- A
On-premises routers have mismatched BGP local preference values for routes received from Google Cloud
Different local preferences cause one path to be preferred for return traffic, while forward traffic may take the other path, leading to asymmetry.
- B
The on-premises routers are using the same AS number causing BGP loop prevention
Why wrong: Using the same AS number is invalid and would prevent BGP sessions from forming; question says sessions are established.
- C
VLAN attachments are configured with different MTU sizes
Why wrong: MTU mismatch can cause fragmentation issues but typically not asymmetrical routing; packets would be dropped rather than via different paths.
- D
Cloud Router is setting different BGP metric attributes for each VLAN attachment
Why wrong: Google Cloud Router does not assign different metrics to different attachments unless configured with custom route propagation; by default, all routes have the same metric.
Quick Answer
The answer is mismatched BGP local preference values on the on-premises routers for routes received from Google Cloud. This is correct because local preference is the first BGP attribute evaluated when selecting the best outbound path from an on-premises network; if R1 has a higher local preference for GCP routes than R2, all outbound traffic will prefer R1, while return traffic from GCP can arrive via either router based on GCP’s routing, creating an asymmetric routing Dedicated Interconnect local preference mismatch that causes packet drops. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of BGP path selection in dual-homed hybrid connectivity, often appearing as a trap where candidates focus on GCP-side routing instead of the on-premises side. A common memory tip is “local preference is the first tiebreaker outbound, not inbound”—think of it as the on-premises router’s “vote” for which link to use for sending traffic, and mismatched votes break symmetry.
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 financial institution is setting up Dedicated Interconnect with Google Cloud. They have two on-premises routers (R1 and R2) each connected to a separate Google Cloud router via VLAN attachments in two different zones (us-central1-a and us-central1-b). The on-premises routers are configured with BGP, and they advertise the corporate prefix 10.0.0.0/8. Google Cloud routers are configured with custom route advertisements. After provisioning, you notice that traffic from some on-premises subnets to GCP experiences asymmetrical routing, causing packet drops. You verify that both BGP sessions are established and that both Cloud Routers have received the 10.0.0.0/8 route. What is the most likely cause of the asymmetrical 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
On-premises routers have mismatched BGP local preference values for routes received from Google Cloud
Asymmetrical routing in a dual-homed Dedicated Interconnect setup occurs when on-premises routers have different BGP local preference values for routes received from Google Cloud. Local preference is the first BGP attribute considered when selecting the best path outbound from the on-premises network. If R1 has a higher local preference for the GCP routes than R2, all outbound traffic from on-premises will prefer R1, while return traffic from GCP may arrive via either router (depending on GCP's routing), causing a mismatch in traffic paths and packet drops.
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.
- ✓
On-premises routers have mismatched BGP local preference values for routes received from Google Cloud
Why this is correct
Different local preferences cause one path to be preferred for return traffic, while forward traffic may take the other path, leading to asymmetry.
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 using the same AS number causing BGP loop prevention
Why it's wrong here
Using the same AS number is invalid and would prevent BGP sessions from forming; question says sessions are established.
- ✗
VLAN attachments are configured with different MTU sizes
Why it's wrong here
MTU mismatch can cause fragmentation issues but typically not asymmetrical routing; packets would be dropped rather than via different paths.
- ✗
Cloud Router is setting different BGP metric attributes for each VLAN attachment
Why it's wrong here
Google Cloud Router does not assign different metrics to different attachments unless configured with custom route propagation; by default, all routes have the same metric.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between BGP attributes that influence inbound traffic (MED, AS-path prepend) versus outbound traffic (local preference), and the trap here is that candidates may incorrectly attribute asymmetrical routing to MED or MTU issues rather than recognizing that local preference mismatch on the on-premises side is the root cause of asymmetric outbound path selection.
Detailed technical explanation
How to think about this question
BGP local preference is an optional non-transitive attribute that is used to influence outbound traffic from an AS. In a dual-homed scenario, if one on-premises router has a higher local preference for GCP routes, all outbound traffic will egress through that router, while GCP's routing (using ECMP or BGP best path selection) may send return traffic via the other router, creating asymmetry. This is a common issue in active/passive or unequal-cost multi-pathing designs where local preference is not consistently configured across redundant BGP peers.
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.
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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: On-premises routers have mismatched BGP local preference values for routes received from Google Cloud — Asymmetrical routing in a dual-homed Dedicated Interconnect setup occurs when on-premises routers have different BGP local preference values for routes received from Google Cloud. Local preference is the first BGP attribute considered when selecting the best path outbound from the on-premises network. If R1 has a higher local preference for the GCP routes than R2, all outbound traffic from on-premises will prefer R1, while return traffic from GCP may arrive via either router (depending on GCP's routing), causing a mismatch in traffic paths and packet drops.
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
1 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. You are designing a hybrid network using Cloud VPN with dynamic routing (BGP) to connect multiple on-premises sites to Google Cloud. What is a best practice to avoid asymmetric routing when you have multiple VPN tunnels from different on-premises routers?
easy- A.Use static routes instead of BGP to have precise control over path selection
- B.Use a different BGP ASN for each on-premises router to ensure uniqueness
- ✓ C.Configure all on-premises routers with the same BGP ASN and enable ECMP on the Cloud Router
- D.Disable ECMP on the Cloud Router to avoid multipath issues
Why C: Option C is correct because using the same BGP ASN on all on-premises routers and enabling ECMP on the Cloud Router allows the Cloud Router to treat multiple BGP sessions as equal-cost paths. This prevents asymmetric routing by ensuring that return traffic can be load-balanced across any available tunnel, while the same ASN avoids BGP loop-prevention mechanisms that would otherwise reject routes from routers with different ASNs.
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Last reviewed: Jun 30, 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.
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