Quick Answer
The answer is to use the ping command with the Don't Fragment (DF) flag set, as this is the definitive tool to test path MTU and identify fragmentation issues. By forcing the packet not to fragment, any link along the path with an MTU smaller than the packet size will drop it and return an ICMP Fragmentation Needed message, allowing you to systematically determine the maximum supported MTU. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept frequently appears in scenarios involving Direct Connect or VPN connectivity, where mismatched MTU values cause silent packet loss—a common trap is assuming the issue is routing or security groups when it is actually a fragmentation problem. A reliable memory tip is to think of the DF flag as a "diagnostic hammer": it breaks the packet on purpose to reveal the weakest link in the path.
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 network engineer is troubleshooting connectivity issues between an EC2 instance in a VPC and an on-premises server over a Direct Connect connection. The engineer has verified that the VPC route tables, Direct Connect virtual interface, and on-premises routing are correctly configured. Which tool should be used to verify the path MTU and identify fragmentation issues?
Use the netstat command
Why wrong: netstat shows connections, not path MTU.
Use the ping command with the DF flag set to test MTU
ping with 'do not fragment' flag can detect MTU issues.
Use the nslookup command
Why wrong: nslookup is for DNS resolution, not MTU.
Use the traceroute command
Why wrong: traceroute shows path but doesn't test MTU effectively.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use the ping command with the DF flag set to test MTU
The ping command with the Don't Fragment (DF) flag set (e.g., `ping -M do -s <size>` on Linux) is the correct tool to verify path MTU because it forces the packet not to be fragmented. If the packet size exceeds the MTU of any link along the path, the router will drop the packet and send an ICMP Fragmentation Needed message back, allowing the engineer to pinpoint the maximum supported MTU and identify fragmentation issues.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Use the netstat command
Why it's wrong here
netstat shows connections, not path MTU.
Use the ping command with the DF flag set to test MTU
Why this is correct
ping with 'do not fragment' flag can detect MTU issues.
Related concept
Read the scenario before looking for a memorised answer.
Use the nslookup command
Use the traceroute command
Why it's wrong here
traceroute shows path but doesn't test MTU effectively.
Common exam traps
The trap here is that candidates often choose traceroute (option D) thinking it shows MTU along the path, but traceroute does not set the DF flag or control payload size to test fragmentation; it only measures hop latency and path, not MTU boundaries.
Trap categories for this question
Command / output trap
netstat shows connections, not path MTU.
Detailed technical explanation
Path MTU discovery (PMTUD) relies on the ICMP Destination Unreachable (Fragmentation Needed) message (Type 3, Code 4) sent by routers when a packet with the DF flag exceeds the next-hop MTU. The ping command with the DF flag and varying packet sizes (e.g., starting at 1472 bytes for a 1500-byte MTU, accounting for 20-byte IP header and 8-byte ICMP header) allows systematic probing. In Direct Connect scenarios, the MTU can be 1500 or 9001 (jumbo frames), and misconfigurations between the VPC, virtual interface, or on-premises router can cause silent packet drops if ICMP is blocked.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Network Management and Operations — This question tests Network Management and Operations — Read the scenario before looking for a memorised answer..
The correct answer is: Use the ping command with the DF flag set to test MTU — The ping command with the Don't Fragment (DF) flag set (e.g., `ping -M do -s <size>` on Linux) is the correct tool to verify path MTU because it forces the packet not to be fragmented. If the packet size exceeds the MTU of any link along the path, the router will drop the packet and send an ICMP Fragmentation Needed message back, allowing the engineer to pinpoint the maximum supported MTU and identify fragmentation issues.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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