The answer is that the VPN spokes do not have custom dynamic route exchange enabled, so routes from Site A are not advertised to Site B. This is the most likely cause because Network Connectivity Center relies on spokes to propagate routes between on-premises sites, and by default, VPN tunnel spokes do not advertise custom dynamic routes unless explicitly configured. Without enabling custom dynamic route exchange, the NCC hub cannot distribute learned routes from one spoke to another, breaking the intended path through Google Cloud. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of NCC spoke route exchange mechanics, often appearing as a trap where candidates assume VPN spokes automatically share routes like VLAN attachments do. A common memory tip is to remember that VPN spokes are “silent by default”—you must flip the custom dynamic route exchange switch to make them talk.
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. 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.
Your company uses Network Connectivity Center (NCC) to manage multiple on-premises sites connected via Cloud VPN and Partner Interconnect. You create a NCC hub and attach spokes (VPN tunnels and VLAN attachments). Traffic between two on-premises sites (Site A and Site B) should flow through Google Cloud. However, traffic is not passing between the sites. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPN spokes do not have custom dynamic route exchange enabled, so routes from Site A are not advertised to Site B.
Option A is correct because NCC routes will only propagate if the spokes are configured with the proper route exchange options. By default, custom dynamic route exchange is not enabled for VPN tunnels; you need to configure advertisement of routes. Option B is wrong because NCC supports VPN and Interconnect. Option C is wrong because NCC uses dynamic routing; static routes are not required. Option D is wrong because VPC networks are not directly involved if using NCC hub-and-spoke.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 need static routes pointing to the Google Cloud VPC for inter-site traffic.
Why it's wrong here
NCC handles dynamic routing.
✓
The VPN spokes do not have custom dynamic route exchange enabled, so routes from Site A are not advertised to Site B.
Why this is correct
NCC requires route exchange to be enabled on spokes to propagate routes.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
Network Connectivity Center does not support Cloud VPN as a spoke type.
The NCC hub must be in the same VPC network as the sites' VPCs.
Why it's wrong here
NCC works across VPCs; it's a global resource.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related PCNE questions on switching, trunking, and access-port configuration.
Configuring network services — This question tests Configuring network services — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The VPN spokes do not have custom dynamic route exchange enabled, so routes from Site A are not advertised to Site B. — Option A is correct because NCC routes will only propagate if the spokes are configured with the proper route exchange options. By default, custom dynamic route exchange is not enabled for VPN tunnels; you need to configure advertisement of routes. Option B is wrong because NCC supports VPN and Interconnect. Option C is wrong because NCC uses dynamic routing; static routes are not required. Option D is wrong because VPC networks are not directly involved if using NCC hub-and-spoke.
What should I do if I get this PCNE question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related PCNE questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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