The correct answer is that the router will fragment the packet before forwarding. When the DF (Don't Fragment) bit is not set, the IPv4 header explicitly permits fragmentation, allowing the router to split the oversized packet into smaller pieces that match the outgoing interface MTU. This is a fundamental behavior of IPv4 fragmentation: the DF bit controls whether a router can break the packet apart, and when it is cleared, fragmentation is allowed. On the CCNA 200-301 v2 exam, this concept tests your understanding of how routers handle MTU mismatches and the role of the DF bit in path MTU discovery. A common trap is assuming fragmentation always happens—remember, if the DF bit were set, the router would drop the packet and send an ICMP Fragmentation Needed message back to the source. For a quick memory tip: “DF = Don’t Fragment, so if DF is 0, go ahead and chop.”
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: iPv4 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header.. 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 host sends an IPv4 packet larger than the outgoing interface MTU, and the DF bit is not set. What will a router normally do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Fragment the packet before forwarding
If fragmentation is allowed, an IPv4 router can fragment a packet to fit the outgoing interface MTU. If DF were set, the router would instead drop the packet and typically send an ICMP message back to the source.
Key principle: IPv4 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Drop the packet without notification
Why it's wrong here
Dropping without notification is not the expected normal behavior when fragmentation is allowed.
When this WOULD be correct
In a different scenario where the question specifies that the DF bit is set, and the packet exceeds the MTU, the router would drop the packet without notification, making this option correct.
✓
Fragment the packet before forwarding
Why this is correct
Correct. The router fragments the packet when DF is not set.
Related concept
IPv4 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header.
✗
Convert the packet into UDP
Why it's wrong here
Routers do not change the transport protocol to solve MTU issues.
When this WOULD be correct
In a different question, if the scenario involved a network device that specifically translates protocols for application-layer communication, such as a gateway or proxy server, the option could be correct. For example, a question might ask what happens when an application requires UDP but receives TCP packets.
✗
Forward it unchanged and let the switch fragment it
Why it's wrong here
Switches do not perform IP fragmentation on behalf of routers.
When this WOULD be correct
In a different scenario where the question specifies that the packet is being processed by a Layer 2 switch that does not perform IP fragmentation, the option could be correct if the switch is configured to handle larger frames and simply forwards the packet without fragmentation.
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.
✓Fragment the packet before forwardingCorrect answer▾
Why this is correct
Correct. The router fragments the packet when DF is not set.
✗Drop the packet without notificationWrong answer — click to see why▾
Why this is wrong here
A router does not drop the packet without notification when the DF bit is not set; instead, it fragments the packet. Dropping without notification only occurs when the DF bit is set and the packet exceeds the MTU, in which case the router sends an ICMP Fragmentation Needed message.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the DF bit is set, and the packet exceeds the MTU, the router would drop the packet without notification, making this option correct.
Why candidates choose this
Students may confuse this scenario with the case where the DF bit is set, leading to packet drop. They might also think that routers always drop oversized packets, but fragmentation is the default behavior when DF is not set.
✗Convert the packet into UDPWrong answer — click to see why▾
Why this is wrong here
Routers operate at Layer 3 (IP) and do not modify the transport layer protocol (e.g., UDP or TCP) to handle MTU issues. Converting a packet to UDP would change the protocol and is not a function of IP fragmentation or any standard routing behavior.
★ When this WOULD be the correct answer
In a different question, if the scenario involved a network device that specifically translates protocols for application-layer communication, such as a gateway or proxy server, the option could be correct. For example, a question might ask what happens when an application requires UDP but receives TCP packets.
Why candidates choose this
A student might think that changing the protocol could reduce packet size or that UDP is used for fragmentation, but this is incorrect. The confusion may arise from the fact that UDP has a smaller header than TCP, but routers do not perform such conversions.
✗Forward it unchanged and let the switch fragment itWrong answer — click to see why▾
Why this is wrong here
Switches operate at Layer 2 and do not perform IP fragmentation. Fragmentation is a Layer 3 function handled by routers or the source host. The router must fragment the packet before forwarding it to the switch.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the packet is being processed by a Layer 2 switch that does not perform IP fragmentation, the option could be correct if the switch is configured to handle larger frames and simply forwards the packet without fragmentation.
Why candidates choose this
Students might think that switches can handle fragmentation because they forward packets, but switches only forward frames based on MAC addresses and do not process IP headers for fragmentation. The term 'switch' is often misassociated with routing functions.
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
Be careful not to confuse the behavior when the DF bit is set with when it is not set. Remember, fragmentation is allowed when DF is not set.
Detailed technical explanation
How to think about this question
IPv4 packets have a maximum transmission unit (MTU) size that limits the largest packet that can be sent on a network link. When a host sends an IPv4 packet larger than the MTU of the outgoing interface, the router must decide how to handle this oversized packet. The 'Don't Fragment' (DF) bit in the IPv4 header controls whether fragmentation is allowed. If the DF bit is not set, the router can fragment the packet into smaller pieces to fit the MTU before forwarding it to the next hop.
The router examines the DF bit to determine its action. If the DF bit is clear (not set), the router fragments the packet into smaller fragments that comply with the MTU size of the outgoing interface. Each fragment is forwarded independently, and the destination host reassembles the fragments. If the DF bit is set, the router cannot fragment the packet and must drop it, sending an ICMP 'Fragmentation Needed' message back to the source. This behavior is fundamental to IPv4 routing and fragmentation rules and is tested in the CCNA exam.
A common exam trap is confusing fragmentation behavior with packet dropping or assuming switches perform fragmentation. Switches operate at Layer 2 and do not fragment IP packets; only routers perform fragmentation. Another trap is thinking the router changes the transport protocol to solve MTU issues, which it does not. Understanding that routers fragment packets only when the DF bit is not set and drop packets otherwise is critical for correctly answering related CCNA questions and for practical network troubleshooting.
KKey Concepts to Remember
IPv4 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header.
When the DF bit is set, routers drop oversized IPv4 packets and send an ICMP 'Fragmentation Needed' message back to the source host.
Switches operate at Layer 2 and do not perform IP fragmentation; fragmentation is exclusively a router function in IPv4 networks.
Routers do not modify the transport layer protocol (such as converting TCP to UDP) to handle MTU size limitations.
Fragmented IPv4 packets are reassembled only at the destination host, not by intermediate routers.
The MTU size defines the maximum packet size that can be transmitted on a network link without fragmentation.
Understanding the DF bit’s role is essential for troubleshooting IPv4 packet delivery issues related to MTU and fragmentation.
ICMP messages generated due to fragmentation issues help the source host adjust packet sizes for successful delivery.
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 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header.
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 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header. 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.
Review iPv4 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — IPv4 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header..
What is the correct answer to this question?
The correct answer is: Fragment the packet before forwarding — If fragmentation is allowed, an IPv4 router can fragment a packet to fit the outgoing interface MTU. If DF were set, the router would instead drop the packet and typically send an ICMP message back to the source.
What should I do if I get this 200-301 question wrong?
Review iPv4 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
IPv4 routers fragment packets that exceed the outgoing interface MTU only if the 'Don't Fragment' (DF) bit is not set in the packet header.
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