The answer is that the Redis cluster security group lacks an inbound rule allowing traffic from the EC2 instance’s security group on port 6379. This is the most likely cause of the connection timeout because security groups are stateful—they automatically allow return traffic for outbound connections, but inbound traffic must be explicitly permitted by a rule in the target security group. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of security group behavior versus network ACLs, a common trap where candidates confuse stateful security groups with stateless NACLs. When you troubleshoot ElastiCache Redis connection timeout issues, always verify that the Redis cluster’s security group has an inbound rule referencing the EC2 security group, not just the EC2’s outbound rule. Memory tip: “Inbound must be invited”—the target security group must explicitly welcome the source.
DBS-C01 Monitoring and Troubleshooting Practice Question
This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. 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 developer is receiving timeout errors when connecting to an Amazon ElastiCache for Redis cluster from an Amazon EC2 instance. The security group for the EC2 instance allows outbound traffic to the Redis cluster's security group on port 6379. The Redis cluster's security group does not allow inbound traffic from the EC2 instance. What is the most likely cause of the timeout?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Redis cluster security group does not have an inbound rule allowing traffic from the EC2 security group on port 6379
Option A is correct because security groups are stateful; for inbound traffic, the target security group must allow the incoming connection. Option B is incorrect because the subnet routing does not block specific ports. Option C is incorrect because NACLs are stateless but the issue is with security groups. Option D is incorrect because the Redis cluster is in a VPC, not on-premises.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 network ACL for the Redis subnet is blocking inbound traffic on port 6379
Why it's wrong here
NACLs are stateless and would also block outbound response if not configured, but security group is the first check.
✓
The Redis cluster security group does not have an inbound rule allowing traffic from the EC2 security group on port 6379
Why this is correct
Inbound rules are required for the target security group.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The subnet route table does not have a route to the Redis cluster
Why it's wrong here
Route tables direct traffic between subnets; they don't block specific ports.
✗
The Redis cluster is not accessible from within the same VPC
Why it's wrong here
ElastiCache clusters are accessible within the VPC if security groups permit.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DBS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The Redis cluster security group does not have an inbound rule allowing traffic from the EC2 security group on port 6379 — Option A is correct because security groups are stateful; for inbound traffic, the target security group must allow the incoming connection. Option B is incorrect because the subnet routing does not block specific ports. Option C is incorrect because NACLs are stateless but the issue is with security groups. Option D is incorrect because the Redis cluster is in a VPC, not on-premises.
What should I do if I get this DBS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DBS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This DBS-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 DBS-C01 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.