ANS-C01 Network Implementation Practice Question
A company has a Direct Connect connection with a private VIF to a VPC. They notice that the BGP session is flapping every few minutes. The network team confirms that the customer router and AWS router are configured correctly. What is the MOST likely cause of the BGP flapping?
⚠ Common exam trap
The ANS-C01 exam often tests the misconception that BGP flapping is always caused by BGP configuration errors (ASN, communities) or missing features like BFD, when in reality Layer 2/3 issues like MTU mismatch are a common root cause in Direct Connect scenarios.
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
✓
MTU mismatch between the customer router and the AWS Direct Connect endpoint
B is correct because an MTU mismatch between the customer router and the AWS Direct Connect endpoint can cause BGP session flapping. When the MTU is mismatched, larger BGP keepalive or update packets may be fragmented or dropped, leading to hold timer expirations and session resets. This occurs even if both routers are configured correctly for BGP, as the issue lies at the Layer 2/3 boundary rather than the BGP configuration itself.
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 BGP community string is not set correctly
Why it's wrong here
Communities affect routing policies, not session stability.
- ✓
MTU mismatch between the customer router and the AWS Direct Connect endpoint
Why this is correct
Packet drops cause BGP session instability.
- ✗
Incorrect BGP ASN configuration on the virtual interface
Why it's wrong here
Would prevent BGP from establishing.
- ✗
Bidirectional Forwarding Detection (BFD) is not enabled
Why it's wrong here
BFD speeds up failure detection, does not cause flapping.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services 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 ANS-C01 exam.