- 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 982
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
Cisco often tests the distinction between IKE phase 1 (ISAKMP) and phase 2 (IPsec) failures, and the trap here is that candidates assume 'tunnels established' means the IPsec SA is up, when in reality the status may reflect only the IKE_SA_INIT phase, and the pre-shared key mismatch prevents full authentication and IPsec SA creation.
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.
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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.
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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.
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