The answer is a failing health probe. The Azure Load Balancer relies entirely on health probes to determine backend instance availability; if the probe is misconfigured—such as using the wrong port, protocol, or HTTP path—or if the backend VMs are not responding to probe requests due to a firewall blocking traffic or the application not listening on the specified port, the load balancer marks both instances as unavailable even though the VMs are running. On the AZ-104 exam, this scenario tests your understanding of how the load balancer’s control plane decouples VM health from application health, and a common trap is assuming a running VM automatically means it is reachable. Remember the mnemonic “Probe or Path” to always check the probe configuration and the application’s response path first when troubleshooting backend unavailability.
AZ-104 Implement and Manage Virtual Networking Practice Question
This AZ-104 practice question tests your understanding of implement and manage virtual networking. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
You have an Azure load balancer in front of two virtual machines. The load balancer reports both instances as unavailable even though the VMs are running. What is the most likely cause?
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 health probe is failing.
The health probe is the mechanism by which the Azure Load Balancer determines the availability of backend instances. If the probe is misconfigured (e.g., wrong port, protocol, path, or interval) or the backend VMs are not responding to the probe requests (e.g., firewall blocking the probe traffic or the application not listening on the specified port), the load balancer marks both instances as 'Unavailable' even though the VMs are running. This is the most common cause of backend pool unavailability.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 health probe is failing.
Why this is correct
A failed health probe causes the load balancer to mark backend instances as unavailable.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The VMs are in an availability set.
Why it's wrong here
Being in an availability set does not make them unavailable to the load balancer.
✗
The storage account uses the Cool tier.
Why it's wrong here
Storage tier has no direct relationship to load balancer backend health.
✗
The subscription has a budget alert.
Why it's wrong here
Budget alerts do not affect load balancer backend availability.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume 'VM is running' means it is automatically available to the load balancer, overlooking the critical role of the health probe configuration and the need for the probe traffic to be allowed by network security rules.
Detailed technical explanation
How to think about this question
The Azure Load Balancer uses a distributed health probing system where each load balancer frontend sends periodic probes (default every 5 seconds) to the configured probe endpoint (e.g., TCP port 80 or HTTP GET /health). If the probe fails consecutively (default threshold is 2 failures), the backend instance is marked as 'Unhealthy' and removed from the rotation. A common subtlety is that the probe traffic originates from the Azure host's virtual IP (168.63.129.16), so any network security group (NSG) rule or host firewall that blocks this IP will cause probe failures, even if the application itself is healthy.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this AZ-104 question in full detail.
Implement and Manage Virtual Networking — This question tests Implement and Manage Virtual Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The health probe is failing. — The health probe is the mechanism by which the Azure Load Balancer determines the availability of backend instances. If the probe is misconfigured (e.g., wrong port, protocol, path, or interval) or the backend VMs are not responding to the probe requests (e.g., firewall blocking the probe traffic or the application not listening on the specified port), the load balancer marks both instances as 'Unavailable' even though the VMs are running. This is the most common cause of backend pool unavailability.
What should I do if I get this AZ-104 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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