CCNA IPv4 DF (Don't Fragment) bit Practice Question
This 200-301 practice question tests your understanding of network services and security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: iPv4 DF (Don't Fragment) bit. 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 packet is larger than the outgoing interface MTU and the DF bit is set in the IPv4 header. What should the router do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Drop the packet and send an ICMP message indicating fragmentation was needed.
If fragmentation is required but DF is set, the router drops the packet and returns an ICMP unreachable message indicating fragmentation was needed.
Key principle: IPv4 DF (Don't Fragment) bit
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Fragment the packet anyway and forward all fragments.
Why it's wrong here
The DF bit explicitly forbids fragmentation.
When this WOULD be correct
In a different scenario where the DF bit is not set, a question might ask what a router should do when a packet exceeds the MTU. In that case, the router could fragment the packet and forward all fragments without violating protocol rules.
✓
Drop the packet and send an ICMP message indicating fragmentation was needed.
Why this is correct
Correct. That behavior supports Path MTU Discovery.
Related concept
IPv4 DF (Don't Fragment) bit
✗
Clear the DF bit and then fragment the packet.
Why it's wrong here
Routers do not rewrite DF this way under normal forwarding.
When this WOULD be correct
In a different scenario where the question specifies that the DF bit is not set, a router could clear the DF bit and fragment the packet to fit the MTU of the outgoing interface. This would be a valid action when fragmentation is permissible.
In a different scenario where the question asks about handling packets that need to be tunneled for secure transmission, and the GRE protocol is explicitly mentioned, the correct answer could involve encapsulating packets in GRE regardless of MTU issues, assuming the context allows for such a solution.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Drop the packet and send an ICMP message indicating fragmentation was needed.Correct answer▾
Why this is correct
Correct. That behavior supports Path MTU Discovery.
✗Fragment the packet anyway and forward all fragments.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because if the DF (Don't Fragment) bit is set, the router cannot fragment the packet, and attempting to do so would violate the IP protocol rules.
★ When this WOULD be the correct answer
In a different scenario where the DF bit is not set, a question might ask what a router should do when a packet exceeds the MTU. In that case, the router could fragment the packet and forward all fragments without violating protocol rules.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of how fragmentation works in relation to the DF bit, leading them to incorrectly assume that routers can override the DF setting.
✗Clear the DF bit and then fragment the packet.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because if the DF (Don't Fragment) bit is set, the router is not allowed to fragment the packet. The correct action is to drop the packet and send an ICMP message indicating that fragmentation is needed.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the DF bit is not set, a router could clear the DF bit and fragment the packet to fit the MTU of the outgoing interface. This would be a valid action when fragmentation is permissible.
Why candidates choose this
Candidates may choose this option because they recall that fragmentation is a common network operation and mistakenly believe that routers can modify packet headers to facilitate forwarding, especially under pressure during an exam.
✗Encapsulate the packet in GRE automatically.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because GRE encapsulation does not address the issue of packet fragmentation; it simply adds a layer of encapsulation without resolving MTU constraints. The router cannot automatically encapsulate a packet in GRE when the DF bit is set and the packet exceeds the MTU.
★ When this WOULD be the correct answer
In a different scenario where the question asks about handling packets that need to be tunneled for secure transmission, and the GRE protocol is explicitly mentioned, the correct answer could involve encapsulating packets in GRE regardless of MTU issues, assuming the context allows for such a solution.
Why candidates choose this
Candidates may find this option tempting because GRE is often associated with tunneling and can handle larger packets, leading to confusion about its role in fragmentation scenarios.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common exam trap is selecting option A, assuming the router will fragment the packet despite the DF bit. Remember, the DF (Don't Fragment) bit explicitly prevents fragmentation. Another trap is option C, thinking the router can clear the DF bit and fragment, which routers do not do. Option D is unrelated to MTU handling and can mislead if you confuse GRE tunneling with fragmentation behavior.
Detailed technical explanation
How to think about this question
In IPv4, the DF (Don't Fragment) bit is used to control packet fragmentation by routers. When a router receives a packet larger than the outgoing interface's MTU, it normally fragments the packet to fit the MTU. However, if the DF bit is set, the router must not fragment the packet. Instead, it drops the packet and sends an ICMP Type 3 Code 4 message (Destination Unreachable, Fragmentation Needed and DF set) back to the source. This mechanism supports Path MTU Discovery (PMTUD), allowing the source host to learn the smallest MTU on the path and adjust packet sizes accordingly. Routers do not clear or modify the DF bit to force fragmentation, nor do they encapsulate packets in GRE tunnels automatically to bypass MTU restrictions. Understanding this behavior is critical for troubleshooting connectivity issues related to MTU and fragmentation in IP networks.
KKey Concepts to Remember
IPv4 DF (Don't Fragment) bit
Path MTU Discovery (PMTUD)
ICMP Fragmentation Needed message
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
IPv4 DF (Don't Fragment) bit
Real-world example
How this comes up in practice
A practitioner preparing for the 200-301 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. IPv4 DF (Don't Fragment) bit Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Network Services and Security — This question tests Network Services and Security — IPv4 DF (Don't Fragment) bit.
What is the correct answer to this question?
The correct answer is: Drop the packet and send an ICMP message indicating fragmentation was needed. — If fragmentation is required but DF is set, the router drops the packet and returns an ICMP unreachable message indicating fragmentation was needed.
What should I do if I get this 200-301 question wrong?
Review iPv4 DF (Don't Fragment) bit, then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
IPv4 DF (Don't Fragment) bit
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