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Question 226 of 497

Configuring network services →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the Cloud Router is in custom advertise mode but does not include the VPC subnets in its custom advertisement list. This is correct because when a Cloud Router is set to custom advertise mode, it only propagates the specific routes you explicitly define; if the VPC subnet ranges are omitted from that list, the on-premises network will receive no routes from Google Cloud, even though the BGP session itself may be established and healthy. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of the distinction between default and custom route advertisement, often appearing as a trap where candidates assume a working BGP session guarantees route propagation. A common memory tip is to think of custom mode as a “whitelist only” behavior—if you don’t add the subnets, they stay invisible to your on-premises peer. Remember: custom mode means you must explicitly advertise every route you want shared, including the VPC subnets.

PCNE Configuring network services Practice Question

This PCNE practice question tests your understanding of configuring network services. 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

Refer to the exhibit.

```
$ gcloud compute routers describe my-router --region us-central1
creationTimestamp: '2023-01-15T10:00:00.000-08:00'
description: Router for on-prem connectivity
id: '1234567890123456789'
kind: compute#router
name: my-router
network: https://www.googleapis.com/compute/v1/projects/my-project/global/networks/default
region: https://www.googleapis.com/compute/v1/projects/my-project/regions/us-central1
bgp:
  asn: 65001
  advertiseMode: CUSTOM
  advertisedGroups:
  - ALL_SUBNETS
  advertisedIpRanges:
  - range: 10.0.1.0/24
    description: On-prem subnet
  keepaliveInterval: 20
```

You have a Cloud Router with the configuration shown. The on-premises network (ASN 65002) is not receiving any routes from Google Cloud. 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.

Question 1hardmultiple choice

Review the full routing breakdown →

Exhibit

Refer to the exhibit.

```
$ gcloud compute routers describe my-router --region us-central1
creationTimestamp: '2023-01-15T10:00:00.000-08:00'
description: Router for on-prem connectivity
id: '1234567890123456789'
kind: compute#router
name: my-router
network: https://www.googleapis.com/compute/v1/projects/my-project/global/networks/default
region: https://www.googleapis.com/compute/v1/projects/my-project/regions/us-central1
bgp:
  asn: 65001
  advertiseMode: CUSTOM
  advertisedGroups:
  - ALL_SUBNETS
  advertisedIpRanges:
  - range: 10.0.1.0/24
    description: On-prem subnet
  keepaliveInterval: 20
```

A
The BGP keepalive interval is set too low
Why wrong: 20 seconds is standard.
B
The BGP session is not established
Why wrong: Session might be up but no routes because of custom mode.
C
The router is in custom advertise mode but does not advertise the VPC subnets
Custom mode overrides default; must explicitly advertise VPC subnets.
D
The on-premises subnet 10.0.1.0/24 is not advertised
Why wrong: It is advertised in custom mode.

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 router is in custom advertise mode but does not advertise the VPC subnets

When a Cloud Router is configured in custom advertise mode, it only advertises the routes explicitly specified in the custom advertisement list. If the VPC subnets are not included in that list, the on-premises network will not receive any routes from Google Cloud, even if the BGP session is established. This is the most likely cause because the question states the on-premises network is not receiving any routes, which aligns with a missing custom advertisement rather than a session or timer issue.

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 BGP keepalive interval is set too low
Why it's wrong here
20 seconds is standard.
✗
The BGP session is not established
Why it's wrong here
Session might be up but no routes because of custom mode.
✓
The router is in custom advertise mode but does not advertise the VPC subnets
Why this is correct
Custom mode overrides default; must explicitly advertise VPC subnets.
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 on-premises subnet 10.0.1.0/24 is not advertised
Why it's wrong here
It is advertised in custom mode.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Google Cloud often tests the misconception that a BGP session being 'Established' guarantees route exchange, but in Google Cloud's custom advertise mode, the session can be up while no routes are advertised due to missing custom advertisement configuration.

Detailed technical explanation

How to think about this question

In Google Cloud, a Cloud Router in custom advertise mode uses the 'bgp: advertise-mode: CUSTOM' setting, which overrides the default automatic advertisement of VPC subnet ranges. The router will only send BGP UPDATE messages for prefixes listed in the 'advertised-ip-ranges' configuration; if this list is empty or missing the VPC subnets, no routes are advertised to the BGP peer. This behavior is distinct from standard BGP where routes are typically advertised from the routing table; here, the Cloud Router acts as a route server that must be explicitly configured to propagate VPC routes.

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?

Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The router is in custom advertise mode but does not advertise the VPC subnets — When a Cloud Router is configured in custom advertise mode, it only advertises the routes explicitly specified in the custom advertisement list. If the VPC subnets are not included in that list, the on-premises network will not receive any routes from Google Cloud, even if the BGP session is established. This is the most likely cause because the question states the on-premises network is not receiving any routes, which aligns with a missing custom advertisement rather than a session or timer issue.

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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Last reviewed: Jun 30, 2026

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