The answer is to check the DNS resolution of the RDS endpoint from the application server. This is correct because after a Multi-AZ failover, the RDS CNAME record updates to point to the new primary instance’s IP address, but client applications may still resolve the old, cached IP, preventing connectivity even though the new primary shows as available in the console. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of how DNS caching can mask a healthy failover, a common trap where administrators focus on security groups or subnet configurations instead of verifying name resolution. Remember that RDS endpoints are DNS names, not static IPs, so always flush the DNS cache or use `nslookup` to confirm the endpoint resolves to the current primary’s IP. A useful memory tip: “Failover is fine, but DNS is the line” — always check resolution before blaming the network.
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 company uses Amazon RDS for PostgreSQL with Multi-AZ deployment. The primary instance fails, and a failover occurs. After the failover, the application is still unable to connect to the database endpoint. The database administrator checks the RDS console and sees that the new primary is in 'available' state. What should the administrator do next to diagnose the connectivity issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the DNS resolution of the RDS endpoint from the application server
Option C is correct because the DNS CNAME of the RDS endpoint should have updated to point to the new primary. If the application is using the old IP or a cached DNS entry, it may not connect. Option A is incorrect because security group rules are usually unchanged. Option B is incorrect because the subnet group is not the issue. Option D is incorrect because the primary is already in available state.
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.
✗
Verify that the security group for the RDS instance allows inbound traffic from the application
Why it's wrong here
Security groups are still in place and usually unchanged after failover.
✗
Check if the subnet group for the RDS instance is correctly configured
Why it's wrong here
Subnet groups are not affected by failover.
✓
Check the DNS resolution of the RDS endpoint from the application server
Why this is correct
The CNAME record should be updated; stale DNS could cause connection failures.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Restart the RDS instance to force a new connection
Why it's wrong here
Restarting is unnecessary and may cause further downtime.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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: Check the DNS resolution of the RDS endpoint from the application server — Option C is correct because the DNS CNAME of the RDS endpoint should have updated to point to the new primary. If the application is using the old IP or a cached DNS entry, it may not connect. Option A is incorrect because security group rules are usually unchanged. Option B is incorrect because the subnet group is not the issue. Option D is incorrect because the primary is already in available state.
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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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