Question 364 of 1,389
CCNA Network Services and Security Practice 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?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Fragment it anyway and forward the pieces
Why it's wrong here
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.
When this WOULD be correct
In a different scenario where a packet is received without the Don't Fragment bit set and exceeds the MTU, a router could fragment it into smaller packets to fit the MTU size and forward the fragments accordingly. This would be a valid situation for option A to be correct.
- ✓
Drop it and typically return an ICMP message indicating fragmentation is needed
Why this is correct
When a router receives an IPv4 packet larger than the outgoing interface MTU and the Don't Fragment (DF) bit is set, it cannot fragment the packet. Instead, it drops the packet and typically sends an ICMPv4 Type 3 Code 4 (Destination Unreachable: fragmentation needed and DF set) message back to the source. This ICMP message includes the MTU of the next-hop link, which allows the sender to perform Path MTU Discovery and adjust its packet size accordingly. This behavior is defined in RFC 792 and RFC 1191, and it prevents silent packet loss by informing the sender of the required size.
- ✗
Translate it to IPv6 and forward it
Why it's wrong here
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.
When this WOULD be correct
In a different scenario where a question asks about a router's behavior when handling a packet that is too large but does not have the Don't Fragment bit set, translating the packet to IPv6 could be a valid option if the router supports IPv6 and is configured to perform such translations.
- ✗
Broadcast it so another router can handle fragmentation
Why it's wrong here
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.
When this WOULD be correct
In a different scenario where the question states that the packet is allowed to be fragmented, a router might broadcast the packet to other routers in a network that supports fragmentation to handle oversized packets collectively. This could occur in a network with specific configurations allowing such behavior.
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.
✓Drop it and typically return an ICMP message indicating fragmentation is neededCorrect answer▾
Why this is correct
When a router receives an IPv4 packet larger than the outgoing interface MTU and the Don't Fragment (DF) bit is set, it cannot fragment the packet. Instead, it drops the packet and typically sends an ICMPv4 Type 3 Code 4 (Destination Unreachable: fragmentation needed and DF set) message back to the source. This ICMP message includes the MTU of the next-hop link, which allows the sender to perform Path MTU Discovery and adjust its packet size accordingly. This behavior is defined in RFC 792 and RFC 1191, and it prevents silent packet loss by informing the sender of the required size.
✗Fragment it anyway and forward the piecesWrong answer — click to see why▾
Why this is wrong here
The router cannot fragment the packet because the Don't Fragment (DF) bit is set. Fragmentation would violate the sender's explicit instruction and is not permitted by the IPv4 specification.
★ When this WOULD be the correct answer
In a different scenario where a packet is received without the Don't Fragment bit set and exceeds the MTU, a router could fragment it into smaller packets to fit the MTU size and forward the fragments accordingly. This would be a valid situation for option A to be correct.
Why candidates choose this
Students may think that routers always fragment oversized packets as a normal forwarding behavior, overlooking the DF bit's role in preventing fragmentation.
✗Translate it to IPv6 and forward itWrong answer — click to see why▾
Why this is wrong here
IPv4-to-IPv6 translation is not a mechanism to handle MTU issues; it is used for interoperability between IPv4 and IPv6 networks. The router does not perform protocol translation to solve a fragmentation problem.
★ When this WOULD be the correct answer
In a different scenario where a question asks about a router's behavior when handling a packet that is too large but does not have the Don't Fragment bit set, translating the packet to IPv6 could be a valid option if the router supports IPv6 and is configured to perform such translations.
Why candidates choose this
Students might confuse the concept of protocol translation with a general 'fix-all' solution, or think that IPv6's larger MTU could bypass the issue, but the packet remains IPv4 and translation is unrelated.
✗Broadcast it so another router can handle fragmentationWrong answer — click to see why▾
Why this is wrong here
Routers do not broadcast packets to offload fragmentation to another router. The forwarding decision is made locally; if the packet cannot be forwarded due to MTU constraints and DF set, it is dropped immediately.
★ When this WOULD be the correct answer
In a different scenario where the question states that the packet is allowed to be fragmented, a router might broadcast the packet to other routers in a network that supports fragmentation to handle oversized packets collectively. This could occur in a network with specific configurations allowing such behavior.
Why candidates choose this
Students may incorrectly believe that broadcasting can help find a router with a larger MTU, but broadcasting is not used for unicast forwarding and does not resolve the fragmentation issue.
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?”
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Last reviewed: May 17, 2026
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