Question 1hardmultiple choice
Full question →A host sends a packet larger than the outgoing interface MTU, and the IPv4 header has the Don't Fragment bit set.
What will a router do with the packet?
hardmultiple choiceObjective-mapped
A host sends a packet larger than the outgoing interface MTU, and the IPv4 header has the Don't Fragment bit set.
What will a router do with the packet?
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 it anyway and forward the pieces
The router cannot simply ignore the Don't Fragment bit. That bit exists specifically to prevent fragmentation. If the router fragmented the packet anyway, it would violate the sender's requirement and defeat the whole point of the flag.
B
Best answer
Drop it and typically return an ICMP message indicating fragmentation is needed
Correct. This is correct. When a packet is too large for the outgoing MTU and fragmentation is not allowed, the router drops the packet and usually informs the sender that fragmentation would be required.
C
Distractor review
Translate it to IPv6 and forward it
Routers do not solve an oversized IPv4 packet by converting it into IPv6. These are different protocols with different headers and forwarding behaviors. The issue here is MTU and fragmentation policy, not an opportunity for protocol conversion.
D
Distractor review
Broadcast it so another router can handle fragmentation
Routers do not broadcast oversized packets to let another router solve the problem. The forwarding decision is made locally. If the packet cannot be forwarded because of the MTU and DF setting, it is dropped.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is to assume that routers will fragment any oversized IPv4 packet regardless of the Don't Fragment bit. Candidates might select the option that routers fragment the packet anyway, ignoring the DF bit. This is incorrect because the DF bit explicitly prevents fragmentation. Another tempting mistake is to think that routers might broadcast the packet to other routers or convert it to IPv6 to solve the problem, which does not happen. Understanding that the router must drop the packet and send an ICMP message is critical to avoid this trap.
Technical deep dive
How to think about this question
IPv4 packets include a header flag called the Don't Fragment (DF) bit, which instructs routers along the path not to fragment the packet under any circumstances. Fragmentation is the process of breaking a large IP packet into smaller pieces so that each piece fits within the Maximum Transmission Unit (MTU) size of the outgoing interface. The MTU is the largest packet size that a network link can transmit without needing fragmentation. When a packet exceeds the MTU of the next hop, routers typically fragment the packet to ensure delivery. However, when the DF bit is set, routers are prohibited from fragmenting the packet. If the packet size exceeds the MTU of the outgoing interface, the router must drop the packet and send an ICMP Type 3 Code 4 message (Destination Unreachable, Fragmentation Needed and DF Set) back to the sender. This behavior is fundamental to Path MTU Discovery (PMTUD), a mechanism that allows the sender to learn the smallest MTU along the path and adjust packet sizes accordingly to avoid fragmentation. A common exam trap is to assume that routers will fragment packets regardless of the DF bit setting. This is incorrect because the DF bit explicitly forbids fragmentation. Another misconception is that routers might broadcast or convert the packet to IPv6 to solve the issue, which is not true. In practice, the router drops the packet and notifies the sender, prompting the sender to reduce packet size. This mechanism ensures efficient network operation and prevents fragmentation-related performance issues.
KKey Concepts to Remember
- The Don't Fragment (DF) bit in the IPv4 header instructs routers not to fragment the packet under any circumstances.
- Routers compare the packet size against the outgoing interface MTU to decide if fragmentation is needed.
- If the packet exceeds the MTU and the DF bit is set, the router drops the packet and sends an ICMP fragmentation-needed message to the sender.
- Path MTU Discovery relies on the DF bit and ICMP messages to dynamically determine the smallest MTU along the path.
- Routers do not fragment packets when the DF bit is set, preserving the sender's requirement to avoid fragmentation.
- Routers do not convert IPv4 packets to IPv6 or broadcast oversized packets to other routers for fragmentation.
- ICMP Type 3 Code 4 messages inform the sender that fragmentation is required but prohibited by the DF bit.
- Dropping packets with the DF bit set and sending ICMP messages helps maintain efficient routing and avoids fragmentation overhead.
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?
The Don't Fragment (DF) bit in the IPv4 header instructs routers not to fragment the packet under any circumstances.
What is the correct answer to this question?
The correct answer is: Drop it and typically return an ICMP message indicating fragmentation is needed — The router drops the packet because the packet is too large for the next link and the sender has explicitly said the packet must not be fragmented. In IPv4, the Don't Fragment bit tells routers not to break the packet into smaller pieces. If the outgoing interface MTU is smaller than the packet size, the router cannot legally forward it as-is and cannot fragment it, so the normal result is to drop the packet and usually send back an ICMP unreachable message indicating that fragmentation is needed. In plain terms, the router is telling the sender, 'This packet is too big for the path you chose, and you told me I am not allowed to split it.' This behavior is central to path MTU discovery.
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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