The answer is that the peer router's firewall is blocking TCP port 179. This is correct because the IDLE state in BGP indicates that the TCP three-way handshake required for session establishment has failed or been reset, and since only one of the two on-prem peers is stuck in IDLE, the issue is isolated to that specific peer rather than a global configuration problem. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of BGP state machine transitions and the distinction between per-peer issues (like firewall rules or IP reachability) versus router-wide misconfigurations. A common trap is assuming a routing protocol or AS number mismatch, but those typically cause ACTIVE or CONNECT states, not IDLE. Remember the memory tip: IDLE means the handshake never started—check layer 4, not layer 3.
PCNE Practice Question: Designing, planning, and prototyping a GCP network
This PCNE practice question tests your understanding of designing, planning, and prototyping a gcp network. 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.
Refer to the exhibit. A network engineer checks the BGP status of a Cloud Router. The on-prem router has two BGP peers configured. What is the most likely cause of the IDLE session for the second peer?
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 peer router's firewall is blocking TCP port 179
The IDLE state in BGP indicates that the session has not been established or has been reset. Since the on-prem router has two BGP peers and only one is IDLE, a per-peer issue like a firewall blocking TCP port 179 on the second peer's router is the most likely cause. This prevents the TCP three-way handshake required for BGP session establishment, leaving the session stuck in IDLE.
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 VLAN attachment is not configured
Why it's wrong here
If VLAN attachment was missing, both sessions would likely fail.
✗
The BGP timers are misconfigured
Why it's wrong here
Timer mismatch typically leads to flapping, not persistent IDLE.
✗
The peer router is not configured with the correct BGP ASN
Why it's wrong here
ASN mismatch usually results in a state other than IDLE.
✓
The peer router's firewall is blocking TCP port 179
Why this is correct
Blocking port 179 prevents TCP connection, causing IDLE.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume an IDLE session is always due to a BGP configuration mismatch (like ASN or timers), but Cisco tests the nuance that a per-peer firewall rule blocking TCP 179 can cause IDLE on only one peer while the other remains established.
Detailed technical explanation
How to think about this question
BGP uses TCP port 179 for session establishment; if a firewall or ACL on the peer router drops TCP SYN packets to this port, the BGP process cannot complete the TCP handshake, and the session remains in IDLE state indefinitely. In Google Cloud, Cloud Router BGP sessions require bidirectional TCP connectivity; a common misconfiguration is allowing TCP 179 only from the Cloud Router's IP but not from the on-prem peer's IP, causing asymmetric filtering. This is distinct from BGP authentication or prefix issues, which manifest after the session is 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.
Designing, planning, and prototyping a GCP network — This question tests Designing, planning, and prototyping a GCP network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The peer router's firewall is blocking TCP port 179 — The IDLE state in BGP indicates that the session has not been established or has been reset. Since the on-prem router has two BGP peers and only one is IDLE, a per-peer issue like a firewall blocking TCP port 179 on the second peer's router is the most likely cause. This prevents the TCP three-way handshake required for BGP session establishment, leaving the session stuck in IDLE.
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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