The answer is to verify BGP timers and hold time settings, as these directly control how long a Cloud Router waits before declaring a peer unreachable, and misconfigured timers are a primary cause of BGP session flapping. When troubleshooting bgp flapping on a cloud router, an MTU mismatch on the VPN tunnel or interconnect is often the hidden culprit—if the MTU is too low, large BGP update packets get silently dropped, causing the hold timer to expire and the session to reset repeatedly. On the Google Professional Cloud Network Engineer exam, this question tests your ability to distinguish between network-layer issues (like MTU) and configuration-layer issues (like timer mismatches), and a common trap is to immediately blame routing policies or firewall rules. Remember the memory tip: “MTU drops, BGP stops”—always check the path’s maximum transmission unit before adjusting timers, because even perfect timer settings won’t fix a silent packet discard.
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 Cloud Router BGP session is flapping. Which TWO actions are appropriate troubleshooting steps?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the MTU size on the VPN tunnel or interconnect.
Option A is correct because an MTU mismatch on the VPN tunnel or interconnect can cause packet fragmentation or drops, leading to BGP session flapping. Cloud Router uses Google's internal infrastructure, and if the MTU is set too low, large BGP update packets may be silently discarded, triggering hold timer expiry and session resets. Checking and adjusting the MTU ensures that BGP packets can traverse the path without fragmentation.
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.
✓
Check the MTU size on the VPN tunnel or interconnect.
Why this is correct
MTU mismatches can cause packet loss and BGP session drops.
Related concept
Read the scenario before looking for a memorised answer.
✗
Delete and recreate the VLAN attachment.
Why it's wrong here
That is a destructive step; better to check configurations first.
✗
Revert all BGP routes to static routes.
Why it's wrong here
This is a drastic measure and not a troubleshooting step.
✓
Verify BGP timers and hold time settings.
Why this is correct
Incorrect timers can cause premature session termination.
Related concept
Read the scenario before looking for a memorised answer.
✗
Increase the bandwidth of the interconnect.
Why it's wrong here
Bandwidth does not cause flapping; flapping is usually due to configuration or link stability.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that increasing bandwidth or recreating attachments resolves BGP flapping, when in fact the issue is typically related to MTU mismatches or BGP timer misconfigurations that affect session stability.
Detailed technical explanation
How to think about this question
BGP session flapping often results from mismatched hold timers or MTU issues that cause keepalive packets to be lost. In Google Cloud, Cloud Router uses BGP-4 (RFC 4271) and relies on TCP port 179; an MTU mismatch can cause TCP segments to be fragmented or dropped, especially when combined with IPsec overhead on VPN tunnels. A real-world scenario involves a VPN tunnel with an MTU of 1400 bytes, where BGP updates exceeding this size are silently dropped, causing the hold timer to expire and the session to reset.
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: Check the MTU size on the VPN tunnel or interconnect. — Option A is correct because an MTU mismatch on the VPN tunnel or interconnect can cause packet fragmentation or drops, leading to BGP session flapping. Cloud Router uses Google's internal infrastructure, and if the MTU is set too low, large BGP update packets may be silently discarded, triggering hold timer expiry and session resets. Checking and adjusting the MTU ensures that BGP packets can traverse the path without fragmentation.
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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