Troubleshooting Direct Connect Private VIF: VPC to On-Premises Connectivity
A company is implementing a hybrid network using AWS Direct Connect. They have a virtual private gateway (VGW) attached to their VPC and a Direct Connect gateway (DXGW) with a private virtual interface (VIF) to their on-premises router. They have established a BGP session between the on-premises router and the VGW. The on-premises network can reach EC2 instances in the VPC, but the VPC instances cannot reach on-premises resources. What is the most likely cause?
Quick Answer
The answer is a missing route in the VPC route table for the on-premises CIDR pointing to the virtual private gateway. When troubleshooting a Direct Connect private VIF return route missing issue, the core concept is that AWS requires an explicit route in the VPC’s route table to direct return traffic back to the on-premises network via the VGW; without it, the VPC instances have no path to respond, even though the BGP session is established and inbound traffic from on-premises succeeds. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of asymmetric routing and the distinction between BGP route propagation and VPC route table configuration—a common trap is assuming that a working BGP session automatically creates the return path. Remember the memory tip: “BGP brings routes in, but the VPC route table lets them out.”
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 VPC route tables lack a route for the on-premises CIDR pointing to the virtual private gateway
For instances in a VPC to reach on-premises resources over Direct Connect, the VPC route tables must have a route for the on-premises CIDR that points to the virtual private gateway (VGW). Without this route, traffic from the VPC to the on-premises network will not be forwarded to the VGW, even though the BGP session is established and the on-premises side can reach the VPC. Option A is incorrect because the VGW is attached to the VPC (otherwise on-premises could not reach EC2 instances). Option B is incorrect because a VPC endpoint for S3 does not cause routing conflicts for on-premises traffic. Option D is incorrect because the BGP session is established and the on-premises CIDR is being advertised (otherwise on-premises could not reach the VPC).
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 virtual private gateway is not attached to the VPC
Why it's wrong here
The VGW is attached (given), so this is not the issue.
- ✗
The VPC has a VPC endpoint for S3 that is causing a routing conflict
Why it's wrong here
VPC endpoints do not interfere with on-premises routing.
- ✓
The VPC route tables lack a route for the on-premises CIDR pointing to the virtual private gateway
Why this is correct
Without a route, VPC instances do not know to send traffic to the VGW for on-premises destinations.
- ✗
The BGP session is not advertising the on-premises CIDR to the VGW
Why it's wrong here
Given that BGP is established and on-premises can reach VPC, the route is likely advertised, but VPC routing is missing.
Visual reference
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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Same concept, more angles
1 more way this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a Direct Connect connection with a single private virtual interface (VIF) to a virtual private gateway (VGW) attached to a VPC. The VPC CIDR is 10.0.0.0/16. The on-premises CIDR is 172.16.0.0/12. The BGP session is established, and the on-premises router is advertising the 172.16.0.0/12 route to the VGW. The VGW is configured to propagate routes to the VPC route tables. However, instances in the VPC cannot reach on-premises resources. The VPC route table shows a propagated route for 172.16.0.0/12 with a target of the VGW. What is the most likely issue?
medium- A.The security groups for the VPC instances do not allow outbound traffic to the on-premises network
- B.The VPC route table does not have a route for the on-premises CIDR
- ✓ C.The on-premises router does not have a route to the VPC CIDR via the Direct Connect interface
- D.The BGP session is not advertising the VPC CIDR to the on-premises router
Why C: The on-premises router must also have a route back to the VPC CIDR via the Direct Connect interface for traffic to flow bidirectionally. Option A is wrong because security groups are stateful, so outbound traffic is automatically allowed. Option B is wrong because the VPC route table does contain the propagated route for 172.16.0.0/12. Option D is wrong because the BGP session is established and the VGW does not need to advertise the VPC CIDR for outbound traffic; the problem is the lack of a return route on the on-premises router.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.