VPC Peering Connectivity Troubleshooting: Routes, Security Groups & NACLs
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?
Quick Answer
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.
⚠ Common exam trap
The ANS-C01 exam often tests the misconception that overlapping CIDRs are the only cause of peering failures, but here the CIDRs are non-overlapping, so candidates must recognize that missing routes and security group/NACL misconfigurations are equally valid causes.
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
✓
The route tables in each VPC do not have routes pointing to the peer VPC's CIDR.
For inter-region VPC peering to function, each VPC must have explicit routes in its route tables pointing to the CIDR block of the peer VPC, with the VPC peering connection as the target. Without these routes, traffic destined for the peer VPC will not be forwarded over the peering link, causing connectivity failure.
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.
- ✓
The network ACLs in VPC A are blocking inbound traffic from VPC B's CIDR.
Why this is correct
NACLs are stateless and must allow both inbound and outbound traffic.
- ✓
The security group rules in VPC A do not allow inbound traffic from VPC B's CIDR.
Why this is correct
Security groups must allow traffic from the peer VPC.
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Same concept, more angles
2 more ways this is tested on ANS-C01
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: 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.
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 C: The most likely cause is that the route tables containing the peering routes are not associated with the subnets where the EC2 instances are located. In AWS, each subnet must have an associated route table. If the main route table has the peering routes but the subnet's custom route table does not, the traffic will not be routed correctly. Since security groups and NACLs allow all traffic and the CIDRs are not overlapping, the only remaining possibility is a route table association issue.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.