The answer is that the on-premises router is not advertising any prefixes over BGP. This is the most likely cause because BGP route advertisement requires the on-premises router to explicitly send its network prefixes to the AWS VPN endpoint; without these advertisements, the VPC has no routes to learn, even though the BGP session itself is established. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that BGP communities are optional and not required for basic route exchange, and that a common trap is assuming the VPN tunnel’s up status guarantees route propagation. When troubleshooting VPN BGP not learning routes on-premises, always verify the router’s network statement or prefix-list configuration first. Memory tip: “No ad, no route” — if the on-premises router doesn’t advertise, the VPC stays empty.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 is migrating its on-premises data center to AWS. The network team needs to establish a site-to-site VPN connection with dynamic routing using BGP. The on-premises router supports BGP but does not support BGP communities. The VPN connection is established, but the VPC does not learn the on-premises routes. What is the most likely cause?
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
✓
The on-premises router is not advertising any prefixes over BGP
Option B is correct because the on-premises router must advertise its routes over the BGP session. Option A is incorrect because BGP communities are not required. Option C is incorrect because the VPN connection is established. Option D is incorrect because BGP does not require static routes.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 VPN tunnel uses pre-shared keys instead of certificates
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The on-premises router is not advertising any prefixes over BGP — Option B is correct because the on-premises router must advertise its routes over the BGP session. Option A is incorrect because BGP communities are not required. Option C is incorrect because the VPN connection is established. Option D is incorrect because BGP does not require static routes.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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Question Discussion
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