- A
The BGP hold timer is set too high on the customer router.
Why wrong: A high hold timer would not prevent the session from coming up; it would only affect session timeout.
- B
The VLAN ID assigned to the VIF is already in use on another VIF.
Why wrong: Duplicate VLAN ID would cause an error during creation, not BGP session failure.
- C
The BGP authentication key does not match between the customer router and AWS.
If BGP MD5 authentication is configured, both sides must have the same key, or the session will not establish.
- D
The Direct Connect interface is in the 'down' state.
Why wrong: The VIF state is 'available', so the interface is up.
Question 728 of 1,705
BGP Session Failure on Direct Connect Private VIF
This ANS-C01 practice question tests your understanding of network design. 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.
A company is setting up a new AWS Direct Connect connection. They have provisioned a 1 Gbps dedicated connection. They need to create a private virtual interface (VIF) to connect to their VPC. The VIF has been created and is in the 'available' state, but the BGP session is not coming up. What is the most likely cause?
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 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
✓
The BGP authentication key does not match between the customer router and AWS.
The most likely cause is a BGP authentication key mismatch. When the BGP session fails to establish but the virtual interface is in the 'available' state, it indicates the Layer 2 (VLAN) and Layer 1 (physical) connectivity are working. BGP authentication uses MD5 (per RFC 2385), and if the pre-shared key configured on the customer router does not exactly match the key specified in the AWS Direct Connect console for the private VIF, the TCP MD5 signature will fail, preventing the BGP session from coming up.
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 BGP hold timer is set too high on the customer router.
Why it's wrong here
A high hold timer would not prevent the session from coming up; it would only affect session timeout.
- ✗
The VLAN ID assigned to the VIF is already in use on another VIF.
- ✓
The BGP authentication key does not match between the customer router and AWS.
Why this is correct
If BGP MD5 authentication is configured, both sides must have the same key, or the session will not establish.
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.
- ✗
The Direct Connect interface is in the 'down' state.
Why it's wrong here
The VIF state is 'available', so the interface is up.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the distinction between Layer 2 (VIF state) and Layer 3 (BGP session) issues, tricking candidates into thinking a 'down' physical interface or VLAN conflict is the cause when the VIF is already in the 'available' state.
Detailed technical explanation
How to think about this question
BGP authentication in AWS Direct Connect uses TCP MD5 signatures as defined in RFC 2385. The key is configured in the AWS console under the VIF's 'BGP authentication key' field and must match exactly on the customer router (e.g., using the 'password' command under 'neighbor' in Cisco IOS). A common real-world mistake is a typo or case sensitivity mismatch, as the key is case-sensitive. Even if the BGP session fails to establish, the VIF status remains 'available' because the VIF itself is provisioned and the Layer 2 circuit is active; BGP is a separate Layer 3 protocol running over that circuit.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The BGP authentication key does not match between the customer router and AWS. — The most likely cause is a BGP authentication key mismatch. When the BGP session fails to establish but the virtual interface is in the 'available' state, it indicates the Layer 2 (VLAN) and Layer 1 (physical) connectivity are working. BGP authentication uses MD5 (per RFC 2385), and if the pre-shared key configured on the customer router does not exactly match the key specified in the AWS Direct Connect console for the private VIF, the TCP MD5 signature will fail, preventing the BGP session from coming up.
What should I do if I get this ANS-C01 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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Last reviewed: Jul 4, 2026
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