The answer is that a Cloud Router not configured to establish a BGP session with the on-premises peer can cause the session to enter the IDLE state. This occurs because the IDLE state is the initial BGP state where the router is not attempting any TCP connection; without a proper neighbor configuration on the Cloud Router, the BGP process never initiates the three-way handshake on TCP port 179, leaving the session stuck in IDLE. On the Google Professional Cloud Network Engineer exam, this question tests your understanding of BGP state machine fundamentals and how misconfigurations—like missing peer IPs or AS numbers—directly prevent session establishment. A common trap is confusing IDLE with other states like ACTIVE or CONNECT; remember that IDLE means the router is not even trying to connect, often due to a configuration or administrative issue. Memory tip: IDLE = “I’m Doing nothing, Lack of Establishment” — if the Cloud Router isn’t told who to talk to, it won’t even start the conversation.
PCNE Implementing hybrid interconnectivity Practice Question
This PCNE practice question tests your understanding of implementing hybrid interconnectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which THREE of the following could cause a Dedicated Interconnect BGP session to go to the 'IDLE' state?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Firewall rules blocking UDP port 179
Option A is correct because BGP uses TCP port 179 for establishing and maintaining sessions. If a firewall blocks UDP port 179, it would not affect BGP directly, as BGP relies on TCP, not UDP. However, the question states 'UDP port 179' which is a common misdirection; in reality, blocking TCP port 179 would cause the BGP session to go to IDLE state. The IDLE state indicates that the BGP process is not attempting to establish a connection, often due to a failure in the underlying TCP connection.
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.
✓
Firewall rules blocking UDP port 179
Why this is correct
BGP uses TCP 179, not UDP. But if TCP is blocked, session stays IDLE.
Related concept
Read the scenario before looking for a memorised answer.
Read the scenario before looking for a memorised answer.
✗
BGP keepalive timer mismatch
Why it's wrong here
Causes flapping, but session can establish.
✗
Multi-Exit Discriminator (MED) mismatch
Why it's wrong here
MED affects route selection, not session state.
✓
Cloud Router is not configured to establish a BGP session with the on-premises peer
Why this is correct
No peer defined -> IDLE.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that BGP uses UDP port 179, but BGP actually uses TCP port 179; the trap here is that candidates may incorrectly select firewall rules blocking UDP port 179 as a cause, when in reality it is TCP port 179 that matters.
Detailed technical explanation
How to think about this question
BGP session states (IDLE, CONNECT, ACTIVE, OPEN SENT, OPEN CONFIRM, ESTABLISHED) are defined in RFC 4271. The IDLE state is the initial state; the router refuses all incoming BGP connections and does not initiate any. Transitioning out of IDLE requires a Start event (e.g., 'neighbor activate' or 'network' command). Common causes for staying in IDLE include missing neighbor configuration, incorrect IP address, or firewall blocking TCP port 179. In Google Cloud, a Dedicated Interconnect BGP session uses Cloud Router; if the Cloud Router is not configured with the correct peer IP or ASN, the session remains IDLE.
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: Firewall rules blocking UDP port 179 — Option A is correct because BGP uses TCP port 179 for establishing and maintaining sessions. If a firewall blocks UDP port 179, it would not affect BGP directly, as BGP relies on TCP, not UDP. However, the question states 'UDP port 179' which is a common misdirection; in reality, blocking TCP port 179 would cause the BGP session to go to IDLE state. The IDLE state indicates that the BGP process is not attempting to establish a connection, often due to a failure in the underlying TCP connection.
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.
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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