The answer is to verify that the on-premises router is advertising the 10.0.1.0/24 prefix via BGP. Even when a VPN tunnel is up and BGP sessions are established, traffic will fail if the on-premises router does not advertise its subnet to GCP’s Cloud Router, because GCP has no route to send return traffic back to the on-premises network. This scenario tests your understanding that a tunnel being up does not guarantee bidirectional reachability—BGP must explicitly advertise the prefixes for route propagation. On the Google Professional Cloud Network Engineer exam, this is a common trap where engineers focus on tunnel status or firewall rules instead of checking BGP advertisements first. A useful memory tip is “Tunnel up, BGP up, but no traffic? Check the advertisements first—no prefix, no path.”
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.
A network engineer is troubleshooting a Cloud VPN tunnel that is not passing traffic. The tunnel status shows as established, and BGP sessions are up. However, traffic from an on-premises subnet (10.0.1.0/24) to a GCP subnet (192.168.1.0/24) is not working. What should the engineer check first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Verify that the on-premises router is advertising the 10.0.1.0/24 prefix via BGP
Option A is correct because even though the VPN tunnel and BGP session are established, traffic may still fail if the on-premises router is not advertising the on-premises subnet (10.0.1.0/24) to GCP via BGP. Without this advertisement, GCP's Cloud Router has no route to reach that subnet, so return traffic from GCP to on-premises is dropped. The first step in troubleshooting is to verify that the on-premises BGP speaker is sending the correct prefix in its UPDATE messages.
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.
✓
Verify that the on-premises router is advertising the 10.0.1.0/24 prefix via BGP
Why this is correct
If the prefix is not advertised, GCP will not have a route to reach it.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Ensure that the Cloud Router is configured with the correct BGP ASN
Why it's wrong here
Incorrect ASN would prevent BGP session from establishing, but it is already up.
✗
Check that GCP has a static route for 10.0.1.0/24 pointing to the VPN tunnel
Why it's wrong here
With dynamic routing, routes are learned via BGP; static routes are not needed.
✗
Check GCP firewall rules to allow ingress from 10.0.1.0/24
Why it's wrong here
Firewall rules control inbound traffic; the issue is outbound from GCP to on-premises.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a 'tunnel established' and 'BGP up' guarantee traffic flow, but the trap here is that route advertisement via BGP is a separate requirement that must be explicitly verified.
Detailed technical explanation
How to think about this question
In Cloud VPN with dynamic routing, the Cloud Router uses BGP to exchange routes with the on-premises peer. GCP only installs routes for prefixes that are advertised by the peer; if the on-premises router does not advertise 10.0.1.0/24, GCP will not have a route for that subnet, causing asymmetric routing or blackholing of return traffic. This is a common misconfiguration where the tunnel and BGP session are up, but the route advertisement is missing or filtered.
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.
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: Verify that the on-premises router is advertising the 10.0.1.0/24 prefix via BGP — Option A is correct because even though the VPN tunnel and BGP session are established, traffic may still fail if the on-premises router is not advertising the on-premises subnet (10.0.1.0/24) to GCP via BGP. Without this advertisement, GCP's Cloud Router has no route to reach that subnet, so return traffic from GCP to on-premises is dropped. The first step in troubleshooting is to verify that the on-premises BGP speaker is sending the correct prefix in its UPDATE messages.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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 →
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.
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.