The answer is that the on-premises router’s local route for 10.0.0.0/8 overrides the BGP-learned /24 routes due to administrative distance. This happens because the local route, whether directly connected or static, carries a much lower administrative distance (typically 0 or 1) than eBGP’s default of 20, so the router prefers the local, less-specific prefix over the more specific BGP routes. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that BGP route selection is not solely about prefix length—administrative distance takes precedence when comparing routes from different sources. A common trap is assuming the more specific /24 routes will automatically win, but the local /8 route’s lower AD causes traffic to stay on-premises instead of traversing the Dedicated Interconnect. Remember the memory tip: “AD first, then length”—administrative distance is the gatekeeper before prefix specificity comes into play.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. 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 has an on-premises data center connected to Google Cloud via a Dedicated Interconnect using VLAN attachments. They have set up a Cloud Router with BGP to exchange routes. The on-premises network advertises a prefix 10.0.0.0/8, and Google Cloud advertises the VPC's subnet ranges (10.0.0.0/24 and 10.0.1.0/24). After configuration, on-premises hosts cannot reach the Google Cloud instances in those subnets. The engineer checks the BGP session status and it is established. The Cloud Router shows that the on-premises prefix is learned, and the on-premises router shows that the specific /24 prefixes are received. However, traffic from on-premises to the Google Cloud subnets is not working. 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 has a local route for 10.0.0.0/8 that overrides the BGP-learned /24 routes.
The most likely cause is that the on-premises router has a local route for the larger prefix 10.0.0.0/8, which is more specific than the BGP-learned /24 routes. Since the local route has a lower administrative distance (typically 0 for directly connected or 1 for static) compared to BGP's administrative distance of 20 for eBGP, the on-premises router prefers the local route and forwards traffic locally instead of sending it over the Dedicated Interconnect to Google Cloud. This results in traffic not reaching the VPC subnets.
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 on-premises router has a local route for 10.0.0.0/8 that overrides the BGP-learned /24 routes.
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The VPN connection needs to be configured for the Interconnect.
Why it's wrong here
The Interconnect is already configured; VPN is separate.
✗
The Google Cloud VPC has a custom static route that overrides the dynamically learned route.
Why it's wrong here
The problem is on the on-premises side, not Google Cloud.
✗
The on-premises firewall is blocking the traffic.
Why it's wrong here
While possible, the routing issue is more likely given the routing table situation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the concept that administrative distance overrides prefix length in route selection when comparing routes from different sources, leading candidates to incorrectly assume that more specific prefixes are always preferred regardless of administrative distance.
Trap categories for this question
Scenario analysis trap
While possible, the routing issue is more likely given the routing table situation.
Detailed technical explanation
How to think about this question
BGP route selection on Cisco routers uses administrative distance to determine the best path when multiple routes to the same destination exist. A local route for 10.0.0.0/8 (e.g., connected or static) has an administrative distance of 0 or 1, while eBGP-learned routes have an administrative distance of 20. Even though the /24 prefixes are more specific, the router's longest-prefix match rule applies only after the best route is selected; if the local route is for the exact prefix or a less specific one, it can still be preferred if it has a lower administrative distance. In this scenario, the on-premises router sees the /24 routes via BGP but prefers the local /8 route because of administrative distance, causing traffic to be forwarded locally.
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 network security — This question tests Implementing network security — 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 has a local route for 10.0.0.0/8 that overrides the BGP-learned /24 routes. — The most likely cause is that the on-premises router has a local route for the larger prefix 10.0.0.0/8, which is more specific than the BGP-learned /24 routes. Since the local route has a lower administrative distance (typically 0 for directly connected or 1 for static) compared to BGP's administrative distance of 20 for eBGP, the on-premises router prefers the local route and forwards traffic locally instead of sending it over the Dedicated Interconnect to Google Cloud. This results in traffic not reaching the VPC subnets.
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.
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.