The answer is that the on-premises router is sending different BGP Multi-Exit Discriminator (MED) values for the same route on the two BGP sessions. This is correct because MED is a metric used to influence inbound traffic to an autonomous system; Cloud Router will always prefer the path with the lower MED value, even when both tunnels are up and advertising identical routes. If one tunnel’s BGP update carries a lower MED, all traffic from Google Cloud will be directed through that tunnel, effectively breaking load balancing. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of BGP path selection and HA VPN behavior—a common trap is assuming equal-cost multipath (ECMP) will automatically distribute traffic, but MED overrides that. A helpful memory tip: “MED decides, lower wins the ride” — the lowest MED value gets all the traffic, so to achieve load balancing, ensure MED values are identical on both BGP sessions.
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 network engineer is troubleshooting an HA VPN setup between Google Cloud and an on-premises data center. The two tunnels are established, and BGP sessions are up on both tunnels. However, traffic from Google Cloud to the on-premises network is only using one tunnel, even though both BGP sessions are advertising the same routes. 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.
The on-premises router is sending different BGP metrics (MED) for the same route on the two BGP sessions.
If MED differs, Cloud Router will prefer lower MED, leading to single-path use.
B
The Cloud Router is not configured for dynamic routing.
Why wrong: Dynamic routing is enabled via BGP; if BGP sessions are up, routing is dynamic.
C
One of the IPsec tunnels is in a dead state.
Why wrong: If a tunnel is dead, BGP would also be down, not established.
D
The on-premises router is setting a higher local preference on one route.
Why wrong: Local preference is used for inbound decisions on the receiving router; it would affect on-premises routing to Google, not Google routing to on-premises.
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 is sending different BGP metrics (MED) for the same route on the two BGP sessions.
When both BGP sessions are up and advertising the same routes, but traffic only uses one tunnel, the most likely cause is that the on-premises router is sending different Multi-Exit Discriminator (MED) values for the same route on the two BGP sessions. MED is a metric that influences inbound traffic to an AS; a lower MED value is preferred. If one tunnel's BGP update carries a lower MED, Google Cloud's Cloud Router will select that path for all traffic, even though both tunnels are functional.
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 on-premises router is sending different BGP metrics (MED) for the same route on the two BGP sessions.
Why this is correct
If MED differs, Cloud Router will prefer lower MED, leading to single-path use.
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 Cloud Router is not configured for dynamic routing.
Why it's wrong here
Dynamic routing is enabled via BGP; if BGP sessions are up, routing is dynamic.
✗
One of the IPsec tunnels is in a dead state.
Why it's wrong here
If a tunnel is dead, BGP would also be down, not established.
✗
The on-premises router is setting a higher local preference on one route.
Why it's wrong here
Local preference is used for inbound decisions on the receiving router; it would affect on-premises routing to Google, not Google routing to on-premises.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between BGP attributes that influence inbound vs. outbound traffic; the trap here is that candidates may confuse MED (inbound metric) with local preference (outbound metric) and incorrectly select Option D, not realizing that local preference set by the on-premises router is not sent to Google Cloud's eBGP peer.
Detailed technical explanation
How to think about this question
MED (metric) is an optional transitive BGP attribute that is exchanged between ASes to influence the entry point for inbound traffic. In Google Cloud HA VPN, each tunnel is a separate BGP session, and the Cloud Router selects the best path based on the lowest MED value when all other path selection criteria (e.g., weight, local preference, AS path length) are equal. A real-world scenario is when an on-premises engineer inadvertently configures different MED values on two redundant routers or interfaces, causing asymmetric traffic flow despite both tunnels being healthy.
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: The on-premises router is sending different BGP metrics (MED) for the same route on the two BGP sessions. — When both BGP sessions are up and advertising the same routes, but traffic only uses one tunnel, the most likely cause is that the on-premises router is sending different Multi-Exit Discriminator (MED) values for the same route on the two BGP sessions. MED is a metric that influences inbound traffic to an AS; a lower MED value is preferred. If one tunnel's BGP update carries a lower MED, Google Cloud's Cloud Router will select that path for all traffic, even though both tunnels are functional.
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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Question Discussion
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