The answer is that the operating system's routing table now uses eth1 as the default gateway, causing asymmetric routing. When a second ENI is attached to an EC2 instance, the Linux or Windows OS often reassigns the default route to the newly attached interface, especially if it receives a default gateway from its subnet’s DHCP options. This means incoming SSH traffic arrives via the original eth0’s public IP and IGW, but the instance’s reply traffic egresses through eth1, which lacks a route back to the internet, breaking the connection. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of multi ENI asymmetric routing and the critical distinction between AWS’s source/destination check and the OS-level routing table. A common trap is assuming the security group or subnet route table is the culprit, but the real issue is that the OS ignores AWS’s logical routing when it has its own default gateway. Memory tip: think “two ENIs, two default gateways—the OS picks the last one attached, breaking the return path.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 has a VPC with a CIDR of 172.16.0.0/16. The VPC has two subnets: subnet A (172.16.1.0/24) and subnet B (172.16.2.0/24). The company launches an EC2 instance in subnet A with a private IP of 172.16.1.10 and a public IP. The instance's security group allows inbound SSH from 0.0.0.0/0. The subnet A's route table has a route to an internet gateway (IGW) for 0.0.0.0/0. The network engineer can SSH into the instance from the internet. The company then attaches a second network interface (eth1) to the instance with an IP from subnet B (172.16.2.20). The engineer wants to use this interface for additional management traffic. After attaching, the engineer can no longer SSH into the instance via the original public IP. 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 operating system's routing table now uses eth1 as the default gateway, causing asymmetric routing.
Option D is correct because when you attach a second ENI, the OS may change the default route, causing traffic to egress via the new interface. Option A is incorrect because the security group is already allowed. Option B is incorrect because the route table is fine. Option C is incorrect because there is no IGW in subnet B by default.
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 operating system's routing table now uses eth1 as the default gateway, causing asymmetric routing.
Why this is correct
Asymmetric routing can cause connection issues.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route table for subnet B does not have a route to the IGW.
Why it's wrong here
eth1 is in subnet B, but the issue is with inbound SSH via original IP.
✗
The security group for eth1 blocks inbound SSH.
Why it's wrong here
Security group for eth1 is separate but can be updated.
✗
The internet gateway is not attached to subnet B.
Why it's wrong here
IGW is attached to VPC, not subnet.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The operating system's routing table now uses eth1 as the default gateway, causing asymmetric routing. — Option D is correct because when you attach a second ENI, the OS may change the default route, causing traffic to egress via the new interface. Option A is incorrect because the security group is already allowed. Option B is incorrect because the route table is fine. Option C is incorrect because there is no IGW in subnet B by default.
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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