ANS-C01 Network Implementation Practice Question
A company has a VPC peering connection between VPC A (10.0.0.0/16) and VPC B (10.1.0.0/16). Both VPCs have route tables that include routes to each other's CIDR blocks via the peering connection. Instances in VPC A can ping instances in VPC B, but traffic to an Application Load Balancer (ALB) in VPC B fails. The ALB is in public subnets with internet-facing scheme. What is the most likely cause?
⚠ Common exam trap
A common mix-up: candidates assume that because ping works, all traffic should work, overlooking that security groups operate at Layer 4 and must explicitly permit the specific protocol and port (e.g., TCP/80 or TCP/443) from the source VPC's CIDR.
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 ALB security group does not allow inbound traffic from VPC A
The most likely cause is that the ALB's security group does not allow inbound traffic from VPC A. Even though the VPC peering connection is properly configured and ICMP traffic (ping) works, HTTP/HTTPS traffic to the ALB is controlled by security groups. The ALB's security group must explicitly permit inbound traffic from the CIDR block of VPC A (10.0.0.0/16) or from the security group of the instances in VPC A. Without this rule, the ALB will drop the traffic, even though the network path is functional.
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 peering connection does not support transitive routing for ALBs
Why it's wrong here
VPC peering supports direct connectivity; ALB can be accessed via private IP.
- ✗
The ALB is internet-facing and cannot be accessed from a peered VPC
Why it's wrong here
Internet-facing ALBs can be accessed from peered VPCs via private IPs with proper security group rules.
- ✗
The route tables in VPC B do not have a route back to VPC A
Why it's wrong here
Route tables include routes to each other's CIDR blocks.
- ✓
The ALB security group does not allow inbound traffic from VPC A
Why this is correct
The security group must allow inbound traffic from the peered VPC's CIDR.
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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.