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.
Your company has a Dedicated Interconnect with two VLAN attachments (vlan-attachment-a and vlan-attachment-b) connected to two different Edge Availability Domains (EADs) in the us-central1 region. Both attachments are associated with a single Cloud Router named 'cr-us-central1'. On-premises, you have two routers (rtr-a and rtr-b) each connected to one VLAN attachment via BGP. The Cloud Router has four BGP sessions: rtr-a (vlan-a), rtr-a (vlan-b), rtr-b (vlan-a), rtr-b (vlan-b) — a full mesh for redundancy. All sessions are established and routes are exchanged. Recently, you added a new on-premises subnet 192.168.100.0/24 and advertised it via BGP from both on-premises routers. However, Google Cloud instances in the VPC cannot reach this subnet. Other on-premises subnets (e.g., 10.0.0.0/8) are reachable. The Cloud Router route table for 'cr-us-central1' shows multiple entries for 192.168.100.0/24, each with different next hops but all with status 'stacked'. There are no BGP route filters configured. What is the most likely cause of the issue?
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 new subnet 192.168.100.0/24 overlaps with an existing VPC subnet.
When a BGP-learned route overlaps with an existing VPC subnet, Google Cloud treats it as a conflict and marks the route as 'stacked' (i.e., not active). The Cloud Router can learn the route, but it will not be installed in the VPC's effective routes because the VPC subnet prefix takes precedence. Since 192.168.100.0/24 is a private IP range that could easily overlap with a subnet in the VPC, this is the most likely cause.
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 Cloud Router has reached its maximum route limit.
Why it's wrong here
Route limit would typically result in a 'limit exceeded' error, not stacked status.
✗
The BGP routes have incorrect MED values causing a conflict.
Why it's wrong here
MED values affect path selection but do not cause stacked status.
✓
The new subnet 192.168.100.0/24 overlaps with an existing VPC subnet.
Why this is correct
When a learned route overlaps with a VPC subnet, the route is stacked and not used to avoid conflict.
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 on-premises routers are not advertising the subnet with a required community tag.
Why it's wrong here
Without route filters, community tags are not required for route acceptance.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume 'stacked' routes indicate a BGP peering or route advertisement issue, when in fact it is a route conflict caused by overlapping prefixes with existing VPC subnets.
Detailed technical explanation
How to think about this question
In Google Cloud, VPC subnets are automatically created with system-generated routes that have a higher priority than dynamically learned routes. When a BGP route overlaps with a VPC subnet, the VPC subnet route takes precedence, and the BGP route is marked as 'stacked' (inactive) in the Cloud Router's route table. This behavior is documented in Google Cloud's route priority model, where system-generated routes have an implicit priority of 0 (highest), while custom and dynamically learned routes have lower priority.
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 — 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 new subnet 192.168.100.0/24 overlaps with an existing VPC subnet. — When a BGP-learned route overlaps with an existing VPC subnet, Google Cloud treats it as a conflict and marks the route as 'stacked' (i.e., not active). The Cloud Router can learn the route, but it will not be installed in the VPC's effective routes because the VPC subnet prefix takes precedence. Since 192.168.100.0/24 is a private IP range that could easily overlap with a subnet in the VPC, this is the most likely cause.
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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