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Question 959 of 1,705

Network Management and Operations →mediumMultiple ChoiceObjective-mapped

Troubleshoot Direct Connect Private VIF Subnet Issues

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.

Network Topology

An engineer is troubleshooting connectivity from on-premises to a VPC via Direct Connect private VIF. The BGP session is up, traffic is flowing, but the on-premises network cannot reach some subnets in the VPC. The VPC CIDR is 10.0.0.0/16. What is the most likely cause based on the exhibit?

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.

Network Topology

A
The BGP session is up, but the on-premises router is not receiving the VPC CIDR route due to missing route propagation on the virtual private gateway.
Why wrong: Route propagation is enabled by default on the VGW, so this is unlikely.
B
The customer router configuration snippet is missing the BGP configuration for those subnets.
Why wrong: The customer router config is not shown in detail.
C
The security groups or network ACLs in the VPC are blocking traffic to those subnets.
Since the VPC CIDR is advertised, reachability issues within the VPC are more likely due to security group or NACL rules.
D
The route filter prefixes only allow the VPC CIDR 10.0.0.0/16, but the VPC has additional CIDRs that are not being advertised.
Why wrong: The question states VPC CIDR is /16, but it could have multiple CIDRs; however, the exhibit shows only one prefix.

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 security groups or network ACLs in the VPC are blocking traffic to those subnets.

The BGP session being up and traffic flowing indicates that routing is properly configured between on-premises and the VPC. The inability to reach only some subnets suggests the issue is at the network security layer within the VPC. Security groups and network ACLs can block traffic to specific subnets while routing remains intact. Therefore, the most likely cause based on the exhibit is that security groups or NACLs are filtering traffic to those subnets.

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 BGP session is up, but the on-premises router is not receiving the VPC CIDR route due to missing route propagation on the virtual private gateway.
Why it's wrong here
Route propagation is enabled by default on the VGW, so this is unlikely.
✗
The customer router configuration snippet is missing the BGP configuration for those subnets.
Why it's wrong here
The customer router config is not shown in detail.
✓
The security groups or network ACLs in the VPC are blocking traffic to those subnets.
Why this is correct
Since the VPC CIDR is advertised, reachability issues within the VPC are more likely due to security group or NACL rules.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The route filter prefixes only allow the VPC CIDR 10.0.0.0/16, but the VPC has additional CIDRs that are not being advertised.
Why it's wrong here
The question states VPC CIDR is /16, but it could have multiple CIDRs; however, the exhibit shows only one prefix.

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.

Trap categories for this question

Command / output trap
The customer router config is not shown in detail.

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.

Visual reference

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 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 security groups or network ACLs in the VPC are blocking traffic to those subnets. — The BGP session being up and traffic flowing indicates that routing is properly configured between on-premises and the VPC. The inability to reach only some subnets suggests the issue is at the network security layer within the VPC. Security groups and network ACLs can block traffic to specific subnets while routing remains intact. Therefore, the most likely cause based on the exhibit is that security groups or NACLs are filtering traffic to those subnets.

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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