The answer is to add the new CIDR block (10.0.3.0/24) to the Direct Connect gateway's allowed prefixes. This is required because the Direct Connect gateway uses an allowed prefixes list to control which VPC CIDRs it advertises to the on-premises router over BGP; simply adding the CIDR to the VPC’s route table does not trigger an automatic update to the gateway’s advertisement. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the explicit prefix authorization mechanism for Direct Connect gateways, often appearing as a trap where candidates mistakenly think updating the VPC route table or checking BGP status alone will fix reachability. A common memory tip is to remember that the Direct Connect gateway acts as a gatekeeper—it only announces prefixes you explicitly allow, so any new VPC CIDR must be manually added to that list. Think of it as "add to allowed, then advertise" to avoid silent routing failures.
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 uses AWS Direct Connect to connect its on-premises data center to a VPC. The connection uses a private virtual interface (VIF) and BGP. The network team recently added a new CIDR block (10.0.3.0/24) to the VPC. They updated the VPC's route table to include a route to the on-premises network. However, the on-premises network cannot reach resources in the new subnet. The BGP session is up, and the Direct Connect gateway is configured. What should the network team do to resolve the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add the new CIDR block (10.0.3.0/24) to the Direct Connect gateway's allowed prefixes.
When adding a new CIDR to a VPC, the Direct Connect gateway does not automatically advertise the new prefix to the on-premises router. The team needs to ensure that the VPC's CIDR is allowed in the Direct Connect gateway's allowed prefixes. Option A is correct because the new CIDR must be added to the allowed prefixes list for the Direct Connect gateway to advertise it via BGP. Option D is incorrect as it would cause traffic to be routed through VPN, not Direct Connect.
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.
✗
Add a static route in the on-premises router pointing to the Direct Connect interface.
Why it's wrong here
Static route would work but does not address the BGP advertisement. The issue is the missing prefix in allowed prefixes.
✗
Set up a new VPN connection as a backup and route traffic over VPN.
Why it's wrong here
Not necessary; the Direct Connect connection is functional, just missing prefix advertisement.
✓
Add the new CIDR block (10.0.3.0/24) to the Direct Connect gateway's allowed prefixes.
Why this is correct
This allows the Direct Connect gateway to advertise the new prefix to the on-premises network via BGP.
A single private VIF can handle multiple CIDRs; no need for a new VIF.
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.
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: Add the new CIDR block (10.0.3.0/24) to the Direct Connect gateway's allowed prefixes. — When adding a new CIDR to a VPC, the Direct Connect gateway does not automatically advertise the new prefix to the on-premises router. The team needs to ensure that the VPC's CIDR is allowed in the Direct Connect gateway's allowed prefixes. Option A is correct because the new CIDR must be added to the allowed prefixes list for the Direct Connect gateway to advertise it via BGP. Option D is incorrect as it would cause traffic to be routed through VPN, not Direct Connect.
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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