- A
Deploy a NAT Gateway in each VPC to force symmetric traffic.
Why wrong: NAT Gateways do not control routing between VPCs and on-premises.
- B
Use AS_PATH prepending on the BGP advertisements from the on-premises router to influence route preference.
AS_PATH prepending makes one path less preferred, ensuring symmetric routing.
- C
Create a Transit Gateway peering attachment between the two VPCs.
Why wrong: Peering attachments connect Transit Gateways, not VPCs directly, and do not resolve asymmetric routing.
- D
Enable VPC Flow Logs to detect and alert on asymmetric flows.
Why wrong: Flow Logs are monitoring tools, not routing controls.
Question 1,367 of 1,705
Resolve Asymmetric Routing with AS_PATH Prepending
This ANS-C01 practice question tests your understanding of network management and operations. 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 multinational corporation is using AWS Transit Gateway to connect multiple VPCs and on-premises networks via AWS Direct Connect and VPN. The network team is experiencing asymmetric routing for traffic between two VPCs that both have routes to the same on-premises network. Which feature should the team implement to resolve this issue?
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
✓
Use AS_PATH prepending on the BGP advertisements from the on-premises router to influence route preference.
AS_PATH prepending allows the on-premises router to artificially lengthen the AS_PATH for specific BGP routes, making those routes less preferred. This influences route selection in the Transit Gateway and VPC route tables, ensuring that traffic from each VPC takes a consistent path and eliminating asymmetric routing.
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.
- ✗
Deploy a NAT Gateway in each VPC to force symmetric traffic.
Why it's wrong here
NAT Gateways do not control routing between VPCs and on-premises.
- ✓
Use AS_PATH prepending on the BGP advertisements from the on-premises router to influence route preference.
Why this is correct
AS_PATH prepending makes one path less preferred, ensuring symmetric routing.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create a Transit Gateway peering attachment between the two VPCs.
Why it's wrong here
Peering attachments connect Transit Gateways, not VPCs directly, and do not resolve asymmetric routing.
- ✗
Enable VPC Flow Logs to detect and alert on asymmetric flows.
Why it's wrong here
Flow Logs are monitoring tools, not routing controls.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse AS_PATH prepending with a general routing policy tool, but the key is that it directly influences BGP best-path selection to break ties and enforce path preference, which is exactly what is needed to fix asymmetric routing in a multi-homed Transit Gateway design.
Detailed technical explanation
How to think about this question
BGP route selection uses the shortest AS_PATH as one of the tie-breakers; by prepending additional AS numbers to routes advertised from one of the on-premises connections, the Transit Gateway will prefer the other path. In a real-world scenario with two Direct Connect connections to the same on-premises network, AS_PATH prepending can be applied selectively per VRF or per prefix to steer traffic without changing the underlying physical topology.
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
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Management and Operations — This question tests Network Management and Operations — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use AS_PATH prepending on the BGP advertisements from the on-premises router to influence route preference. — AS_PATH prepending allows the on-premises router to artificially lengthen the AS_PATH for specific BGP routes, making those routes less preferred. This influences route selection in the Transit Gateway and VPC route tables, ensuring that traffic from each VPC takes a consistent path and eliminating asymmetric routing.
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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Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways 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 network engineer is designing a hybrid network with AWS Direct Connect and a VPN backup. The company has multiple VPCs connected to an AWS Transit Gateway. The on-premises network advertises the same prefixes over both Direct Connect (via private VIF) and VPN (via BGP). The engineer wants to ensure that traffic from the VPCs to on-premises prefers the Direct Connect path. What should the engineer do?
medium- A.Set a higher local preference on the Transit Gateway for Direct Connect routes
- B.Decrease the AS path length on the Direct Connect BGP advertisements
- ✓ C.Configure AS path prepending on the VPN BGP advertisements from on-premises
- D.Set a higher MED on the Direct Connect BGP advertisements
Why C: To prefer the Direct Connect path, the engineer should make the VPN path less preferred in BGP path selection. AS path prepending on the VPN BGP advertisements artificially increases the AS path length, making the VPN route less preferred compared to the Direct Connect route (which has a shorter AS path). Option A is incorrect because local preference is a BGP attribute typically set on the receiving router within an AS, not on the Transit Gateway; the Transit Gateway does not set local preference for routes received from on-premises. Option B is incorrect because you cannot decrease the AS path length below the actual path; the Direct Connect path already has the minimum path length. Option D is incorrect because setting a higher MED (Multi-Exit Discriminator) on the Direct Connect advertisements would make them less preferred (lower MED is better), which is the opposite of what is desired.
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Last reviewed: Jun 11, 2026
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