ANS-C01 Network Design Practice Question
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?
⚠ Common exam trap
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.
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.
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.
Why it's wrong here
Duplicate VLAN ID would cause an error during creation, not BGP session failure.
- ✓
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.
- ✗
The Direct Connect interface is in the 'down' state.
Why it's wrong here
The VIF state is 'available', so the interface is up.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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