The answer is that both firewall rules and VPC subnet routes must be verified, as the root cause of dedicated interconnect traffic fails despite active state is almost always a missing ingress firewall rule or an absent subnet route. Even when a Dedicated Interconnect VLAN attachment shows ACTIVE and Cloud Router successfully exchanges routes with on-premises, the GCP VM remains unreachable because the VPC firewall must explicitly permit incoming traffic from the on-premises source range, and the subnet’s route must exist in the VPC routing table for the destination VM. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that BGP route exchange alone does not guarantee traffic flow—it only enables path discovery. A common trap is assuming an ACTIVE attachment and learned routes mean connectivity is complete, but without firewall and subnet route configuration, packets are silently dropped. Remember the memory tip: “Active link, learned routes, but no flow? Check the firewall and the route table—two keys unlock the door.”
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 Dedicated Interconnect VLAN attachment is in ACTIVE state. The Cloud Router has learned routes from on-premises, and the on-premises router has learned routes from GCP. However, traffic from on-premises to a GCP VM fails. 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
✓
Both A and C.
The correct answer is D because both firewall rules and VPC subnet routes must be in place for traffic to reach a GCP VM. Even if the VLAN attachment is ACTIVE and routes are exchanged, the VPC firewall must permit ingress from the on-premises range, and the VPC subnet routes must exist in the routing table for the VM's subnet. Without both, traffic will be dropped or not forwarded.
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.
✗
Check firewall rules on the GCP VPC for ingress from the on-premises range.
Why it's wrong here
This is one possible cause, but not the only one to check.
✗
Confirm that the Cloud Router is advertising the on-premises subnets back to the VPC.
Why it's wrong here
This is about outbound traffic; but issue is inbound.
✗
Verify that the VPC subnet routes exist in the routing table.
Why it's wrong here
Subnet routes exist by default; less likely to be missing.
✓
Both A and C.
Why this is correct
Both firewall rules and Cloud Router advertisement must be verified.
Clue confirmation
The clue word "first" 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
Google Cloud often tests the misconception that route exchange alone guarantees connectivity, but in GCP, both firewall rules and subnet routes are mandatory for traffic to reach a VM, even when the interconnect is ACTIVE and BGP sessions are established.
Detailed technical explanation
How to think about this question
In GCP, a Dedicated Interconnect VLAN attachment uses a Cloud Router to exchange BGP routes. For traffic from on-premises to reach a GCP VM, the VPC must have a subnet route for the VM's IP range (either a system-generated route or a custom route), and the VPC firewall must allow ingress from the on-premises source IP range. Even if BGP routes are learned, the VM will not receive traffic if the firewall denies it or if the subnet route is missing (e.g., if the VM is in a subnet that was deleted or not properly associated).
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
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.
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: Both A and C. — The correct answer is D because both firewall rules and VPC subnet routes must be in place for traffic to reach a GCP VM. Even if the VLAN attachment is ACTIVE and routes are exchanged, the VPC firewall must permit ingress from the on-premises range, and the VPC subnet routes must exist in the routing table for the VM's subnet. Without both, traffic will be dropped or not forwarded.
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.
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Question Discussion
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