PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring 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 has configured a HA VPN between Google Cloud and an on-premises data center using two tunnels with separate Cloud Routers and BGP sessions in active/active mode. Each Cloud Router is configured to learn routes from the on-premises side and advertise VPC subnets. Recently, one of the tunnels experienced a physical link failure and went down. The security team notices that the remaining tunnel is still up and passing traffic, but some routes that were learned via the failed tunnel are no longer present in the routing table of that Cloud Router. The on-premises administrator confirms that the routes are still being advertised from the local router. What is the impact on traffic to the on-premises network?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Traffic destined to subnets only reachable via the failed tunnel will be dropped.
In an active/active HA VPN with separate Cloud Routers and BGP sessions, each Cloud Router maintains its own independent routing table. When a tunnel fails, the BGP session associated with that Cloud Router goes down, causing all routes learned via that session to be withdrawn. Even though the on-premises router continues to advertise the routes, the Cloud Router that lost its BGP session will no longer have those routes in its routing table. Traffic destined to subnets that were only reachable through the failed tunnel will be dropped because the remaining Cloud Router does not have a route for those subnets.
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 failed tunnel will automatically be restored by BGP when the link comes back.
Why it's wrong here
This is true but irrelevant to the immediate impact; the question asks for the impact after failure.
✗
The Cloud Router will initiate a failover to a secondary BGP session.
Why it's wrong here
There is no secondary BGP session; the other Cloud Router continues independently.
✓
Traffic destined to subnets only reachable via the failed tunnel will be dropped.
Why this is correct
The missing routes mean those destinations are unreachable until the tunnel recovers or routes are learned via the other tunnel.
Related concept
Read the scenario before looking for a memorised answer.
✗
All traffic will continue to flow through the remaining tunnel without any disruption.
Why it's wrong here
Traffic to subnets only reachable via the failed tunnel will be dropped.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that active/active HA VPN provides full redundancy for all routes, but in reality, each Cloud Router independently learns routes, and a tunnel failure only drops traffic to subnets unique to that tunnel's BGP session.
Detailed technical explanation
How to think about this question
In Google Cloud HA VPN with active/active mode, each tunnel is associated with a separate Cloud Router and a separate BGP session. The Cloud Router uses BGP to learn routes from the on-premises side and installs them in its routing table with a next hop pointing to the tunnel interface. When the tunnel fails, the BGP session goes down, and the Cloud Router immediately withdraws all routes learned via that session (per BGP RFC 4271). The remaining Cloud Router's routing table is unaffected, but it only contains routes learned via its own BGP session, which may not include all on-premises subnets if the on-premises router is advertising different prefixes per session.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Traffic destined to subnets only reachable via the failed tunnel will be dropped. — In an active/active HA VPN with separate Cloud Routers and BGP sessions, each Cloud Router maintains its own independent routing table. When a tunnel fails, the BGP session associated with that Cloud Router goes down, causing all routes learned via that session to be withdrawn. Even though the on-premises router continues to advertise the routes, the Cloud Router that lost its BGP session will no longer have those routes in its routing table. Traffic destined to subnets that were only reachable through the failed tunnel will be dropped because the remaining Cloud Router does not have a route for those subnets.
What should I do if I get this PCSE 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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