The answer is that Bidirectional Forwarding Detection (BFD) is not enabled on the BGP session. Without BFD, the BGP session relies on its default keepalive timers—typically 60 seconds—to detect link failures, which is far too slow to catch transient issues like micro-bursts or brief packet loss. This delay causes routes to be intermittently withdrawn and re-advertised, leading to dropped traffic even though the BGP state shows 'Established'. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how BFD provides sub-second failure detection (default 300ms) to ensure consistent BGP route exchange over Dedicated Interconnect. A common trap is assuming an 'Established' BGP session guarantees stable routing; in reality, BFD is the missing piece for fast convergence. Memory tip: think "BFD = BGP Failure Detection"—without it, your routes are blind to quick hiccups.
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 is setting up a Dedicated Interconnect connection between their on-premises network and Google Cloud. They have configured a VLAN attachment and assigned a Cloud Router with BGP sessions. They notice that traffic is being dropped intermittently. The BGP session status shows 'Established' but routes are not being exchanged consistently. 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
✓
Bidirectional Forwarding Detection (BFD) is not enabled on the BGP session
When BFD is not enabled on a BGP session, the BGP keepalive timers (typically 60 seconds) are used to detect failures, which can cause intermittent traffic drops because BGP does not detect link failures quickly enough. With BFD enabled (default interval of 300ms), failures are detected in sub-seconds, preventing route flapping and ensuring consistent route exchange. The 'Established' BGP state with inconsistent route exchange is a classic symptom of BFD being absent, as routes may be withdrawn and re-advertised due to transient link issues that BGP alone cannot react to fast enough.
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.
✓
Bidirectional Forwarding Detection (BFD) is not enabled on the BGP session
Why this is correct
Without BFD, BGP may remain Established while the data plane is down, causing dropped traffic.
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 firewall is blocking BGP port 179
Why it's wrong here
Blocking BGP would prevent the session from establishing, not cause intermittent issues.
✗
The Cloud Router has reached the maximum number of routes
Why it's wrong here
Route limits cause rejection of new routes, not intermittent drops of existing traffic.
✗
The MTU on the VLAN attachment is set too low
Why it's wrong here
MTU issues cause packet drops or fragmentation, not BGP route exchange inconsistency.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a BGP session in 'Established' state guarantees stable route exchange, but the trap here is that BFD is required for fast failure detection in cloud interconnect scenarios, and its absence causes intermittent route flapping that does not break the BGP session itself.
Detailed technical explanation
How to think about this question
BFD operates independently of BGP, using a lightweight UDP-based echo mechanism (RFC 5880) to detect forwarding path failures in under one second. In Google Cloud, BFD is configured on the Cloud Router BGP session with a default detection multiplier of 3 and a minimum interval of 300ms, meaning a failure is declared after 900ms of missed packets. Without BFD, BGP relies on its hold timer (default 180 seconds), which is far too slow for detecting intermittent link issues in hybrid interconnectivity scenarios, leading to route withdrawal and re-advertisement that causes traffic to drop.
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.
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: Bidirectional Forwarding Detection (BFD) is not enabled on the BGP session — When BFD is not enabled on a BGP session, the BGP keepalive timers (typically 60 seconds) are used to detect failures, which can cause intermittent traffic drops because BGP does not detect link failures quickly enough. With BFD enabled (default interval of 300ms), failures are detected in sub-seconds, preventing route flapping and ensuring consistent route exchange. The 'Established' BGP state with inconsistent route exchange is a classic symptom of BFD being absent, as routes may be withdrawn and re-advertised due to transient link issues that BGP alone cannot react to fast enough.
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
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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. A company has a Dedicated Interconnect connection between their on-premises data center and Google Cloud. They are experiencing intermittent connectivity issues on a specific VLAN attachment. The VLAN attachment is configured with a single Cloud Router and BGP sessions are established. Which two steps should they take to troubleshoot the issue? (Choose two.)
medium
✓ A.Verify the BGP session status on the Cloud Router.
B.Check the MTU configuration on the on-premises router.
C.Verify the physical interconnect location and cable connections.
✓ D.Review the VLAN attachment status in the Google Cloud Console.
E.Review the Cloud NAT configuration for the VPC network.
Why A: Option A is correct because BGP session status on the Cloud Router directly indicates whether the routing protocol is functioning correctly between the on-premises router and Google Cloud. Intermittent connectivity on a VLAN attachment often stems from BGP flapping or session drops, which can be verified by checking the BGP state (e.g., Established, Idle, or Active) and associated error counters in the Google Cloud Console or via gcloud commands.
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Question Discussion
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