The answer is that the on-premises router is not advertising the on-premises prefix to GCP via BGP. This is the most likely cause because BGP prefix advertisement for return traffic on Interconnect is the mechanism that tells Google Cloud which networks exist on the on-premises side. Without this advertisement, GCP has no route in its routing tables for the on-premises subnet, so when it tries to send return traffic, the packets have no valid next hop and are dropped by the on-premises firewall as unroutable. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of asymmetric routing and the bidirectional nature of BGP sessions over Dedicated Interconnect. A common trap is assuming that because traffic flows one way, the route is automatically known in the reverse direction. Remember: BGP is a two-way street—if you don’t advertise your prefix, GCP cannot send traffic back. Memory tip: “No advertise, no return.”
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 has a Dedicated Interconnect connection from their on-premises data center to GCP. They have set up BGP sessions over VLAN attachments to peer with their VPC. Traffic from on-premises to GCP works, but return traffic from GCP to on-premises is dropped at the on-premises firewall. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The on-premises router is not advertising the on-premises prefix to GCP via BGP.
The most likely cause is that the on-premises router is not advertising the on-premises prefix to GCP via BGP. For return traffic from GCP to be routed correctly, GCP must learn the on-premises network prefix through a BGP advertisement from the on-premises router. Without this advertisement, GCP has no route to send traffic back, causing the firewall to drop the packets as they have no valid path.
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 VLAN attachment is in a different region than the GCP resources.
Why it's wrong here
Region mismatch does not cause packet drops; it affects latency.
✗
A GCP firewall rule is blocking the return traffic.
The MTU size is inconsistent between the on-premises router and GCP.
Why it's wrong here
MTU mismatch might cause fragmentation but not specific return traffic drops.
✓
The on-premises router is not advertising the on-premises prefix to GCP via BGP.
Why this is correct
If the on-premises prefix is not advertised, GCP routes return traffic through the internet, causing asymmetric routing and firewall drops.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that return traffic issues are caused by GCP firewall rules or MTU mismatches, when the actual root cause is a missing or incorrect BGP prefix advertisement from the on-premises side.
Detailed technical explanation
How to think about this question
In BGP over Dedicated Interconnect, the on-premises router must advertise the exact prefixes it wants GCP to route back to it. GCP installs these as static routes in the VPC's routing table only if the BGP session is established and the prefix is accepted. If the on-premises router fails to advertise the prefix (e.g., due to a missing network statement in the BGP configuration or a route-map filtering the prefix), GCP will not have a route for the return traffic, causing it to be dropped at the on-premises firewall as it arrives without a matching forwarding entry.
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.
Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The on-premises router is not advertising the on-premises prefix to GCP via BGP. — The most likely cause is that the on-premises router is not advertising the on-premises prefix to GCP via BGP. For return traffic from GCP to be routed correctly, GCP must learn the on-premises network prefix through a BGP advertisement from the on-premises router. Without this advertisement, GCP has no route to send traffic back, causing the firewall to drop the packets as they have no valid path.
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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Question Discussion
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