- A
The Cloud Router BGP AS number is duplicated
Why wrong: Duplicate AS numbers would affect BGP after tunnel is up, but the tunnel itself would still establish.
- B
The pre-shared keys do not match
Mismatched pre-shared keys would cause IKE negotiation to fail, resulting in no response from Google side.
- C
The on-premises firewall is blocking UDP port 500
Why wrong: If IKE packets are being sent, UDP 500 is not blocked outbound, but responses might be blocked if not allowed inbound.
- D
The VPN tunnel is configured as policy-based instead of route-based
Why wrong: Policy-based vs route-based affects traffic selectors, not IKE negotiation; tunnels would still establish.
Question 757 of 1,000
PCNE Implementing Hybrid Interconnectivity Practice Question
This PCNE practice question tests your understanding of implementing hybrid interconnectivity. 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 configures an HA VPN with two tunnels to an on-premises network using IKEv2 and pre-shared keys. After configuration, the tunnels show as established, but no traffic flows. The Cloud Router BGP sessions are not established. The on-premises firewall logs show IKE packets are being sent but no response. 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 pre-shared keys do not match
B is correct because the on-premises firewall logs show IKE packets being sent but no response, which indicates that the IKEv2 pre-shared key mismatch causes the IKE authentication phase to fail. Even though the tunnel status shows as established (likely due to a stale or misconfigured status check), the BGP sessions cannot form because the IKE security association (SA) is not fully authenticated, preventing the IPsec SA from being created and thus blocking all traffic, including BGP packets.
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 Router BGP AS number is duplicated
Why it's wrong here
Duplicate AS numbers would affect BGP after tunnel is up, but the tunnel itself would still establish.
- ✓
The pre-shared keys do not match
Why this is correct
Mismatched pre-shared keys would cause IKE negotiation to fail, resulting in no response from Google side.
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 on-premises firewall is blocking UDP port 500
Why it's wrong here
If IKE packets are being sent, UDP 500 is not blocked outbound, but responses might be blocked if not allowed inbound.
- ✗
The VPN tunnel is configured as policy-based instead of route-based
Why it's wrong here
Policy-based vs route-based affects traffic selectors, not IKE negotiation; tunnels would still establish.
Common exam traps
Common exam trap: answer the scenario, not the keyword
In Google Cloud HA VPN, the tunnel status may show as established after IKE_SA_INIT but before full authentication. Candidates often mistakenly assume that the IPsec SA is up, but the pre-shared key mismatch prevents full authentication and IPsec SA creation, which is why BGP sessions fail despite the tunnel status indicating established.
Detailed technical explanation
How to think about this question
In IKEv2, the pre-shared key is used during the IKE_AUTH exchange to authenticate the peers; a mismatch causes the responder to silently drop the request (no response) after the IKE_SA_INIT phase completes. This differs from IKEv1, where a PSK mismatch might generate an error notification. The Cloud Router BGP sessions rely on the IPsec tunnel being fully operational, which requires successful IKE authentication and IPsec SA establishment.
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 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.
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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.
- →
Implementing Hybrid Interconnectivity — study guide chapter
Learn the concepts, then practise the questions
- →
Implementing Hybrid Interconnectivity 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 Hybrid Interconnectivity — This question tests Implementing Hybrid Interconnectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The pre-shared keys do not match — B is correct because the on-premises firewall logs show IKE packets being sent but no response, which indicates that the IKEv2 pre-shared key mismatch causes the IKE authentication phase to fail. Even though the tunnel status shows as established (likely due to a stale or misconfigured status check), the BGP sessions cannot form because the IKE security association (SA) is not fully authenticated, preventing the IPsec SA from being created and thus blocking all traffic, including BGP packets.
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
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 →
Keep practising
More PCNE practice questions
- An organization is migrating to Google Cloud and requires connectivity between their on-premises network and VPC. They p…
- A company is migrating on-premises DNS to Google Cloud. They have a hybrid network using Cloud VPN and want to resolve o…
- A network engineer is configuring a Cloud Router for BGP peering with an on-premises router over a VPN tunnel. The on-pr…
- A company uses Cloud NAT to allow private instances to reach the internet. They notice that egress traffic from Compute…
- You are setting up Partner Interconnect with a service provider that offers both Layer 2 and Layer 3 options. Your on-pr…
- Match each VPC networking concept to its definition.
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.