Troubleshooting BGP Session Flaps with Graceful Restart

PCNE By islever Editorial Last updated

Configuring a site-to-site IPSec VPN. Maintaining and troubleshooting connectivity issues.

Short answer

C Ensure that graceful restart is enabled on the on-premises router to preserve the routing table and stabilize BGP sessions experiencing frequent flaps.

Question

Your organization's on-premises networking team is reporting frequent BGP session flaps toward your Google Cloud environment. You need to review the BGP configuration. What should you do?
  1. Switch to static routing.
  2. Increase the BGP hold timer to 36000 seconds max.
  3. Ensure that graceful restart is enabled on the on-premises router. Correct Answer
  4. Ask the on-premises team to enable Bidirectional Forwarding Detection (BFD).

Key Takeaways

The main trap is confusing BFD with Graceful Restart; BFD detects faults faster and worsens flaps, whereas Graceful Restart maintains stability during transient disruptions.

To resolve BGP session flaps in a hybrid cloud setup, enable Graceful Restart on the on-premises router. This preserves the forwarding table during session resets, unlike BFD which can accelerate drops.

Common mistake: Choosing D (Enable BFD) is the most common mistake. Candidates often associate BFD with 'better' connectivity monitoring, not realizing that its sub-second detection sensitivity makes it counterproductive for flapping sessions.

Expert Analysis

The scenario describes BGP session flaps, which are rapid, repeated state changes between established and idle states. The goal is to review the configuration to stabilize these sessions.

Graceful Restart (Option C) is the correct solution. According to Google Cloud documentation for troubleshooting BGP sessions, enabling Graceful Restart on the on-premises router allows the router to preserve its forwarding table while the BGP session resets. This prevents traffic blackholing during brief disruptions and stabilizes the session by ensuring that routing information is not lost when the neighbor drops and re-establishes the connection. This directly addresses the instability caused by flapping.

Bidirectional Forwarding Detection (BFD) (Option D) is a common trap. While BFD is excellent for fast failure detection, it accelerates the detection of link failures. In a flapping scenario, BFD would cause the BGP session to drop faster at the slightest hint of trouble, potentially exacerbating the flapping issue rather than resolving it. Comments in the source data correctly identify that BFD is the "opposite" of what is needed here because it reduces tolerance for transient errors.

Static Routing (Option A) is not a review of BGP configuration but a complete architectural change, which is disproportionate and likely inappropriate if dynamic routing is required.

Hold Timer (Option B) is incorrect because setting a hold timer to 36000 seconds (10 hours) is unrealistic and violates standard BGP operational practices; it does not address the root cause of the flap.

Official Reference

https://cloud.google.com/network-connectivity/docs/router/support/troubleshoot-bgp-sessions#on-prem-router-experiences-bgp-flap

This link opens the official vendor documentation in a new tab.

Exam Strategy

When troubleshooting BGP flaps, distinguish between fault detection mechanisms and fault tolerance mechanisms. Use Graceful Restart or Keepalive tuning to tolerate transient issues. Reserve BFD only for scenarios where you need immediate, precise failure detection on stable links. If the question mentions 'flapping' or 'instability', avoid options that increase detection speed like BFD.

Frequently Asked Questions

Why is BFD bad for BGP flapping?

BFD detects failures in milliseconds. If a link is unstable, BFD will trigger a session drop almost instantly, making the BGP flap worse instead of allowing time for recovery.

What does Graceful Restart do?

It allows a router to retain its forwarding table while a BGP session resets, preventing traffic loss during brief network disruptions or configuration changes.

More PCNE FAQ →

Community Votes

C
50%
D
50%

50% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 9 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 9

  1. glb2 👍 2 Answered D
    BFD (Bidirectional Forwarding Detection) is a lightweight protocol designed to detect faults between two forwarding engines connected by a link. It works independently of routing protocols like BGP but can be used to accelerate BGP failure detection.
  2. RKS_2021 👍 2 Answered C
    https://cloud.google.com/network-connectivity/docs/router/support/troubleshoot-bgp-sessions
  3. 0d2fdfa 👍 1 Answered C
    C is the correct answer Bidirectional Forwarding Detection (BFD) is actually the opposite of what you want here. BFD is designed for sub-second failure detection. If you have a flapping session, enabling BFD would make the router drop the session even faster at the slightest hint of trouble, potentially making the flapping worse. A & B are out of question.
  4. researched_answer_boi 👍 1 Answered C
    Option "C", when applied during configuration review, would solve the issue according to https://docs.cloud.google.com/network-connectivity/docs/router/support/troubleshoot-bgp-sessions#on-prem-router-experiences-bgp-flap.
  5. RKS_2021 👍 1
    C is the solution, but question is about review the configuration, therefore D is correct.
Show 4 more comments
  1. ulkerra 👍 1 Answered C
    A graceful restart feature will help the router to preserve routing table.
  2. RKS_2021 👍 1 Answered D
    question is about review the configuration, therefore D is correct.
  3. 1f01b87 👍 1 Answered D
    D is the correct answer
  4. n2183712847 👍 1 Answered D
    Feel like it is D

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