- A
The instance becomes unreachable from on-premises.
Why wrong: On-premises still has connectivity.
- B
Return traffic from the instance to on-premises is routed through Direct Connect instead of staying within the VPC.
More specific route is preferred.
- C
Traffic destined to the instance is blackholed.
Why wrong: Traffic will still flow but via on-premises.
- D
Traffic is load-balanced between the VPC and Direct Connect.
Why wrong: No load balancing; route selection is deterministic.
Question 588 of 1,705
Specific BGP Route Overlapping VPC CIDR: Asymmetric Traffic Impact
This ANS-C01 practice question tests your understanding of network implementation. 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 has a Direct Connect connection with a private VIF to a VPC. The on-premises network uses BGP to advertise its prefixes. Recently, the on-premises router started advertising a more specific route (10.0.0.0/24) that overlaps with the VPC's CIDR (10.0.0.0/16). What is the impact on traffic destined to 10.0.0.5?
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
✓
Return traffic from the instance to on-premises is routed through Direct Connect instead of staying within the VPC.
When an on-premises router advertises a more specific route (10.0.0.0/24) via BGP over a Direct Connect private VIF, the VPC propagates this route into its route tables. For traffic destined to 10.0.0.5, the VPC now has a more specific route (10.0.0.0/24) pointing to the virtual private gateway (VGW), overriding the local VPC route (10.0.0.0/16). This causes return traffic from the instance to on-premises to be routed through Direct Connect instead of staying within the VPC, as the VPC's route table prefers the longest prefix match.
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 instance becomes unreachable from on-premises.
Why it's wrong here
On-premises still has connectivity.
- ✓
Return traffic from the instance to on-premises is routed through Direct Connect instead of staying within the VPC.
Why this is correct
More specific route is preferred.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Traffic destined to the instance is blackholed.
Why it's wrong here
Traffic will still flow but via on-premises.
- ✗
Traffic is load-balanced between the VPC and Direct Connect.
Why it's wrong here
No load balancing; route selection is deterministic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that a more specific BGP route will cause the instance to become unreachable or blackholed, but the actual impact is asymmetric routing where return traffic is forced out through Direct Connect.
Detailed technical explanation
How to think about this question
Under the hood, AWS VPC route tables use the longest prefix match algorithm, so a /24 route always wins over a /16 route regardless of the target (e.g., local vs. VGW). This behavior is consistent with RFC 4632 and can lead to asymmetric routing if the on-premises network does not have a corresponding route back to the VPC. In real-world scenarios, this often occurs when on-premises administrators inadvertently advertise overlapping prefixes, causing traffic to hairpin through Direct Connect and incurring additional latency and costs.
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Return traffic from the instance to on-premises is routed through Direct Connect instead of staying within the VPC. — When an on-premises router advertises a more specific route (10.0.0.0/24) via BGP over a Direct Connect private VIF, the VPC propagates this route into its route tables. For traffic destined to 10.0.0.5, the VPC now has a more specific route (10.0.0.0/24) pointing to the virtual private gateway (VGW), overriding the local VPC route (10.0.0.0/16). This causes return traffic from the instance to on-premises to be routed through Direct Connect instead of staying within the VPC, as the VPC's route table prefers the longest prefix match.
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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Last reviewed: Jul 4, 2026
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