CCNA Network Infrastructure and Connectivity Practice Question
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?
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
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 the packet without sending an ICMP error message is not the normal behavior when fragmentation is permitted, and it would leave the sender unaware of the loss. If the DF bit were set and the packet exceeded the outgoing MTU, the router would send ICMP Destination Unreachable, Fragmentation Needed (Type 3, Code 4) back to the source; but when DF=0, fragmentation is performed instead. A silent drop would be a misconfiguration, not a valid routing decision.
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
When the outgoing interface has a smaller MTU than the incoming packet and the IP header's Do Not Fragment (DF) bit is 0, the router correctly fragments the packet into smaller Layer 3 pieces that each fit the outgoing MTU. Each fragment gets its own IP header with the same identification field, a fragment offset, and the More Fragments flag set as needed, allowing the destination host to reassemble the original packet. This is the standard expected IPv4 behavior for an intermediate router.
- ✗
Convert the packet into UDP
Why it's wrong here
Converting the transport layer protocol to UDP would not solve an IP MTU issue because IP fragmentation operates at Layer 3, independent of whether TCP or UDP carries the payload. Moreover, a router cannot silently rewrite the transport header without breaking end-to-end protocol state, and UDP would still be subject to the same 1500-byte MTU limit. Thus, this option is invalid and would corrupt the communication.
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
A switch is a Layer 2 device that forwards frames based on MAC addresses, and it cannot perform IP fragmentation because that would require modifying the Layer 3 packet content inside the frame. If the router forwarded the oversized packet unchanged to the switch, the frame would simply exceed the outgoing interface's MTU and be dropped by the switch or the next device. IPv4 fragmentation is exclusively the responsibility of Layer 3 devices such as routers at the point where a packet crosses an MTU boundary.
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
When the outgoing interface has a smaller MTU than the incoming packet and the IP header's Do Not Fragment (DF) bit is 0, the router correctly fragments the packet into smaller Layer 3 pieces that each fit the outgoing MTU. Each fragment gets its own IP header with the same identification field, a fragment offset, and the More Fragments flag set as needed, allowing the destination host to reassemble the original packet. This is the standard expected IPv4 behavior for an intermediate router.
✗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?”
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
Go deeper
Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
Internet Protocol
Internet Protocol (IP) is the set of rules that governs how data is addressed, routed, and sent from one device to another across networks, including the internet.
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.