The answer is that the update-source loopback0 command is unsupported for Cloud Router BGP sessions, making it the most likely cause of the routing failure. Cloud Router requires BGP peering to use the primary IP address of the interface directly connected to the VLAN attachment, not a loopback address. Because the loopback interface is not directly reachable by the on-premises router, the BGP session cannot establish, and traffic from on-premises to Google Cloud will not route via the interconnect. On the Google Professional Cloud Network Engineer exam, this question tests your understanding of Cloud Router’s BGP implementation limitations, which differ from traditional router configurations. A common trap is assuming that standard BGP features like update-source loopback are supported in Google Cloud, but they are not. Remember the memory tip: “Cloud Router BGP is direct, not loopback—no source trick, just the link’s IP.”
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.
Exhibit
Refer to the exhibit.
```
# Cloud Router BGP configuration
router bgp 65001
neighbor 169.254.0.1 remote-as 64512
neighbor 169.254.0.1 ebgp-multihop 2
neighbor 169.254.0.1 update-source loopback0
address-family ipv4 unicast
neighbor 169.254.0.1 route-map SET-MED in
neighbor 169.254.0.1 route-map SET-LOCAL-PREF out
!
route-map SET-MED permit 10
set metric 100
!
route-map SET-LOCAL-PREF permit 10
set local-preference 200
```
A network engineer configured a Cloud Router with the BGP configuration shown. The on-premises router (AS 64512) is peering with the Cloud Router (AS 65001) over a Dedicated Interconnect VLAN attachment. The engineer notices that traffic from on-premises to Google Cloud is not being routed via this interconnect as expected. 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 update-source loopback0 command is invalid for Cloud Router BGP sessions
Option C is correct because Cloud Router does not support the `update-source loopback0` command. BGP sessions on Cloud Router must use the primary IP address of the interface that is directly connected to the VLAN attachment; loopback interfaces are not supported for BGP peering. This command would cause the Cloud Router to attempt to source BGP packets from a loopback address that is not reachable by the on-premises router, breaking the BGP session.
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 route-map SET-MED sets the MED attribute incorrectly
Why it's wrong here
MED is valid for eBGP, but the issue is the peering not working.
✗
The ebgp-multihop 2 command is not supported on Cloud Router
Why it's wrong here
Cloud Router supports ebgp-multihop, but it is unnecessary for directly connected peers.
✓
The update-source loopback0 command is invalid for Cloud Router BGP sessions
Why this is correct
Cloud Router requires the BGP session to use the link-local IP address, not a loopback.
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 local-preference is set in the outbound direction, which is not allowed
Why it's wrong here
Local preference can be set outbound to influence routes sent to on-premises, but not the root cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that Cloud Router supports the same BGP commands as a physical Cisco router, leading candidates to overlook the fact that Cloud Router is a managed service with a restricted feature set, particularly regarding interface sourcing and multihop capabilities.
Detailed technical explanation
How to think about this question
Cloud Router implements BGP-4 (RFC 4271) but with restrictions: it only supports eBGP, requires the peer IP to be directly connected (no multihop), and mandates that the BGP update source must be the interface IP of the VLAN attachment. Loopback interfaces are not exposed for BGP peering because Cloud Router operates in a virtualized environment where the underlying network infrastructure expects BGP packets to originate from the same subnet as the VLAN attachment. In real-world scenarios, using an incorrect update source would cause the BGP session to remain in Idle or Active state, as the TCP connection cannot be established.
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.
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 update-source loopback0 command is invalid for Cloud Router BGP sessions — Option C is correct because Cloud Router does not support the `update-source loopback0` command. BGP sessions on Cloud Router must use the primary IP address of the interface that is directly connected to the VLAN attachment; loopback interfaces are not supported for BGP peering. This command would cause the Cloud Router to attempt to source BGP packets from a loopback address that is not reachable by the on-premises router, breaking the BGP session.
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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Question Discussion
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