The answer is to advertise more specific prefixes over Direct Connect and less specific prefixes over the VPN. This works because BGP route selection always prefers the longest prefix match, so a /24 advertised via Direct Connect will be chosen over a /16 from the VPN, fixing asymmetric routing by forcing all traffic to use Direct Connect when available. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of BGP path selection and how to manipulate routing without removing the VPN backup—a common trap is thinking you need to disable the VPN or adjust AS_PATH prepending, but prefix specificity is simpler and more reliable. Remember the memory tip: "Specific wins the path"—the more precise the subnet mask, the more preferred the route, ensuring Direct Connect handles the traffic.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 using AWS Direct Connect to connect its on-premises data center to AWS. The company has a single hosted virtual interface (VIF) with a private VIF to a VPC. The network team notices that traffic from on-premises to AWS is asymmetric—some packets go through the Direct Connect while others use a VPN backup. The team wants all traffic to use Direct Connect when available. What should they do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Advertise more specific prefixes (e.g., /24) over Direct Connect and less specific (e.g., /16) over VPN.
Option D is correct because BGP route selection prefers more specific prefixes (longest prefix match) over less specific ones. By advertising a more specific prefix (e.g., /24) over Direct Connect and a less specific prefix (e.g., /16) over the VPN, the on-premises router will prefer the Direct Connect path for the more specific route, ensuring all traffic uses Direct Connect when available. This leverages BGP's prefix-length-based path selection without requiring removal of the VPN backup.
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.
✗
Disable AS_PATH prepending on the Direct Connect BGP session.
Why it's wrong here
Disabling prepending could make on-premises routes less preferred, not more.
✗
Convert the private VIF to a public VIF.
Why it's wrong here
Changing VIF type does not address route preference.
✗
Remove the VPN connection to force all traffic through Direct Connect.
Why it's wrong here
Removing the VPN eliminates backup, which is not desired.
✓
Advertise more specific prefixes (e.g., /24) over Direct Connect and less specific (e.g., /16) over VPN.
Why this is correct
Correct: More specific BGP routes are preferred, so Direct Connect will be used when available.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume AS_PATH prepending or removing the VPN is the solution, but the key is understanding that BGP's longest prefix match overrides all other path selection attributes, making prefix specificity the most reliable way to enforce traffic flow.
Detailed technical explanation
How to think about this question
BGP route selection uses the longest prefix match rule first; a more specific prefix (e.g., 10.0.0.0/24) is always preferred over a less specific one (e.g., 10.0.0.0/16), regardless of other BGP attributes like AS_PATH or MED. In AWS Direct Connect scenarios, you can control prefix advertisement granularity by advertising specific subnets over Direct Connect and summarizing over the VPN, ensuring the Direct Connect path is always chosen for those subnets. This technique is commonly used in hybrid networking to enforce active/passive routing without relying on BGP attributes that may be overridden by local preferences.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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: Advertise more specific prefixes (e.g., /24) over Direct Connect and less specific (e.g., /16) over VPN. — Option D is correct because BGP route selection prefers more specific prefixes (longest prefix match) over less specific ones. By advertising a more specific prefix (e.g., /24) over Direct Connect and a less specific prefix (e.g., /16) over the VPN, the on-premises router will prefer the Direct Connect path for the more specific route, ensuring all traffic uses Direct Connect when available. This leverages BGP's prefix-length-based path selection without requiring removal of the VPN backup.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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