The answer is that each private VIF requires a separate BGP session, and configuring only one session is the most likely reason only one VPC is reachable. This is because multiple private VIFs are logically isolated Layer 2 constructs over the same Direct Connect connection, and each VIF must have its own unique BGP session and VLAN ID to exchange routes with the on-premises router. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of how Direct Connect virtual interfaces operate independently, even when sharing a single physical port. A common trap is assuming a single BGP session can serve multiple VIFs, but the control plane must remain separate per VIF. Remember the memory tip: one VIF, one BGP session — no sharing allowed.
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 has a Direct Connect connection with two private virtual interfaces (VIFs) to two different VPCs. The company wants to use the same Direct Connect connection for both VPCs, but the on-premises router only has one physical port. The network engineer configures a single BGP session over a VLAN-tagged interface. After configuration, only one VPC is reachable. What is the most likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Each private VIF requires a separate BGP session, and only one session was configured
Option C is correct because each private VIF requires a unique BGP session and VLAN ID. Using a single BGP session cannot serve two VIFs because the VIFs are logically separate. Option A is wrong because the same BGP ASN can be used on both VIFs if the Direct Connect gateway is used. Option B is wrong because there is no requirement to use the same VLAN. Option D is wrong because the BGP session can be established with different peer IPs.
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.
✗
The on-premises router is using the same BGP ASN for both VIFs, which is not allowed
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
Common exam traps
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: Each private VIF requires a separate BGP session, and only one session was configured — Option C is correct because each private VIF requires a unique BGP session and VLAN ID. Using a single BGP session cannot serve two VIFs because the VIFs are logically separate. Option A is wrong because the same BGP ASN can be used on both VIFs if the Direct Connect gateway is used. Option B is wrong because there is no requirement to use the same VLAN. Option D is wrong because the BGP session can be established with different peer IPs.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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