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Question 941 of 1,000
Implementing a Virtual Private Cloud →hardMultiple ChoiceObjective-mapped

Why Is Traffic Not Flowing Through HA VPN? — One BGP Session Down | Google PCNE

This PCNE practice question tests your understanding of implementing a virtual private cloud. 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.

Exhibit

$ gcloud compute routers describe my-router --region=us-central1
bgp:
  asn: 65000
  advertiseMode: DEFAULT
  advertisedGroups:
  - ALL_SUBNETS
  advertisedIpRanges:
  - range: 10.0.0.0/16
  - range: 10.1.0.0/16
  keepaliveInterval: 20
  session Range: 169.254.0.0/16
bgpPeers:
- interfaceName: vpn-tunnel-1
  ipAddress: 169.254.0.1
  peerIpAddress: 169.254.0.2
  peerAsn: 65001
  advertisedRoutePriority: 100
  status: UP
  statusReason: ''
- interfaceName: vpn-tunnel-2
  ipAddress: 169.254.0.5
  peerIpAddress: 169.254.0.6
  peerAsn: 65001
  advertisedRoutePriority: 100
  status: DOWN
  statusReason: 'Connectivity error'

Refer to the exhibit. A company uses a Cloud Router with two BGP sessions for an HA VPN to on-premises. Traffic is not flowing correctly to the on-premises network. What is the most likely issue?

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.

Exhibit

$ gcloud compute routers describe my-router --region=us-central1
bgp:
  asn: 65000
  advertiseMode: DEFAULT
  advertisedGroups:
  - ALL_SUBNETS
  advertisedIpRanges:
  - range: 10.0.0.0/16
  - range: 10.1.0.0/16
  keepaliveInterval: 20
  session Range: 169.254.0.0/16
bgpPeers:
- interfaceName: vpn-tunnel-1
  ipAddress: 169.254.0.1
  peerIpAddress: 169.254.0.2
  peerAsn: 65001
  advertisedRoutePriority: 100
  status: UP
  statusReason: ''
- interfaceName: vpn-tunnel-2
  ipAddress: 169.254.0.5
  peerIpAddress: 169.254.0.6
  peerAsn: 65001
  advertisedRoutePriority: 100
  status: DOWN
  statusReason: 'Connectivity error'

Quick Answer

The answer is that the BGP session with vpn-tunnel-2 is down, causing asymmetric routing. When one BGP session in an HA VPN goes down, traffic from on-premises may still arrive at the healthy tunnel, but return traffic from Google Cloud is routed through the failed tunnel, breaking flow. This tests your understanding of how Cloud Router relies on both BGP sessions to maintain symmetric routing for an HA VPN. On the Google Professional Cloud Network Engineer exam, this scenario is a classic trap: candidates often blame ASN or priority values, but the real issue is that a single downed session forces traffic to take an unbalanced path. Remember, HA VPN requires both tunnels and their BGP sessions to be healthy for proper traffic flow. A quick memory tip: “One down, route around—but not symmetric.”

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 BGP session with vpn-tunnel-2 is down, causing asymmetric routing.

The exhibit shows that vpn-tunnel-2's BGP session is in a state other than 'established' (likely down), while vpn-tunnel-1 is established. A down BGP session on one tunnel can cause asymmetric routing or loss of connectivity, especially if traffic is sent over the down tunnel. Option B correctly identifies this. Option A is incorrect because the advertised route priority (MED) of 100 is not the issue; the problem is the session state. Option C is incorrect because keepalive interval 20 seconds is standard. Option D is incorrect because private ASN 65000 is allowed in Cloud Router BGP configurations.

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 advertised route priority is too low.

    Why it's wrong here

    Priority 100 is acceptable.

  • ✓

    The BGP session with vpn-tunnel-2 is down, causing asymmetric routing.

    Why this is correct

    A down BGP session can disrupt proper route advertisement and traffic flow.

    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 keepalive interval is too high.

    Why it's wrong here

    20 seconds is a standard keepalive interval.

  • ✗

    The ASN 65000 is private and not allowed.

    Why it's wrong here

    Private ASNs are allowed for Cloud Router.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

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.
  • Use explanations to understand the rule behind the answer.

TExam Day Tips

  • Underline the problem statement mentally.
  • 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.

Quick reference

Asymmetric Encryption Algorithm Comparison

AlgorithmKey ExchangeSignaturesEquivalent Security KeyNotes
RSA-3072YesYes128-bitWidely deployed; slow for bulk data
ECDSA P-256NoYes128-bitFast signatures; standard TLS certs
ECDH / ECDHEYesNo128-bitPerfect forward secrecy in TLS 1.3
DH / DHEYesNo128-bit (3072-bit key)Replaced by ECDHE in modern TLS
Ed25519NoYes~128-bitSSH keys, modern PKI

What to study next

Got this wrong? Here's your next step.

Identify which PCNE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this PCNE question test?

Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The BGP session with vpn-tunnel-2 is down, causing asymmetric routing. — The exhibit shows that vpn-tunnel-2's BGP session is in a state other than 'established' (likely down), while vpn-tunnel-1 is established. A down BGP session on one tunnel can cause asymmetric routing or loss of connectivity, especially if traffic is sent over the down tunnel. Option B correctly identifies this. Option A is incorrect because the advertised route priority (MED) of 100 is not the issue; the problem is the session state. Option C is incorrect because keepalive interval 20 seconds is standard. Option D is incorrect because private ASN 65000 is allowed in Cloud Router BGP configurations.

What should I do if I get this PCNE question wrong?

Identify which PCNE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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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Last reviewed: Jun 24, 2026

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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.