The answer is a BGP keepalive or hold timer mismatch on the backup Cloud Router. This occurs because the backup router expects a keepalive message within a shorter interval than the BGP peer sends, causing the hold timer to expire and forcing the router to withdraw all learned routes—even though the TCP session remains established. The 30-second withdrawal window is a classic symptom, often matching a misconfigured hold timer set to one-third of the default 90 seconds. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of BGP timer mechanics within Cloud Router and Cloud VPN redundancy setups; a common trap is assuming a session drop is required for route withdrawal. Remember the memory tip: “Hold timer expires, routes expire—session stays, routes go away.”
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. 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 uses Cloud VPN tunnels to connect multiple sites to Google Cloud. They have a primary and a backup tunnel for redundancy, each with a different Cloud Router (both in the same region). BGP sessions are established on both routers. The network team notices that during a failover test, traffic fails over to the backup tunnel but then after 30 seconds, the backup tunnel traffic stops and does not recover until the primary tunnel comes back. The engineer finds that the backup Cloud Router is advertising the same routes as the primary, but the backup tunnel's BGP session shows that the routes are being withdrawn after 30 seconds. Additionally, the BGP session remains established. 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.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The backup Cloud Router's BGP session is experiencing a keepalive timeout due to incorrect timers.
The backup Cloud Router's BGP session remains established, but routes are withdrawn after 30 seconds. This is classic behavior of a BGP keepalive or hold timer mismatch: if the backup router expects a keepalive within a shorter interval than the peer sends, the hold timer expires, causing the router to withdraw all routes learned from that peer while keeping the TCP session alive (or re-establishing it). The 30-second interval matches the default BGP hold timer (90 seconds) divided by three, suggesting a timer misconfiguration on the backup router.
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 backup Cloud Router is configured with a lower MED value, causing the routes to be withdrawn.
Why it's wrong here
MED affects path selection, not route withdrawal.
✓
The backup Cloud Router's BGP session is experiencing a keepalive timeout due to incorrect timers.
Why this is correct
Keepalive timer mismatch can cause the on-premises router to withdraw routes while the session remains established from Google's perspective.
Clue confirmation
The clue words "most likely", "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The backup tunnel is using a static routing method instead of dynamic BGP.
The backup tunnel's Cloud Router is in a different region, and the routes are not propagated globally.
Why it's wrong here
Cloud Router is regional, but routes are propagated globally via VPC.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume route withdrawal always indicates a BGP session failure, but Google Cloud often tests the nuance that hold timer expiration can cause route withdrawal while the TCP session remains established (or quickly re-establishes), leading to the mistaken belief that the session is stable when routes are actually being withdrawn.
Detailed technical explanation
How to think about this question
BGP keepalive and hold timers are negotiated during the OPEN message exchange; the hold timer is set to the smaller of the two peers' configured values, and keepalives are sent at one-third of the hold timer. If one side's hold timer is misconfigured to a very low value (e.g., 30 seconds), the peer must send keepalives every 10 seconds; if the peer sends them less frequently (e.g., every 30 seconds), the hold timer expires, causing the router to withdraw all routes from that peer and reset the session. In Google Cloud, Cloud Router BGP timers can be customized, but the default hold time is 90 seconds; a misconfiguration to 30 seconds would cause the observed 30-second withdrawal.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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 network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The backup Cloud Router's BGP session is experiencing a keepalive timeout due to incorrect timers. — The backup Cloud Router's BGP session remains established, but routes are withdrawn after 30 seconds. This is classic behavior of a BGP keepalive or hold timer mismatch: if the backup router expects a keepalive within a shorter interval than the peer sends, the hold timer expires, causing the router to withdraw all routes learned from that peer while keeping the TCP session alive (or re-establishing it). The 30-second interval matches the default BGP hold timer (90 seconds) divided by three, suggesting a timer misconfiguration on the backup router.
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", "primary". 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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Question Discussion
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