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Question 413 of 497

Implementing hybrid interconnectivity →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the on-premises routers do not use AS path prepending to prefer the local DC’s path for the prefix. Without AS path prepending, both DCs advertise the same CIDR with equal AS path length, so Cloud Router’s default route priority treats them as equally preferred, leading to suboptimal path selection after failover. When both tunnels re-establish, GCP traffic may still be routed to the remote DC (e.g., DC2) instead of the local DC (DC1), causing increased latency due to cross-country transit. On the Google Professional Cloud Network Engineer exam, this tests your understanding of BGP AS path prepending for failover and latency control in hybrid cloud designs—a common trap is assuming that simply re-establishing a tunnel restores optimal routing. Remember: equal AS path length means equal preference; artificially lengthen the backup path to force local preference. Memory tip: “Prepend to prevent the long way around.”

PCNE Implementing hybrid interconnectivity Practice Question

This PCNE practice question tests your understanding of implementing hybrid interconnectivity. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 two on-premises data centers (DC1 and DC2) connected to Google Cloud via two separate VPN tunnels to the same VPC. Each tunnel terminates on a different Cloud VPN gateway (gateway1 in us-east1, gateway2 in us-west1). The on-premises routers advertise the same CIDR 172.16.0.0/12 from both DCs. Cloud Router is configured with BGP and uses default route priority. You notice that after a failover event where one tunnel goes down, traffic continues to flow, but there is a significant increase in latency for traffic coming from GCP to on-premises. You verify that both tunnels have re-established. What is the most likely cause of the increased latency?

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.

Question 1hardmultiple choice

Open the full BGP breakdown →

A
The on-premises routers are using site-to-site VPN between themselves causing a routing loop
Why wrong: A site-to-site VPN between DCs would not directly affect GCP routing; traffic from GCP would still use the advertised paths.
B
The on-premises routers do not use AS path prepending to prefer the local DC's path for the prefix
Without AS path prepending, GCP may choose a suboptimal path (e.g., sending DC1 traffic via DC2) if the routes have equal AS path length, causing increased latency.
C
The Cloud VPN tunnels are using different preshared keys
Why wrong: Preshared keys affect authentication but not routing or latency.
D
Bidirectional Forwarding Detection (BFD) is not enabled on the Cloud VPN tunnels
Why wrong: BFD speeds up failure detection, but the latency issue persists after both tunnels are up, so BFD is not the cause.

Full breakdown with real-world context →

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

✓

The on-premises routers do not use AS path prepending to prefer the local DC's path for the prefix

When both on-premises routers advertise the same CIDR (172.16.0.0/12) to Google Cloud via BGP, Cloud Router selects the path with the shorter AS path length by default. Without AS path prepending on the backup DC's router, both routes have equal AS path length, causing Cloud Router to load-balance or pick a suboptimal path after failover. After the tunnel re-establishes, traffic from GCP may still be routed to the remote DC (e.g., DC2) instead of the local DC (DC1), resulting in higher latency due to cross-country or inter-DC transit.

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 routers are using site-to-site VPN between themselves causing a routing loop
Why it's wrong here
A site-to-site VPN between DCs would not directly affect GCP routing; traffic from GCP would still use the advertised paths.
✓
The on-premises routers do not use AS path prepending to prefer the local DC's path for the prefix
Why this is correct
Without AS path prepending, GCP may choose a suboptimal path (e.g., sending DC1 traffic via DC2) if the routes have equal AS path length, causing increased latency.
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 VPN tunnels are using different preshared keys
Why it's wrong here
Preshared keys affect authentication but not routing or latency.
✗
Bidirectional Forwarding Detection (BFD) is not enabled on the Cloud VPN tunnels
Why it's wrong here
BFD speeds up failure detection, but the latency issue persists after both tunnels are up, so BFD is not the cause.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Google Cloud often tests the misconception that increased latency after failover is due to a routing loop or BFD misconfiguration, when the real issue is the lack of AS path prepending to influence BGP path selection for the same prefix advertised from multiple locations.

Detailed technical explanation

How to think about this question

AS path prepending is a BGP attribute manipulation technique where the local AS number is repeated multiple times in the AS_PATH to make a route less preferred. In Google Cloud, Cloud Router uses the BGP best path selection algorithm, which prefers the shortest AS path length when all other attributes (local preference, MED, etc.) are equal. Without prepending, both DCs' routes appear equally attractive, and Cloud Router may not consistently prefer the geographically closer DC, leading to suboptimal routing and higher latency after failover.

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: The on-premises routers do not use AS path prepending to prefer the local DC's path for the prefix — When both on-premises routers advertise the same CIDR (172.16.0.0/12) to Google Cloud via BGP, Cloud Router selects the path with the shorter AS path length by default. Without AS path prepending on the backup DC's router, both routes have equal AS path length, causing Cloud Router to load-balance or pick a suboptimal path after failover. After the tunnel re-establishes, traffic from GCP may still be routed to the remote DC (e.g., DC2) instead of the local DC (DC1), resulting in higher latency due to cross-country or inter-DC transit.

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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