
Common exam traps
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.
Trap categories for this question
Command / output trap
If the peering connection is not active, the route tables would show the target as 'blackhole', but the question implies routes are added, so likely active.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route tables in VPC A do not have a route to VPC B CIDR pointing to the peering connection — VPC peering does not support transitive routing. If VPC A tries to reach VPC B via a CIDR that is not directly peered, traffic will not be forwarded. The route tables must have specific routes for the peered VPC CIDR. The issue likely is that the route tables are missing the route to the peer, or the route is pointing to an incorrect target.
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: "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?
CIDR notation defines the prefix length.
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