Dedicated Interconnect: BFD for Consistent BGP Route Exchange
A company is setting up a Dedicated Interconnect connection between their on-premises network and Google Cloud. They have configured a VLAN attachment and assigned a Cloud Router with BGP sessions. They notice that traffic is being dropped intermittently. The BGP session status shows 'Established' but routes are not being exchanged consistently. What is the most likely cause?
Quick Answer
The answer is that Bidirectional Forwarding Detection (BFD) is not enabled on the BGP session. Without BFD, the BGP session relies on its default keepalive timers—typically 60 seconds—to detect link failures, which is far too slow to catch transient issues like micro-bursts or brief packet loss. This delay causes routes to be intermittently withdrawn and re-advertised, leading to dropped traffic even though the BGP state shows 'Established'. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how BFD provides sub-second failure detection (default 300ms) to ensure consistent BGP route exchange over Dedicated Interconnect. A common trap is assuming an 'Established' BGP session guarantees stable routing; in reality, BFD is the missing piece for fast convergence. Memory tip: think "BFD = BGP Failure Detection"—without it, your routes are blind to quick hiccups.
⚠ Common exam trap
Google Cloud often tests the misconception that a BGP session in 'Established' state guarantees stable route exchange, but the trap here is that BFD is required for fast failure detection in cloud interconnect scenarios, and its absence causes intermittent route flapping that does not break the BGP session itself.
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
✓
Bidirectional Forwarding Detection (BFD) is not enabled on the BGP session
When BFD is not enabled on a BGP session, the BGP keepalive timers (typically 60 seconds) are used to detect failures, which can cause intermittent traffic drops because BGP does not detect link failures quickly enough. With BFD enabled (default interval of 300ms), failures are detected in sub-seconds, preventing route flapping and ensuring consistent route exchange. The 'Established' BGP state with inconsistent route exchange is a classic symptom of BFD being absent, as routes may be withdrawn and re-advertised due to transient link issues that BGP alone cannot react to fast enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Bidirectional Forwarding Detection (BFD) is not enabled on the BGP session
Why this is correct
Without BFD, BGP may remain Established while the data plane is down, causing dropped traffic.
- ✗
The on-premises firewall is blocking BGP port 179
Why it's wrong here
Blocking BGP would prevent the session from establishing, not cause intermittent issues.
- ✗
The Cloud Router has reached the maximum number of routes
Why it's wrong here
Route limits cause rejection of new routes, not intermittent drops of existing traffic.
- ✗
The MTU on the VLAN attachment is set too low
Why it's wrong here
MTU issues cause packet drops or fragmentation, not BGP route exchange inconsistency.
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Same concept, more angles
1 more way this is tested on PCNE
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a Dedicated Interconnect connection between their on-premises data center and Google Cloud. They are experiencing intermittent connectivity issues on a specific VLAN attachment. The VLAN attachment is configured with a single Cloud Router and BGP sessions are established. Which two steps should they take to troubleshoot the issue? (Choose two.)
medium- ✓ A.Verify the BGP session status on the Cloud Router.
- B.Check the MTU configuration on the on-premises router.
- C.Verify the physical interconnect location and cable connections.
- ✓ D.Review the VLAN attachment status in the Google Cloud Console.
- E.Review the Cloud NAT configuration for the VPC network.
Why A: BGP session status on the Cloud Router directly indicates whether the routing protocol is functioning correctly between the on-premises router and Google Cloud. Intermittent connectivity on a VLAN attachment often stems from BGP flapping or session drops, which can be verified by checking the BGP state (e.g., Established, Idle, or Active) and associated error counters in the Google Cloud Console or via gcloud commands.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.