- A
Route tables in one VPC do not have a route to the peered VPC CIDR
A missing route in one VPC prevents return traffic, causing one-way connectivity.
- B
The security group in the peered VPC is blocking ICMP traffic
Why wrong: Security group rules are stateful and would block traffic both ways.
- C
The VPC CIDR blocks overlap
Why wrong: Overlapping CIDRs would prevent peering altogether.
- D
The VPC peering connection is in a 'pending-acceptance' state
Why wrong: If pending, neither side would work.
Question 1,424 of 1,705
One-Way VPC Peering Connectivity? Check Missing Route Tables
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 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?
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.
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
✓
Route tables in one VPC do not have a route to the peered VPC CIDR
The most likely cause is that the route tables in one VPC lack a route pointing to the peered VPC's CIDR block. For a VPC peering connection to function bidirectionally, each VPC must have a route in its route table that directs traffic destined for the other VPC's CIDR to the peering connection (pcx-*). Without this route, packets from the source VPC to the peered VPC will be dropped, even if the peering connection itself is active and the security groups allow traffic.
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.
- ✓
Route tables in one VPC do not have a route to the peered VPC CIDR
Why this is correct
A missing route in one VPC prevents return traffic, causing one-way connectivity.
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 security group in the peered VPC is blocking ICMP traffic
Why it's wrong here
Security group rules are stateful and would block traffic both ways.
- ✗
The VPC CIDR blocks overlap
Why it's wrong here
Overlapping CIDRs would prevent peering altogether.
- ✗
The VPC peering connection is in a 'pending-acceptance' state
Why it's wrong here
If pending, neither side would work.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that security groups or network ACLs are the primary cause of one-way connectivity issues, but the real trap is that route table misconfiguration is the most common reason for asymmetric traffic flow in VPC peering scenarios.
Detailed technical explanation
How to think about this question
VPC peering relies on route table entries to direct traffic; without a route, the source VPC's subnet will send packets to the internet gateway or virtual private gateway instead of the peering connection, causing a black hole. The ping works from one side because that VPC has the correct route, while the other VPC does not. This is a common misconfiguration when setting up cross-account or cross-region peering, as route tables must be manually updated in each VPC.
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
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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: Route tables in one VPC do not have a route to the peered VPC CIDR — The most likely cause is that the route tables in one VPC lack a route pointing to the peered VPC's CIDR block. For a VPC peering connection to function bidirectionally, each VPC must have a route in its route table that directs traffic destined for the other VPC's CIDR to the peering connection (pcx-*). Without this route, packets from the source VPC to the peered VPC will be dropped, even if the peering connection itself is active and the security groups allow traffic.
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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Last reviewed: Jul 4, 2026
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