The answer is that the BGP session will fail or behave unpredictably. This occurs because BGP’s loop prevention mechanism, defined in RFC 4271 Section 9.3, requires each router in a peering session to have a unique ASN; when a router receives a route update containing its own ASN, it interprets that as a routing loop and drops the session or rejects the routes. On the Google Professional Cloud Network Engineer exam, this concept tests your understanding of BGP duplicate ASN behavior in hybrid cloud scenarios, where a private ASN like 64512 is often reused by mistake between on-premises routers and Cloud Routers. A common trap is assuming that private ASNs are interchangeable or that Cloud Router will automatically override the conflict, but the exam expects you to know that identical ASNs break the eBGP peering entirely. Memory tip: think of BGP’s ASN as a unique ID badge—if two guards at the same door flash the same badge, the system assumes one is an imposter and shuts the door.
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 on-premises router uses BGP ASN 64512. The Cloud Router is also configured with ASN 64512. When the BGP peering is established, what behavior is expected?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The BGP session will fail or behave unpredictably.
BGP requires each router in a peering session to have a unique ASN to properly enforce loop prevention and path selection. When both the on-premises router and Cloud Router use the same ASN 64512, the BGP session will fail or behave unpredictably because each router will see its own ASN in received updates, triggering the BGP loop detection mechanism (RFC 4271, Section 9.3) and causing the session to drop or routes to be rejected.
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 session works but routes are not exchanged.
Why it's wrong here
Without proper ASN uniqueness, routes may not be accepted.
✗
Cloud Router will automatically prepend its ASN to avoid conflict.
Why it's wrong here
Cloud Router does not automatically prepend; it advertises routes with its own ASN.
✓
The BGP session will fail or behave unpredictably.
Why this is correct
Same ASN on both sides causes BGP to reject the session (at least one side will see a loop).
Related concept
Read the scenario before looking for a memorised answer.
✗
The BGP session works normally since ASN 64512 is a private ASN.
Why it's wrong here
Private ASN still must be unique among peers.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that private ASNs (64512-65535) are exempt from BGP loop detection, but in reality, BGP treats all ASNs equally for loop prevention, and duplicate ASNs will cause the session to fail.
Detailed technical explanation
How to think about this question
Under the hood, when a BGP router receives an OPEN message containing the same ASN as its own, it sends a NOTIFICATION message with error code 'OPEN Message Error' and subcode 'Bad Peer AS', terminating the session. In real-world scenarios, this often occurs during migration or misconfiguration when both sides use the default private ASN (e.g., 64512) without coordination, leading to unexpected BGP session flapping or complete failure.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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 BGP session will fail or behave unpredictably. — BGP requires each router in a peering session to have a unique ASN to properly enforce loop prevention and path selection. When both the on-premises router and Cloud Router use the same ASN 64512, the BGP session will fail or behave unpredictably because each router will see its own ASN in received updates, triggering the BGP loop detection mechanism (RFC 4271, Section 9.3) and causing the session to drop or routes to be rejected.
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.
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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