The correct answer is to confirm that the BGP ASN configured on the on-premises router matches the one provided by AWS. This is because BGP session establishment relies on a mutual agreement on the Autonomous System Number; if the ASNs do not match, the routers will reject the peering attempt, keeping the session in an idle or active state regardless of physical or VLAN health. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this BGP ASN mismatch scenario is a classic troubleshooting trap, often buried in a question where you have already verified physical connectivity and VLANs, testing your understanding that BGP is a layer-3 protocol independent of IP addressing or VPC CIDRs. A common mistake is to check route propagation or VPC settings, but the core issue is always the ASN handshake. Memory tip: think of BGP as a phone call—if you dial the wrong area code (ASN), the call never connects, no matter how good your signal (physical link) is.
ANS-C01 Network Management and Operations Practice Question
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 company uses AWS Direct Connect to connect its on-premises data center to AWS. The network engineer is troubleshooting connectivity issues and notices that the BGP session between the on-premises router and the AWS Direct Connect virtual interface is down. The engineer has verified the physical connectivity and the VLAN configuration. What should the engineer check next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Confirm that the BGP ASN configured on the on-premises router matches the one provided by AWS.
Option C is correct because BGP session establishment depends on matching the BGP ASN on both sides. If they do not match, the session will not come up. Option A is wrong because CIDR size does not affect BGP. Option B is wrong because the VPC CIDR is not used in the BGP peering process. Option D is wrong because the virtual interface type (private or public) is not the direct cause of BGP failure.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Ensure the virtual interface type is set to 'public'.
Why it's wrong here
Incorrect: Virtual interface type does not directly affect BGP session state.
✗
Verify that the on-premises router is advertising a CIDR block smaller than /24.
Why it's wrong here
Incorrect: CIDR size is not a BGP session requirement.
✓
Confirm that the BGP ASN configured on the on-premises router matches the one provided by AWS.
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related ANS-C01 questions on switching, trunking, and access-port configuration.
Network Management and Operations — This question tests Network Management and Operations — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Confirm that the BGP ASN configured on the on-premises router matches the one provided by AWS. — Option C is correct because BGP session establishment depends on matching the BGP ASN on both sides. If they do not match, the session will not come up. Option A is wrong because CIDR size does not affect BGP. Option B is wrong because the VPC CIDR is not used in the BGP peering process. Option D is wrong because the virtual interface type (private or public) is not the direct cause of BGP failure.
What should I do if I get this ANS-C01 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related ANS-C01 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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