This CV0-004 practice question tests your understanding of operations and support. 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 systems administrator is troubleshooting a network connectivity issue between two peered VPCs in different regions. The VPC peering connection is established and the route tables are configured. However, instances cannot communicate. What should be checked NEXT?
Why wrong: VPC peering does not use route propagation; routes must be added manually.
B
The network ACLs at the subnet level
Why wrong: NACLs are stateless and can also block traffic, but security groups are more commonly the issue; however, checking security groups is a standard next step.
C
The security group rules on the instances
Security groups are stateful firewalls that can block inter-VPC traffic if not explicitly allowed.
D
The MTU settings on the instances
Why wrong: MTU mismatch could cause packet loss but is not the most likely next step for general 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 security group rules on the instances
Since the VPC peering connection is established and route tables are configured, the next likely cause is that security group rules on the instances are blocking traffic. Security groups act as virtual firewalls at the instance level, and by default they deny all inbound traffic; explicit rules must allow traffic from the peered VPC's CIDR or security group. This is a common oversight when peering VPCs across regions, as security groups do not automatically permit cross-peering 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.
✗
The route propagation of the peering connection
Why it's wrong here
VPC peering does not use route propagation; routes must be added manually.
✗
The network ACLs at the subnet level
Why it's wrong here
NACLs are stateless and can also block traffic, but security groups are more commonly the issue; however, checking security groups is a standard next step.
✓
The security group rules on the instances
Why this is correct
Security groups are stateful firewalls that can block inter-VPC traffic if not explicitly allowed.
Related concept
Read the scenario before looking for a memorised answer.
✗
The MTU settings on the instances
Why it's wrong here
MTU mismatch could cause packet loss but is not the most likely next step for general connectivity failure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on network ACLs or route propagation first, forgetting that security groups are the most granular and commonly misconfigured firewall layer that can silently block traffic even when routes and ACLs are correct.
Detailed technical explanation
How to think about this question
Security groups are stateful, meaning that if you allow inbound traffic from a peered VPC, the outbound return traffic is automatically allowed regardless of outbound rules. However, you must explicitly add an inbound rule for the peered VPC's CIDR or security group ID; simply having the peering connection and routes does not open the instance's firewall. In cross-region VPC peering, security group references cannot use the peered VPC's security group ID if the VPCs are in different regions—you must use CIDR ranges instead.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Operations and Support — This question tests Operations and Support — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The security group rules on the instances — Since the VPC peering connection is established and route tables are configured, the next likely cause is that security group rules on the instances are blocking traffic. Security groups act as virtual firewalls at the instance level, and by default they deny all inbound traffic; explicit rules must allow traffic from the peered VPC's CIDR or security group. This is a common oversight when peering VPCs across regions, as security groups do not automatically permit cross-peering traffic.
What should I do if I get this CV0-004 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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Question Discussion
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