The correct answer is to verify that route propagation is enabled on the VPC route tables. Even when BGP sessions are established and the VPN tunnels are up, the routes learned from the on-premises network via BGP will not be installed into the VPC route tables unless route propagation is explicitly turned on for those tables. This is a common oversight because engineers often assume that an established BGP session automatically populates the routing table, but AWS requires this manual step to add the propagated routes. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the separation between BGP peering and route table integration, frequently appearing as a trap where candidates focus on tunnel status or static routes instead. A helpful memory tip is "BGP up does not mean routes in the table—propagation is the missing cable."
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 VPC with an AWS Site-to-Site VPN connection to their on-premises network. The VPN uses dynamic routing with BGP. The on-premises network is advertising a specific route to the VPC. However, instances in the VPC cannot reach the on-premises network. The VPN tunnels are up and BGP sessions are established. What should the engineer check?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Route propagation is enabled on the VPC route tables.
Option C is correct because even if BGP routes are received, they must be propagated to the route tables. If route propagation is not enabled, the routes are not added. Option A is incorrect; static routes are not necessary for dynamic routing. Option B is incorrect because the VPN connection routes are already configured. Option D is incorrect because the tunnels are up, so the configuration is likely correct.
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 tunnel options include the correct encryption algorithms.
Why it's wrong here
Tunnels are up, so encryption is fine.
✗
The on-premises router is advertising the route with the correct ASN.
Why it's wrong here
BGP session is established, so ASN is likely correct.
✓
Route propagation is enabled on the VPC route tables.
Why this is correct
Without route propagation, BGP routes are not added to route tables.
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: Route propagation is enabled on the VPC route tables. — Option C is correct because even if BGP routes are received, they must be propagated to the route tables. If route propagation is not enabled, the routes are not added. Option A is incorrect; static routes are not necessary for dynamic routing. Option B is incorrect because the VPN connection routes are already configured. Option D is incorrect because the tunnels are up, so the configuration is likely correct.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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