- A
Configure the on-premises router at DC1 to set a lower MED value for its advertised routes to make DC1 preferred
Lower MED makes DC1 preferred; after failover, when DC1 recovers, its lower MED will cause traffic to return.
- B
Change the VPC routing mode from auto to custom to force route re-evaluation
Why wrong: Routing mode does not affect BGP path selection.
- C
Disable the BGP session on DC2's Cloud Router during normal operation and re-enable it only when DC1 fails
Why wrong: Not a best practice; manual intervention is not reliable.
- D
On the Google Cloud Router for DC1, apply a route policy to set a higher local preference for routes learned from DC1
Why wrong: Higher local preference will prefer DC1, but local preference is applied to routes received by Cloud Router; this could work, but the simplest and most common practice is to use MED.
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 large enterprise has a multi-site on-premises network with two data centers (DC1 and DC2) connected via a private WAN. They are migrating critical applications to Google Cloud and have established a Dedicated Interconnect at each data center, each with a single VLAN attachment (vlan-attachment-1 from DC1, vlan-attachment-2 from DC2) connected to a single VPC network in us-central1. Each VLAN attachment uses a separate Cloud Router (router-us-central1-dc1 and router-us-central1-dc2) with BGP sessions advertising the same on-premises prefixes. The VPC has auto-mode subnet ranges. They notice that traffic from Google Cloud to on-premises is flowing only through DC1, and when DC1's interconnect fails, traffic fails over to DC2, but after DC1 recovers, traffic does not return to DC1. The on-premises routers are advertising the same prefixes with equal MED values. What is the most likely cause and the best corrective action?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"best"Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Configure the on-premises router at DC1 to set a lower MED value for its advertised routes to make DC1 preferred
Option A is correct because with equal MED values from both on-premises routers, Google Cloud's Cloud Routers will prefer the first learned route and stick to it due to the default BGP best-path selection behavior (no tie-breaking based on router ID or other attributes when routes are identical). When DC1's interconnect fails, the route is withdrawn, and traffic fails over to DC2. After DC1 recovers, the route is re-advertised, but Cloud Router does not automatically preempt the existing DC2 route because the BGP best-path selection is stable and does not re-evaluate unless the current best path is withdrawn. Setting a lower MED on DC1's advertised routes makes DC1's route more preferred, ensuring traffic returns to DC1 after recovery.
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.
- ✓
Configure the on-premises router at DC1 to set a lower MED value for its advertised routes to make DC1 preferred
Why this is correct
Lower MED makes DC1 preferred; after failover, when DC1 recovers, its lower MED will cause traffic to return.
Clue confirmation
The clue words "best", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Change the VPC routing mode from auto to custom to force route re-evaluation
Why it's wrong here
Routing mode does not affect BGP path selection.
- ✗
Disable the BGP session on DC2's Cloud Router during normal operation and re-enable it only when DC1 fails
Why it's wrong here
Not a best practice; manual intervention is not reliable.
- ✗
On the Google Cloud Router for DC1, apply a route policy to set a higher local preference for routes learned from DC1
Why it's wrong here
Higher local preference will prefer DC1, but local preference is applied to routes received by Cloud Router; this could work, but the simplest and most common practice is to use MED.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that BGP automatically re-evaluates the best path when a new route is received, but in practice, BGP is stable and only switches to a new path if the current best path is withdrawn, which is why adjusting MED (or other attributes) is necessary to force traffic back to the primary link after recovery.
Detailed technical explanation
How to think about this question
In BGP, when multiple paths have equal MED and are from different autonomous systems (AS), the path with the lowest neighbor address (or router ID) is typically chosen, but Google Cloud's Cloud Router implements a stable path selection that does not flap. The MED attribute is a hint to external neighbors about the preferred path into an AS, and lowering MED on DC1's advertisements makes DC1's path more preferred over DC2's. This behavior is defined in RFC 4271 and is commonly used in multi-homed designs to control inbound traffic flow without relying on AS path prepending or other attributes.
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.
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FAQ
Questions learners often ask
What does this PCNE question test?
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: Configure the on-premises router at DC1 to set a lower MED value for its advertised routes to make DC1 preferred — Option A is correct because with equal MED values from both on-premises routers, Google Cloud's Cloud Routers will prefer the first learned route and stick to it due to the default BGP best-path selection behavior (no tie-breaking based on router ID or other attributes when routes are identical). When DC1's interconnect fails, the route is withdrawn, and traffic fails over to DC2. After DC1 recovers, the route is re-advertised, but Cloud Router does not automatically preempt the existing DC2 route because the BGP best-path selection is stable and does not re-evaluate unless the current best path is withdrawn. Setting a lower MED on DC1's advertised routes makes DC1's route more preferred, ensuring traffic returns to DC1 after recovery.
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: "best", "most likely". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.
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