The answer is that the VLAN ID is already in use on a different attachment on the same interconnect. This is the most likely cause because a Dedicated Interconnect VLAN attachment entering a DEFECTIVE state while the underlying interconnect connection remains ACTIVE points to a Layer 2 configuration conflict rather than a physical link issue. VLAN IDs must be unique per interconnect, so if the specified VLAN ID is already allocated to another attachment, the new attachment cannot establish proper Layer 2 connectivity, resulting in the DEFECTIVE state. On the Google Professional Cloud Network Engineer exam, this scenario tests your ability to differentiate between interconnect-level and VLAN-level faults—a common trap is assuming the physical link is down when the interconnect shows ACTIVE. Remember the memory tip: “Active link, defective VLAN? Check for a VLAN ID collision.”
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.
An engineer runs the command above to check the status of a Dedicated Interconnect VLAN attachment. The state shows DEFECTIVE. The associated interconnect connection is in ACTIVE state. 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 VLAN ID is already in use on a different attachment on the same interconnect
A Dedicated Interconnect VLAN attachment showing DEFECTIVE state while the interconnect connection itself is ACTIVE indicates a configuration conflict at the VLAN level. The most common cause is that the VLAN ID specified for this attachment is already allocated to another VLAN attachment on the same interconnect, as VLAN IDs must be unique per interconnect. This conflict prevents the attachment from establishing proper Layer 2 connectivity, resulting in a DEFECTIVE state.
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 ID is already in use on a different attachment on the same interconnect
Why this is correct
Duplicate VLANIDs cause the attachment to be DEFECTIVE.
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 is not configured with a BGP session for this attachment
Why it's wrong here
Missing BGP session would not cause DEFECTIVE state; attachment would still be operational.
✗
The maximum number of VLAN attachments for this interconnect has been exceeded
Why it's wrong here
Exceeding attachment limit would prevent creation, not cause DEFECTIVE state after creation.
✗
The data center power is down
Why it's wrong here
If the interconnect itself is ACTIVE, data center power is likely fine.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between Layer 2 attachment health and Layer 3 BGP session status — candidates mistakenly assume a BGP misconfiguration causes the attachment to be DEFECTIVE, but the attachment state is independent of BGP and reflects only the VLAN-level connectivity.
Detailed technical explanation
How to think about this question
In Google Cloud, a Dedicated Interconnect VLAN attachment represents a 802.1q VLAN trunked over the physical interconnect. Each VLAN attachment must use a unique VLAN ID (1-4094) per interconnect to avoid collisions in the Layer 2 domain. When a duplicate VLAN ID is detected, the attachment enters a DEFECTIVE state because the underlying switch or router cannot properly isolate traffic for that VLAN. This is verified by checking the attachment's detailed status via `gcloud compute interconnects attachments describe` which will show a `state` of `DEFECTIVE` and often a `detailedState` of `VLAN_ID_CONFLICT`.
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 VLAN ID is already in use on a different attachment on the same interconnect — A Dedicated Interconnect VLAN attachment showing DEFECTIVE state while the interconnect connection itself is ACTIVE indicates a configuration conflict at the VLAN level. The most common cause is that the VLAN ID specified for this attachment is already allocated to another VLAN attachment on the same interconnect, as VLAN IDs must be unique per interconnect. This conflict prevents the attachment from establishing proper Layer 2 connectivity, resulting in a DEFECTIVE state.
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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