The answer is to verify that VPC firewall rules allow traffic from the on-premises subnets and to confirm that a valid route exists for the Compute Engine instance IP ranges. This is correct because even when a ping succeeds to the VLAN attachment IP—which confirms Layer 2 and BGP session health—traffic to instances requires a matching subnet route in the VPC’s routing table, as the VPC router drops packets without a destination route. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of the separation between interconnect connectivity and VPC routing, a common trap where candidates assume a successful VLAN ping implies full instance reachability. Remember that the VLAN attachment IP is a Google-managed address on the interconnect, not an instance, so routing and firewall rules are separate layers. A useful memory tip is “Ping the link, not the instance—routes and firewalls are the difference.”
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 company is troubleshooting connectivity issues between their on-premises network and Google Cloud over a Dedicated Interconnect. They can ping the VLAN attachment IP but cannot reach Compute Engine instances. Which TWO checks should they perform?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Confirm that the subnet routes for the instance IP ranges are present in the VPC
Option B is correct because for on-premises traffic to reach Compute Engine instances over Dedicated Interconnect, the VPC must have a subnet route (either automatically created or custom static/dynamic route) that matches the instance IP ranges. Without this route, packets from the on-premises network will be dropped by the VPC router, even if the VLAN attachment is reachable.
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 network has IAM permissions to access instances
Why it's wrong here
IAM is not a factor in network connectivity.
✓
Confirm that the subnet routes for the instance IP ranges are present in the VPC
Why this is correct
Routes must exist for return traffic.
Related concept
Read the scenario before looking for a memorised answer.
✓
Verify that VPC firewall rules allow traffic from the on-premises subnets
Check that BGP sessions are established between Cloud Router and on-premises router
Why it's wrong here
BGP is not strictly required if static routes are used.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that pinging the VLAN attachment IP confirms end-to-end connectivity to instances, but in reality it only confirms BGP session health and Layer 3 reachability to the Cloud Router interface, not the VPC routing or firewall rules required for instance access.
Detailed technical explanation
How to think about this question
When a Dedicated Interconnect is used, Cloud Router establishes BGP sessions with the on-premises router to exchange routes. The VLAN attachment IP is a link-local address (169.254.x.x) used for BGP peering; pinging it confirms Layer 3 connectivity and BGP session health. However, even with BGP up, traffic to Compute Engine instances requires that the VPC has a route for the instance subnet (e.g., a dynamic route advertised by the on-premises router or a static route) and that VPC firewall rules permit the traffic. A common real-world scenario is when an on-premises router advertises a default route but the VPC lacks a specific route for the instance subnet, causing asymmetric routing or blackholing.
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.
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: Confirm that the subnet routes for the instance IP ranges are present in the VPC — Option B is correct because for on-premises traffic to reach Compute Engine instances over Dedicated Interconnect, the VPC must have a subnet route (either automatically created or custom static/dynamic route) that matches the instance IP ranges. Without this route, packets from the on-premises network will be dropped by the VPC router, even if the VLAN attachment is reachable.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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