The answer is that Cloud Router’s BGP is not configured for multipath, so it selects only one best path per prefix. By default, BGP evaluates attributes like MED, local preference, and AS-path length to choose a single optimal route; without enabling multipath (via the `maximum-paths` or `bgp multipath` command), Cloud Router will not distribute traffic across multiple VLAN attachments even when equal-cost paths exist. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how BGP path selection interacts with Dedicated Interconnect and VLAN attachments—a common trap is assuming that multiple BGP sessions automatically load-balance traffic. The key insight is that BGP multipath must be explicitly configured to allow ECMP across attachments. Memory tip: think “BGP picks one path by default; you must tell it to share the load.”
PCNE Practice Question: Designing, planning, and prototyping a GCP network
This PCNE practice question tests your understanding of designing, planning, and prototyping a gcp network. 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 organization is connecting their on-premises data center to GCP using Dedicated Interconnect with multiple VLAN attachments. They have configured Cloud Router with BGP sessions for each VLAN attachment. They notice that traffic from GCP to on-premises is not load-balanced across the attachments; instead, all traffic uses a single link. 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
✓
Cloud Router's BGP is not configured for multipath; it selects only one best path per prefix.
Cloud Router uses BGP to exchange routes with the on-premises router. By default, BGP selects only a single best path per prefix based on the lowest MED, weight, local preference, AS-path length, or IGP metric. Without BGP multipath enabled (e.g., 'maximum-paths' or 'bgp multipath' configuration), Cloud Router will not load-balance traffic across multiple VLAN attachments even if multiple equal-cost paths are available. This causes all traffic to egress through a single link.
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.
✓
Cloud Router's BGP is not configured for multipath; it selects only one best path per prefix.
Why this is correct
BGP multipath must be enabled to load-balance across multiple equal-cost paths.
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 router is not advertising the same routes over all BGP sessions.
Why it's wrong here
If routes are different, GCP would use the most specific; but the issue is load balancing, not route advertisement.
✗
The VLAN attachments have different bandwidths, causing BGP to prefer the higher bandwidth link.
Why it's wrong here
BGP does not consider bandwidth by default; it uses AS path, local preference, etc.
✗
The BGP keepalive timers are misconfigured, causing session flapping.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume BGP automatically load-balances across multiple equal-cost paths, but BGP requires explicit multipath configuration to enable ECMP; otherwise, it selects only one best path per prefix.
Detailed technical explanation
How to think about this question
BGP multipath (e.g., 'maximum-paths ebgp 2' or 'bgp multipath' in Cisco IOS) allows a router to install multiple equal-cost paths into the routing table for the same prefix, enabling ECMP load balancing. In Google Cloud, Cloud Router supports BGP multipath when multiple VLAN attachments advertise the same routes with identical BGP attributes (AS-path length, MED, local preference). Without this configuration, the router selects only the best path, and all traffic follows that single link, even if other links are idle.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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.
Designing, planning, and prototyping a GCP network — This question tests Designing, planning, and prototyping a GCP network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Cloud Router's BGP is not configured for multipath; it selects only one best path per prefix. — Cloud Router uses BGP to exchange routes with the on-premises router. By default, BGP selects only a single best path per prefix based on the lowest MED, weight, local preference, AS-path length, or IGP metric. Without BGP multipath enabled (e.g., 'maximum-paths' or 'bgp multipath' configuration), Cloud Router will not load-balance traffic across multiple VLAN attachments even if multiple equal-cost paths are available. This causes all traffic to egress through a single link.
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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