- A
The Cloud VPN tunnel is down.
Why wrong: If the tunnel were down, outbound traffic would not reach on-prem.
- B
The MTU is set to 1500 on the VPN tunnel.
Why wrong: MTU 1500 is default; GCP supports up to 1460 for VM instances, but that would cause fragmentation, not dropping.
- C
The instance's firewall is blocking ICMP.
Why wrong: Connectivity is likely tested with TCP/UDP; ICMP blocking is not the most likely cause.
- D
The on-premises router is not advertising the return route via BGP, causing asymmetric routing.
Return traffic may take a different path, which GCP drops because it doesn't match the expected ingress interface.
Question 81 of 1,000
PCNE Implementing VPC Instances Practice Question
This PCNE practice question tests your understanding of implementing vpc instances. 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.
An engineer is troubleshooting connectivity from a Compute Engine instance (internal IP: 10.0.0.2) to an on-premises server (IP: 203.0.113.5) over a Cloud VPN tunnel. The traffic reaches the on-premises network, but the return traffic is dropped. 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 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 on-premises router is not advertising the return route via BGP, causing asymmetric routing.
For asymmetric routing, GCP may drop return traffic if it does not match the forwarding route. Cloud VPN supports dynamic routing (BGP) which advertises the on-premises prefix. However, if the on-premises router forwards return traffic through a different path (e.g., internet), GCP's firewall drops it because the return packet does not come via the VPN tunnel (asymmetric).
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 Cloud VPN tunnel is down.
Why it's wrong here
If the tunnel were down, outbound traffic would not reach on-prem.
- ✗
The MTU is set to 1500 on the VPN tunnel.
- ✗
The instance's firewall is blocking ICMP.
- ✓
The on-premises router is not advertising the return route via BGP, causing asymmetric routing.
Why this is correct
Return traffic may take a different path, which GCP drops because it doesn't match the expected ingress interface.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
- Use explanations to understand the rule behind the answer.
TExam Day Tips
- Underline the problem statement mentally.
- 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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
What to study next
Got this wrong? Here's your next step.
Identify which PCNE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
- →
Implementing VPC Instances — study guide chapter
Learn the concepts, then practise the questions
- →
Implementing VPC Instances practice questions
Targeted practice on this topic area only
- →
All PCNE questions
1,000 questions across all exam domains
- →
Google Professional Cloud Network Engineer study guide
Full concept coverage aligned to exam objectives
- →
PCNE practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related PCNE practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Configuring Network Services practice questions
Practise PCNE questions linked to Configuring Network Services.
Implementing Hybrid Interconnectivity practice questions
Practise PCNE questions linked to Implementing Hybrid Interconnectivity.
Managing, Monitoring, and Optimising Network Operations practice questions
Practise PCNE questions linked to Managing, Monitoring, and Optimising Network Operations.
Designing, Planning, and Prototyping a GCP Network practice questions
Practise PCNE questions linked to Designing, Planning, and Prototyping a GCP Network.
Implementing VPC Instances practice questions
Practise PCNE questions linked to Implementing VPC Instances.
Implementing network security practice questions
Practise PCNE questions linked to Implementing network security.
Implementing a Virtual Private Cloud practice questions
Practise PCNE questions linked to Implementing a Virtual Private Cloud.
PCNE fundamentals practice questions
Practise PCNE questions linked to PCNE fundamentals.
PCNE scenario practice questions
Practise PCNE questions linked to PCNE scenario.
PCNE troubleshooting practice questions
Practise PCNE questions linked to PCNE troubleshooting.
Practice this exam
Start a free PCNE practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this PCNE question test?
Implementing VPC Instances — This question tests Implementing VPC Instances — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The on-premises router is not advertising the return route via BGP, causing asymmetric routing. — For asymmetric routing, GCP may drop return traffic if it does not match the forwarding route. Cloud VPN supports dynamic routing (BGP) which advertises the on-premises prefix. However, if the on-premises router forwards return traffic through a different path (e.g., internet), GCP's firewall drops it because the return packet does not come via the VPN tunnel (asymmetric).
What should I do if I get this PCNE question wrong?
Identify which PCNE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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
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 →
Last reviewed: Jul 4, 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.
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.