- A
The tunnel is in a failed state
Why wrong: If tunnel is down both BGP sessions would typically fail; but the question says one session works, so tunnel is likely up.
- B
The on-premises router has incorrect ASN configured for that BGP session
Incorrect ASN (Autonomous System Number) on one BGP session could prevent route exchange while the other session works.
- C
The on-premises router has a firewall blocking BGP updates only on one IP address
Why wrong: Firewall blocking would affect both directions but since one session works, it's less likely to be selective BGP blocking.
- D
The Cloud Router is set to advertisement mode 'Custom' and does not advertise all subnets
Why wrong: Advertisement mode would affect routes advertised, not the ability to receive routes.
Quick Answer
The answer is an incorrect ASN configured for the BGP session on the on-premises router. This is the most likely cause of a BGP ASN mismatch route not received scenario because Google Cloud’s HA VPN requires each Cloud Router instance to use the same peer ASN as the on-premises side; if one session has a mismatched ASN, BGP will reject the routes or fail to establish, while the correctly configured session works fine. On the Google Professional Cloud Network Engineer exam, this tests your understanding of BGP session configuration within HA VPN, often appearing as a trap where you might suspect IP addressing or firewall rules instead. The key insight is that each Cloud Router instance has a unique BGP IP, but the peer ASN must be identical across both sessions. Memory tip: “Same ASN, different IPs” — the ASN is the identity, the IP is just the address.
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.
A company has set up an HA VPN tunnel between their on-premises router and a Cloud Router in Google Cloud. The on-premises router establishes BGP sessions to both Cloud Router instances, but the routes learned from one Cloud Router instance are not being received. The other instance works fine. 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 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
The on-premises router has incorrect ASN configured for that BGP session
The most likely cause is that the on-premises router has an incorrect ASN configured for the BGP session with the failing Cloud Router instance. In Google Cloud HA VPN, each Cloud Router instance uses a unique BGP IP address but both must use the same peer ASN as configured on the on-premises side. If the ASN mismatch occurs, BGP will not establish or will reject routes, while the other session with the correct ASN works fine.
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 tunnel is in a failed state
Why it's wrong here
If tunnel is down both BGP sessions would typically fail; but the question says one session works, so tunnel is likely up.
- ✓
The on-premises router has incorrect ASN configured for that BGP session
Why this is correct
Incorrect ASN (Autonomous System Number) on one BGP session could prevent route exchange while the other session works.
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 has a firewall blocking BGP updates only on one IP address
- ✗
The Cloud Router is set to advertisement mode 'Custom' and does not advertise all subnets
Why it's wrong here
Advertisement mode would affect routes advertised, not the ability to receive routes.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the concept that BGP session establishment and route exchange are separate phases, and an ASN mismatch specifically prevents route exchange even if the tunnel is up, leading candidates to incorrectly blame tunnel failure or firewall rules.
Detailed technical explanation
How to think about this question
BGP sessions require matching ASNs on both ends; a mismatch causes the OPEN message to be rejected with a NOTIFICATION error (code 2, subcode 2). In Google Cloud HA VPN, each Cloud Router instance has its own BGP IP address but shares the same peer ASN configuration, so an ASN misconfiguration on the on-premises router for one session will prevent route exchange for that specific session only. This is a common misconfiguration when manually configuring multiple BGP sessions with different peer IPs but forgetting to update the ASN.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Implementing hybrid interconnectivity — study guide chapter
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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: The on-premises router has incorrect ASN configured for that BGP session — The most likely cause is that the on-premises router has an incorrect ASN configured for the BGP session with the failing Cloud Router instance. In Google Cloud HA VPN, each Cloud Router instance uses a unique BGP IP address but both must use the same peer ASN as configured on the on-premises side. If the ASN mismatch occurs, BGP will not establish or will reject routes, while the other session with the correct ASN works fine.
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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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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