Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
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 choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
Drop the packet without notification
Dropping without notification is not the expected normal behavior when fragmentation is allowed.
B
Best answer
Fragment the packet before forwarding
Correct. The router fragments the packet when DF is not set.
C
Distractor review
Convert the packet into UDP
Routers do not change the transport protocol to solve MTU issues.
D
Distractor review
Forward it unchanged and let the switch fragment it
Switches do not perform IP fragmentation on behalf of routers.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that if a packet exceeds the MTU, the router will drop it regardless of the DF bit setting. Many candidates mistakenly believe fragmentation is not allowed or that switches handle fragmentation. Another common mistake is thinking the router changes the packet’s transport protocol (e.g., converting it to UDP) to fit the MTU. The key is to remember that routers fragment packets only when the DF bit is not set and drop packets with the DF bit set, sending an ICMP message. Misunderstanding this leads to incorrect answers and confusion about how IPv4 fragmentation works in routing.
Technical deep dive
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.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
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?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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