Question 1,383 of 1,389
CCNA IPv4 DF (Don't Fragment) bit Practice Question
A packet is larger than the outgoing interface MTU and the DF bit is set in the IPv4 header. What should the router do?
⚠ Common exam trap
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.
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 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.
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 the packet anyway and forward all fragments.
Why it's wrong here
The DF bit is an explicit instruction to not fragment the datagram, so doing so anyway is a direct violation of RFC 791. Since the outgoing link cannot carry the frame, the router must drop the packet instead of forwarding fragments. Fragmenting despite DF would also prevent the source from learning the path MTU and adjusting appropriately.
When this WOULD be correct
In a different scenario where the DF bit is not set, a question might ask what a router should do when a packet exceeds the MTU. In that case, the router could fragment the packet and forward all fragments without violating protocol rules.
- ✓
Drop the packet and send an ICMP message indicating fragmentation was needed.
Why this is correct
When the DF bit is set and the packet exceeds the outgoing interface MTU, the router cannot fragment it without violating the IPv4 specification. The only compliant action is to discard the packet and return an ICMP Type 3, Code 4 message carrying the MTU of the next-hop link. This error informs the source so it can reduce its segment size, which is the basis of Path MTU Discovery.
- ✗
Clear the DF bit and then fragment the packet.
Why it's wrong here
A router does not have permission to clear the DF bit on behalf of the source; fragmentation behavior is controlled solely by the original packet's flags. If the DF bit is cleared, the router may fragment, but it never alters that flag in normal forwarding. Rewriting DF would break the end-to-end contract that the source requested and could hide PMTUD failures from the sender.
When this WOULD be correct
In a different scenario where the question specifies that the DF bit is not set, a router could clear the DF bit and fragment the packet to fit the MTU of the outgoing interface. This would be a valid action when fragmentation is permissible.
- ✗
Encapsulate the packet in GRE automatically.
Why it's wrong here
GRE is only used when an explicit tunnel has been configured; it is not a fallback action for oversized packets. Encapsulating the packet into GRE would actually add an additional 4-byte GRE header plus IP header, making the MTU problem worse. Automatic GRE encapsulation has no role in standard IP MTU handling and would require tunnel endpoints to be defined first.
When this WOULD be correct
In a different scenario where the question asks about handling packets that need to be tunneled for secure transmission, and the GRE protocol is explicitly mentioned, the correct answer could involve encapsulating packets in GRE regardless of MTU issues, assuming the context allows for such a solution.
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 the packet and send an ICMP message indicating fragmentation was needed.Correct answer▾
Why this is correct
When the DF bit is set and the packet exceeds the outgoing interface MTU, the router cannot fragment it without violating the IPv4 specification. The only compliant action is to discard the packet and return an ICMP Type 3, Code 4 message carrying the MTU of the next-hop link. This error informs the source so it can reduce its segment size, which is the basis of Path MTU Discovery.
✗Fragment the packet anyway and forward all fragments.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because if the DF (Don't Fragment) bit is set, the router cannot fragment the packet, and attempting to do so would violate the IP protocol rules.
★ When this WOULD be the correct answer
In a different scenario where the DF bit is not set, a question might ask what a router should do when a packet exceeds the MTU. In that case, the router could fragment the packet and forward all fragments without violating protocol rules.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of how fragmentation works in relation to the DF bit, leading them to incorrectly assume that routers can override the DF setting.
✗Clear the DF bit and then fragment the packet.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because if the DF (Don't Fragment) bit is set, the router is not allowed to fragment the packet. The correct action is to drop the packet and send an ICMP message indicating that fragmentation is needed.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the DF bit is not set, a router could clear the DF bit and fragment the packet to fit the MTU of the outgoing interface. This would be a valid action when fragmentation is permissible.
Why candidates choose this
Candidates may choose this option because they recall that fragmentation is a common network operation and mistakenly believe that routers can modify packet headers to facilitate forwarding, especially under pressure during an exam.
✗Encapsulate the packet in GRE automatically.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because GRE encapsulation does not address the issue of packet fragmentation; it simply adds a layer of encapsulation without resolving MTU constraints. The router cannot automatically encapsulate a packet in GRE when the DF bit is set and the packet exceeds the MTU.
★ When this WOULD be the correct answer
In a different scenario where the question asks about handling packets that need to be tunneled for secure transmission, and the GRE protocol is explicitly mentioned, the correct answer could involve encapsulating packets in GRE regardless of MTU issues, assuming the context allows for such a solution.
Why candidates choose this
Candidates may find this option tempting because GRE is often associated with tunneling and can handle larger packets, leading to confusion about its role in fragmentation scenarios.
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
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.
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