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Question 1,602 of 1,705

Network Security, Compliance and Governance →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the on-premises network is not advertising VPC-B’s CIDR to the VPC via BGP, so VPC-B lacks a return route to on-premises. This is because VPC peering and on-premises route propagation are separate mechanisms: while the on-premises routers have a static route for VPC-B’s CIDR pointing to the Direct Connect virtual interface, BGP must also advertise that CIDR into the VPC’s route tables for return traffic to flow. Without that BGP advertisement, VPC-B’s instances send replies toward the on-premises network but have no route back, breaking connectivity. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how BGP over Direct Connect interacts with VPC peering—a common trap is assuming static routes on-premises are automatically propagated to the VPC side. Remember: for hybrid connectivity, both directions of route propagation must be explicitly configured; a static route on-premises does not create a BGP advertisement into the VPC. Memory tip: “BGP both ways or no play.”

ANS-C01 Network Security, Compliance and Governance Practice Question

This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 hybrid network architecture with an AWS Direct Connect connection between its on-premises data center and an Amazon VPC. The VPC has a single private subnet with Amazon EC2 instances running a critical application. The on-premises network uses BGP to advertise a route for the VPC's CIDR (10.0.0.0/16) to the on-premises routers. Recently, the company added a new application in a second VPC (VPC-B) with CIDR 10.1.0.0/16 and peered it with the original VPC (VPC-A). After the peering, users on-premises can still reach resources in VPC-A, but cannot reach resources in VPC-B. The VPC-A route table has a route for VPC-B's CIDR pointing to the peering connection. The VPC-B route table has a route for VPC-A's CIDR pointing to the peering connection. The on-premises routers have a static route for VPC-B's CIDR pointing to the Direct Connect virtual interface. What is the most likely cause of the issue?

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.

Question 1hardmultiple choice

Open the full BGP breakdown →

A
The VPC peering connection is not properly configured.
Why wrong: The peering is configured correctly as VPCs can communicate with each other.
B
The on-premises network is not advertising VPC-B's CIDR to the VPC via BGP, so VPC-B does not have a route back to on-premises.
VPC-B needs a route to on-premises CIDR pointing to the virtual private gateway, but that route must be propagated via BGP or added manually.
C
The security group on VPC-B instances is blocking inbound traffic from on-premises IP ranges.
Why wrong: Security groups are stateful and would not block return traffic if outbound is allowed.
D
The virtual private gateway does not have route propagation enabled for VPC-B.
Why wrong: Route propagation is for the VGW to propagate routes to VPC route tables; VPC-B is not attached to the same VGW.

Full breakdown with real-world context →

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

✓

The on-premises network is not advertising VPC-B's CIDR to the VPC via BGP, so VPC-B does not have a route back to on-premises.

Option C is correct. The on-premises network does not know how to reach VPC-B because VPC-B's CIDR is not advertised via BGP over Direct Connect. The static route on on-premises routers is not propagated to the VPC side, so VPC-B does not have a return route to on-premises. Option A is incorrect because the peering connection is working between VPCs. Option B is incorrect because security groups are stateful and would allow return traffic. Option D is incorrect because route propagation for the virtual private gateway does not apply to VPC-B if it is not attached to the same VGW.

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 VPC peering connection is not properly configured.
Why it's wrong here
The peering is configured correctly as VPCs can communicate with each other.
✓
The on-premises network is not advertising VPC-B's CIDR to the VPC via BGP, so VPC-B does not have a route back to on-premises.
Why this is correct
VPC-B needs a route to on-premises CIDR pointing to the virtual private gateway, but that route must be propagated via BGP or added manually.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The security group on VPC-B instances is blocking inbound traffic from on-premises IP ranges.
Why it's wrong here
Security groups are stateful and would not block return traffic if outbound is allowed.
✗
The virtual private gateway does not have route propagation enabled for VPC-B.
Why it's wrong here
Route propagation is for the VGW to propagate routes to VPC route tables; VPC-B is not attached to the same VGW.

Common exam traps

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The on-premises network is not advertising VPC-B's CIDR to the VPC via BGP, so VPC-B does not have a route back to on-premises. — Option C is correct. The on-premises network does not know how to reach VPC-B because VPC-B's CIDR is not advertised via BGP over Direct Connect. The static route on on-premises routers is not propagated to the VPC side, so VPC-B does not have a return route to on-premises. Option A is incorrect because the peering connection is working between VPCs. Option B is incorrect because security groups are stateful and would allow return traffic. Option D is incorrect because route propagation for the virtual private gateway does not apply to VPC-B if it is not attached to the same VGW.

What should I do if I get this ANS-C01 question wrong?

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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