The answer is to advertise a more specific prefix over Direct Connect and a less specific prefix over VPN, while setting a higher MED value on the VPN routes. This works because BGP always prefers the most specific route match, so the /24 prefix over Direct Connect will be chosen over the /23 prefix from the VPN, and the higher MED on the VPN routes further ensures Direct Connect is preferred when both paths are available. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of BGP route preference and failover design, often appearing as a trap where candidates mistakenly rely on AS_PATH length or local preference instead of prefix length and MED. A common memory tip is “specificity first, then MED” — think of the more specific route as the “direct” path, and MED as the tiebreaker that pushes traffic away from the VPN.
ANS-C01 Network Design Practice Question
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 runs a critical application on Amazon EC2 instances in a VPC. The application receives data from an on-premises data center over an AWS Direct Connect connection. The company wants to add redundant connectivity using a VPN connection over the internet. They need to ensure that traffic from on-premises to AWS uses the Direct Connect connection when it is healthy, and only fails over to the VPN if Direct Connect fails. Which configuration achieves this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Advertise a more specific prefix over Direct Connect and a less specific prefix over VPN, and set a higher MED value on the VPN routes.
Option A is correct. By advertising a more specific prefix (e.g., /24) over Direct Connect and a less specific prefix (e.g., /23) over VPN, BGP will prefer the more specific route. Additionally, setting a higher MED value on the VPN routes makes Direct Connect routes preferred. Option B is incorrect because longer AS_PATH in BGP makes a route less preferred. Option C is incorrect because a lower local preference makes a route less preferred. Option D is incorrect because a higher weight makes a route more preferred, which would prefer VPN.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Advertise a more specific prefix over Direct Connect and a less specific prefix over VPN, and set a higher MED value on the VPN routes.
Why this is correct
BGP prefers more specific prefixes. Higher MED on VPN makes Direct Connect routes preferred when both are available.
Related concept
Static NAT maps one inside address to one outside address.
✗
Advertise the same prefix over both connections but prepend AS_PATH multiple times on the Direct Connect routes.
Why it's wrong here
Longer AS_PATH makes a route less preferred, so this would prefer VPN over Direct Connect.
✗
Set a higher BGP weight on the VPN routes.
Why it's wrong here
Higher weight makes a route more preferred, so this would prefer VPN.
✗
Set a lower BGP local preference on the Direct Connect routes.
Why it's wrong here
Lower local preference makes a route less preferred, so this would prefer VPN.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related ANS-C01 NAT questions on configuration and troubleshooting.
Network Design — This question tests Network Design — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Advertise a more specific prefix over Direct Connect and a less specific prefix over VPN, and set a higher MED value on the VPN routes. — Option A is correct. By advertising a more specific prefix (e.g., /24) over Direct Connect and a less specific prefix (e.g., /23) over VPN, BGP will prefer the more specific route. Additionally, setting a higher MED value on the VPN routes makes Direct Connect routes preferred. Option B is incorrect because longer AS_PATH in BGP makes a route less preferred. Option C is incorrect because a lower local preference makes a route less preferred. Option D is incorrect because a higher weight makes a route more preferred, which would prefer VPN.
What should I do if I get this ANS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related ANS-C01 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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