- A
The VPC subnets in us-central1 are using smaller prefixes that are not being summarized by the Cloud Router.
Why wrong: Route advertisement is not about summarization; Cloud Router advertises all subnets in its region.
- B
The on-premises London routers are not receiving routes for the VPC subnets in us-central1, so they send traffic via the internet.
Cloud Router only sends routes for subnets in its own region to its BGP peers; global routing mode does not change this.
- C
The Cloud Router in europe-west1 is prepending AS path for routes coming from us-central1, making them less preferred.
Why wrong: The London routers do not receive those routes at all.
- D
The on-premises New York routers are setting a lower MED for routes to us-central1, causing London to deprefer them.
Why wrong: MED is per-AS and does not affect London's choice if they don't see the routes.
Quick Answer
The answer is that the on-premises London routers are not receiving routes for the VPC subnets in us-central1, causing traffic to fall back to the internet. This occurs because, while the VPC is in global dynamic routing mode and the Cloud Router in europe-west1 learns all VPC subnets, it must explicitly advertise those us-central1 prefixes to the London peers via BGP. If those advertisements are missing—perhaps due to a missing route advertisement configuration or a route filter—London has no path over the Dedicated Interconnect and defaults to the internet. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how Cloud Routers handle route propagation across regions with Dedicated Interconnect, a common trap where engineers assume global routing mode automatically shares routes with on-premises. The key insight is that Cloud Routers only advertise routes they are explicitly configured to share with BGP peers, even if they learn them from the VPC. Memory tip: “Global mode learns, but BGP advertises—if you don’t tell it, the traffic tells you it’s going the wrong way.”
PCNE Implementing hybrid interconnectivity Practice Question
This PCNE practice question tests your understanding of implementing hybrid interconnectivity. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 multinational company is migrating workloads to Google Cloud and requires a hybrid connectivity solution between their on-premises data centers in New York and London and Google Cloud regions us-central1 and europe-west1. Each data center has a pair of redundant border routers. The network team has set up a Dedicated Interconnect connection in each Google Cloud region, with two VLAN attachments per region (total 4 attachments). Each VLAN attachment is associated with a separate Cloud Router. The Cloud Routers in us-central1 are configured with BGP sessions to the on-premises routers in New York, and the Cloud Routers in europe-west1 peer with the London routers. The VPC is in 'global' dynamic routing mode. After deployment, traffic from on-premises London to Google Cloud in us-central1 takes a suboptimal path: it goes from London to us-central1 via the internet instead of using the Dedicated Interconnect in europe-west1 and then internal Google Cloud backbone. All BGP sessions are up, and routes are being exchanged. The on-premises routers are advertising all their subnets. The Cloud Routers are learning the on-premises prefixes. What is the most likely cause of this suboptimal routing?
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 choices
Why each option matters
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 London routers are not receiving routes for the VPC subnets in us-central1, so they send traffic via the internet.
Option B is correct because the on-premises London routers are not receiving routes for the VPC subnets in us-central1. Since the Cloud Routers in europe-west1 only peer with London, they must advertise the us-central1 VPC subnets to London via BGP. If those routes are missing, London has no path via the Dedicated Interconnect and falls back to the internet. The VPC is in global dynamic routing mode, so Cloud Routers in both regions learn all VPC subnets, but the europe-west1 Cloud Router must explicitly advertise them to London.
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 VPC subnets in us-central1 are using smaller prefixes that are not being summarized by the Cloud Router.
Why it's wrong here
Route advertisement is not about summarization; Cloud Router advertises all subnets in its region.
- ✓
The on-premises London routers are not receiving routes for the VPC subnets in us-central1, so they send traffic via the internet.
Why this is correct
Cloud Router only sends routes for subnets in its own region to its BGP peers; global routing mode does not change this.
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.
- ✗
The Cloud Router in europe-west1 is prepending AS path for routes coming from us-central1, making them less preferred.
Why it's wrong here
The London routers do not receive those routes at all.
- ✗
The on-premises New York routers are setting a lower MED for routes to us-central1, causing London to deprefer them.
Why it's wrong here
MED is per-AS and does not affect London's choice if they don't see the routes.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume global dynamic routing automatically ensures all Cloud Routers advertise all VPC subnets to all on-premises peers, but in reality, each Cloud Router must be explicitly configured to advertise the prefixes it learns from the VPC to its BGP neighbors.
Detailed technical explanation
How to think about this question
In a global dynamic routing mode VPC, Cloud Routers in all regions learn all VPC subnets via the VPC's internal routing. However, each Cloud Router only advertises routes to its BGP peers if the routes are present in its routing table and the BGP session is configured to advertise them. If the europe-west1 Cloud Router is not configured to advertise the us-central1 subnets (e.g., due to missing route advertisement or a prefix-list filter), London routers never receive those prefixes and must use a default route or internet path. This is a common misconfiguration when using multiple Cloud Routers with selective advertisement.
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 — study guide chapter
Learn the concepts, then practise the questions
- →
Implementing hybrid interconnectivity practice questions
Targeted practice on this topic area only
- →
All PCNE questions
497 questions across all exam domains
- →
Google Professional Cloud Network Engineer study guide
Full concept coverage aligned to exam objectives
- →
PCNE practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related PCNE practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Designing, planning, and prototyping a GCP network practice questions
Practise PCNE questions linked to Designing, planning, and prototyping a GCP network.
Implementing hybrid interconnectivity practice questions
Practise PCNE questions linked to Implementing hybrid interconnectivity.
Configuring network services practice questions
Practise PCNE questions linked to Configuring network services.
Implementing network security practice questions
Practise PCNE questions linked to Implementing network security.
Implementing a Virtual Private Cloud practice questions
Practise PCNE questions linked to Implementing a Virtual Private Cloud.
PCNE fundamentals practice questions
Practise PCNE questions linked to PCNE fundamentals.
PCNE scenario practice questions
Practise PCNE questions linked to PCNE scenario.
PCNE troubleshooting practice questions
Practise PCNE questions linked to PCNE troubleshooting.
Practice this exam
Start a free PCNE practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this PCNE question test?
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: The on-premises London routers are not receiving routes for the VPC subnets in us-central1, so they send traffic via the internet. — Option B is correct because the on-premises London routers are not receiving routes for the VPC subnets in us-central1. Since the Cloud Routers in europe-west1 only peer with London, they must advertise the us-central1 VPC subnets to London via BGP. If those routes are missing, London has no path via the Dedicated Interconnect and falls back to the internet. The VPC is in global dynamic routing mode, so Cloud Routers in both regions learn all VPC subnets, but the europe-west1 Cloud Router must explicitly advertise them to London.
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 →
Same concept, more angles
2 more ways this is tested on PCNE
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An organization is using Cloud VPN with dynamic routing (BGP) to connect their on-premises network to Google Cloud. They notice that traffic from Google Cloud to on-premises is not using the VPN tunnel but instead going through the internet. They have verified that the VPN tunnel is up and BGP sessions are established. Which configuration issue is most likely causing this behavior?
hard- A.The Cloud Router BGP IP address is misconfigured
- B.The pre-shared key for the VPN tunnel is mismatched
- C.The on-premises BGP ASN is incorrect
- ✓ D.The on-premises router is not advertising the on-premises CIDR via BGP
Why D: Option D is correct because if the on-premises router does not advertise the on-premises CIDR via BGP, the Cloud Router will not have a route to that network in its routing table. Even though the VPN tunnel is up and BGP sessions are established, without the specific prefix being advertised, Google Cloud will use its default route (0.0.0.0/0) to send traffic to on-premises over the internet instead of through the VPN tunnel.
Variation 2. An organization has established a Dedicated Interconnect to Google Cloud. They can ping instances in a VPC subnet but cannot connect to a service running on a different subnet within the same VPC. What is the most likely cause?
medium- A.The MTU is too small.
- B.The firewall rules are blocking the traffic.
- C.The VLAN attachment is down.
- ✓ D.The BGP routes for that subnet are not advertised.
Why D: The Dedicated Interconnect is up and the organization can ping instances in one subnet, which confirms that Layer 2 and basic Layer 3 connectivity (via the VLAN attachment) are working. However, the inability to reach a service on a different subnet within the same VPC indicates that the on-premises router does not have a route to that specific subnet. This is most likely because the BGP session is not advertising the prefix for that subnet, or the on-premises router is not receiving the route via BGP. Without the route, traffic from on-premises to that subnet will be dropped.
Keep practising
More PCNE practice questions
- An organization is migrating to Google Cloud and requires connectivity between their on-premises network and VPC. They p…
- A company is migrating on-premises DNS to Google Cloud. They have a hybrid network using Cloud VPN and want to resolve o…
- A network engineer is configuring a Cloud Router for BGP peering with an on-premises router over a VPN tunnel. The on-pr…
- A company uses Cloud NAT to allow private instances to reach the internet. They notice that egress traffic from Compute…
- Match each VPC networking concept to its definition.
- Drag and drop the steps to troubleshoot a VPN tunnel that is not passing traffic into the correct order.
Last reviewed: Jun 25, 2026
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.