- A
Cloud CDN cached responses from the old load balancer may still be served until their cache TTL expires.
Cloud CDN caches content at edge locations; if the cache TTL has not expired, users may receive the old content even after DNS changes.
- B
The old load balancer's SSL certificate is still cached by clients.
Why wrong: SSL certificates are not cached at client-side; the browser receives the new certificate from the new load balancer.
- C
The DNS change has not propagated globally despite the TTL expiring.
Why wrong: The engineer confirmed the TTL has expired, so DNS should be updated globally.
- D
The old load balancer's IP address is still being served by Google's edge network due to anycast.
Why wrong: Anycast ensures that the DNS change points to the new IP; the old IP is no longer advertised.
Quick Answer
The answer is that Cloud CDN cached responses from the old load balancer may still be served until their cache TTL expires. This occurs because Cloud CDN caches content at Google’s edge nodes based on HTTP cache-control headers or a default cache TTL, which operate independently of DNS resolution. Even after the DNS TTL has expired and the new record propagates, users hitting a stale edge cache will receive the old load balancer’s content until that cache entry’s TTL also expires. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that DNS and HTTP caching are separate layers—a common trap is assuming a DNS change instantly flushes all cached responses. The key distinction is that DNS TTL governs how long a client caches the IP address, while cache TTL governs how long Cloud CDN holds the response object. For the exam, remember the memory tip: “DNS points the way, but cache TTL holds the payload.”
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. 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 has deployed an external HTTPS load balancer with a Cloud CDN backend. The load balancer uses a managed SSL certificate. Recently, the company updated their DNS record to point to a different IP address of a new load balancer. After the change, some users are still being served from the old load balancer's cache. The network engineer has confirmed that the DNS TTL has expired. What is the most likely cause of this issue?
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 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
Cloud CDN cached responses from the old load balancer may still be served until their cache TTL expires.
Cloud CDN caches content at Google's edge caches based on the cache-control headers or default cache TTL. When the DNS record is updated to point to a new load balancer, the old load balancer's cached responses may still be served from edge caches until their cache TTL expires, even after the DNS TTL has expired. This is because Cloud CDN caches are independent of DNS resolution and are governed by HTTP caching rules.
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.
- ✓
Cloud CDN cached responses from the old load balancer may still be served until their cache TTL expires.
Why this is correct
Cloud CDN caches content at edge locations; if the cache TTL has not expired, users may receive the old content even after DNS changes.
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 old load balancer's SSL certificate is still cached by clients.
Why it's wrong here
SSL certificates are not cached at client-side; the browser receives the new certificate from the new load balancer.
- ✗
The DNS change has not propagated globally despite the TTL expiring.
Why it's wrong here
The engineer confirmed the TTL has expired, so DNS should be updated globally.
- ✗
The old load balancer's IP address is still being served by Google's edge network due to anycast.
Why it's wrong here
Anycast ensures that the DNS change points to the new IP; the old IP is no longer advertised.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between DNS TTL (which controls how long DNS records are cached by resolvers) and HTTP cache TTL (which controls how long content is cached by CDN edge nodes), leading candidates to incorrectly attribute the issue to DNS propagation rather than CDN cache expiration.
Detailed technical explanation
How to think about this question
Cloud CDN uses a distributed cache hierarchy where edge caches store responses based on the cache key (typically the URL and headers). Even after DNS changes, cached objects remain in edge caches until their max-age (from Cache-Control header) or the default TTL (e.g., 1 hour for uncacheable responses) expires. This can lead to a 'cache stampede' if the old cache is not purged manually via cache invalidation or by setting a short max-age during the migration.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this PCNE question test?
Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Cloud CDN cached responses from the old load balancer may still be served until their cache TTL expires. — Cloud CDN caches content at Google's edge caches based on the cache-control headers or default cache TTL. When the DNS record is updated to point to a new load balancer, the old load balancer's cached responses may still be served from edge caches until their cache TTL expires, even after the DNS TTL has expired. This is because Cloud CDN caches are independent of DNS resolution and are governed by HTTP caching rules.
What should I do if I get this PCNE 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.
About these practice questions
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Last reviewed: Jun 30, 2026
This PCNE practice question is part of Courseiva's free Google Cloud 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 PCNE exam.
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