ANS-C01 Network Management and Operations Practice Question
A company is using AWS Direct Connect with a private VIF to connect to a VPC. The on-premises network team reports that they can ping the VPC's private IP addresses but cannot establish TCP connections to an EC2 instance's private IP. The security groups and NACLs are configured to allow the traffic. What is the most likely cause of this issue?
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 EC2 instance's operating system firewall is blocking TCP traffic
Since ICMP echo requests (pings) are working but TCP connections fail, the issue lies at Layer 4 or above. The EC2 instance's operating system firewall (e.g., iptables, Windows Firewall) can be configured to allow ICMP while blocking inbound TCP, which matches the reported symptoms. Security groups and NACLs are correctly configured, so they are not the cause. Option A is correct. Option B is incorrect because a down VIF would block all traffic, including pings. Option C is the opposite—if the on-premises firewall were blocking ICMP, pings would fail. Option D is incorrect because a missing route would break all traffic, not just TCP.
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 EC2 instance's operating system firewall is blocking TCP traffic
Why this is correct
OS firewall can block TCP while allowing ICMP.
- ✗
The Direct Connect virtual interface is in a down state
Why it's wrong here
Down VIF would affect all traffic including ping.
- ✗
The on-premises firewall is blocking ICMP but not TCP
Why it's wrong here
The problem statement says pings work, so ICMP is not blocked.
- ✗
The VPC route table is missing a route for the on-premises CIDR
Why it's wrong here
Missing route would affect ping and TCP alike.
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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.