The answer is asymmetric routing caused by a higher local preference on the on-premises BGP speaker. When the on-premises router advertises routes with a local preference of 200 versus the Cloud Router’s default of 100, Google Cloud prefers the on-premises path for return traffic, while forward traffic from on-premises may still take the Cloud VPN tunnel. This mismatch creates asymmetric routing, where stateful firewalls—such as on-premises firewalls or Google Cloud firewall rules—drop packets that don’t match an existing session, leading to intermittent drops. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of BGP path selection and how local preference influences traffic flow in hybrid connectivity. A common trap is assuming the issue is a tunnel flap or route propagation delay, but the real culprit is the firewall state table. Memory tip: “Higher local pref pulls return traffic home, but forward traffic doesn’t follow—firewalls drop the mismatch.”
PCNE Implementing hybrid interconnectivity Practice Question
This PCNE practice question tests your understanding of implementing hybrid interconnectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 Hybrid Connectivity setup using Cloud VPN with dynamic routing (BGP). They notice that traffic from their on-premises network to Google Cloud is intermittently dropping. The on-premises BGP speaker is sending routes with a higher local preference (200) than the Google Cloud router (default 100). What is the most likely cause of the intermittent drops?
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
✓
Asymmetric routing is causing traffic to be dropped by stateful firewalls
The on-premises BGP speaker is sending routes with a higher local preference (200) than the default on Cloud Router (100). This makes the on-premises route preferred for return traffic from Google Cloud, but the forward traffic from on-premises may still use the Cloud VPN tunnel. This asymmetry causes stateful firewalls (e.g., on-premises firewall or Google Cloud firewall) to drop packets that do not match an existing session, leading to intermittent drops.
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.
✗
AS path prepending is causing route flapping
Why it's wrong here
AS path prepending would affect path selection but not cause intermittent drops directly.
✓
Asymmetric routing is causing traffic to be dropped by stateful firewalls
Why this is correct
Higher local preference can cause asymmetric routing, leading to stateful 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.
✗
Cloud Router is not configured for ECMP
Why it's wrong here
ECMP is not related to local preference; Cloud Router supports ECMP by default.
✗
The BGP MED attribute is misconfigured
Why it's wrong here
MED is used for inbound traffic; local preference affects outbound traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that BGP attributes like local preference only affect inbound traffic, when in fact local preference influences outbound path selection from the router's perspective, and a mismatch between on-premises and cloud can cause asymmetric routing that stateful firewalls drop.
Detailed technical explanation
How to think about this question
Local preference is a well-known BGP attribute used to influence outbound traffic from an AS; a higher local preference is preferred. In this scenario, the on-premises router advertises a route with local preference 200 to Google Cloud, making that route preferred for return traffic from Google Cloud, while forward traffic from on-premises may still use the Cloud VPN tunnel (which has default local preference 100). This creates asymmetric routing, where stateful firewalls (e.g., on-premises firewall or Google Cloud firewall) track session state and drop packets that arrive on a different interface than the original session, causing intermittent drops. The issue is exacerbated if the firewall expects symmetric traffic flow.
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: Asymmetric routing is causing traffic to be dropped by stateful firewalls — The on-premises BGP speaker is sending routes with a higher local preference (200) than the default on Cloud Router (100). This makes the on-premises route preferred for return traffic from Google Cloud, but the forward traffic from on-premises may still use the Cloud VPN tunnel. This asymmetry causes stateful firewalls (e.g., on-premises firewall or Google Cloud firewall) to drop packets that do not match an existing session, leading to intermittent drops.
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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