Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
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 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
Fragment the packet anyway and forward all fragments.
The DF bit explicitly forbids fragmentation.
B
Best answer
Drop the packet and send an ICMP message indicating fragmentation was needed.
Correct. That behavior supports Path MTU Discovery.
C
Distractor review
Clear the DF bit and then fragment the packet.
Routers do not rewrite DF this way under normal forwarding.
D
Distractor review
Encapsulate the packet in GRE automatically.
GRE is unrelated to standard MTU handling.
Common exam trap
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.
Technical deep dive
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.
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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 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?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
Discussion
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