The correct answer is that the switch floods the frame out all ports in the same VLAN except the incoming port. This happens because the switch’s MAC address table does not contain an entry for the destination MAC address, making it an unknown unicast frame; lacking a specific egress port, the switch must forward the frame to all other ports within that broadcast domain to ensure the destination receives it, while avoiding sending it back out the port where it arrived. On the CCNA 200-301 v2 exam, this concept tests your understanding of Layer 2 switching logic and the distinction between known and unknown unicast frames, often appearing in questions about flooding versus forwarding. A common trap is confusing unknown unicast flooding with broadcast traffic—remember that broadcasts are sent to all ports including the incoming port, whereas unknown unicast flooding excludes the ingress port. Memory tip: think “UUF” for Unknown Unicast Flooding—the switch “Uses Every Exit” except the one the frame came in on.
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: a Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A switch receives a unicast frame for a destination MAC address that is not yet in its MAC address table. What does the switch do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Floods the frame out all ports in the same VLAN except the incoming port
An unknown unicast frame is flooded within the VLAN because the switch does not yet know which port leads to the destination MAC. The frame is not sent back out the receiving port.
Key principle: A Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Drops the frame immediately
Why it's wrong here
Dropping is not the standard behavior for unknown unicast on a normal switch.
When this WOULD be correct
In a scenario where a switch is configured to drop all unicast frames for security reasons, such as in a highly restrictive network policy exam question, this option could be correct. For example, if the question specifies that the switch is in a security mode that prohibits unknown unicast traffic, dropping the frame would be the expected behavior.
✓
Floods the frame out all ports in the same VLAN except the incoming port
Why this is correct
Correct. Unknown unicast is flooded within the VLAN.
Related concept
A Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions.
✗
Sends the frame to the default gateway first
Why it's wrong here
A Layer 2 switch does not send unknown unicast to the default gateway for forwarding decisions.
When this WOULD be correct
In a scenario where a question asks about the behavior of a Layer 3 device, such as a router, when it receives a packet destined for an unknown IP address, the correct answer could involve sending the packet to the default gateway for further processing. This would clarify the role of the default gateway in routing decisions.
✗
Converts the frame to a broadcast packet
Why it's wrong here
The switch does not rewrite the packet as a broadcast; it floods the existing unicast frame.
When this WOULD be correct
If the question asked about a network device that explicitly converts unicast frames to broadcast for specific purposes, such as a network appliance designed to propagate certain types of traffic, then this option could be correct. For example, a device configured to broadcast certain control messages might convert unicast frames accordingly.
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.
✓Floods the frame out all ports in the same VLAN except the incoming portCorrect answer▾
Why this is correct
Correct. Unknown unicast is flooded within the VLAN.
✗Drops the frame immediatelyWrong answer — click to see why▾
Why this is wrong here
Switches are designed to forward unknown unicast frames by flooding, not dropping them. Dropping would occur only if the frame is malformed or security features like port security are violated.
★ When this WOULD be the correct answer
In a scenario where a switch is configured to drop all unicast frames for security reasons, such as in a highly restrictive network policy exam question, this option could be correct. For example, if the question specifies that the switch is in a security mode that prohibits unknown unicast traffic, dropping the frame would be the expected behavior.
Why candidates choose this
Students might think that if a switch doesn't know the destination, it should discard the frame to avoid unnecessary traffic, but this is incorrect because flooding ensures delivery.
✗Sends the frame to the default gateway firstWrong answer — click to see why▾
Why this is wrong here
A Layer 2 switch operates at the data link layer and does not involve the default gateway for forwarding decisions. The default gateway is used for routing between VLANs or subnets, not for unknown unicast flooding within a VLAN.
★ When this WOULD be the correct answer
In a scenario where a question asks about the behavior of a Layer 3 device, such as a router, when it receives a packet destined for an unknown IP address, the correct answer could involve sending the packet to the default gateway for further processing. This would clarify the role of the default gateway in routing decisions.
Why candidates choose this
Students may confuse the switch's behavior with that of a host or router, where unknown destinations are sent to the default gateway. However, switches flood unknown unicasts within the VLAN.
✗Converts the frame to a broadcast packetWrong answer — click to see why▾
Why this is wrong here
Switches flood the original unicast frame unchanged; they do not modify the frame type to broadcast. Broadcasting would change the destination MAC to FF:FF:FF:FF:FF:FF, which is not the case for unknown unicast flooding.
★ When this WOULD be the correct answer
If the question asked about a network device that explicitly converts unicast frames to broadcast for specific purposes, such as a network appliance designed to propagate certain types of traffic, then this option could be correct. For example, a device configured to broadcast certain control messages might convert unicast frames accordingly.
Why candidates choose this
The term 'flood' might be misinterpreted as 'broadcast', but flooding means sending the frame out all ports except the incoming port while preserving the original unicast destination MAC.
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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Do not confuse switch flooding behavior for unknown unicast frames with dropping or routing behaviors.
Detailed technical explanation
How to think about this question
In Ethernet switching, a switch maintains a MAC address table that maps MAC addresses to specific switch ports. When a switch receives a unicast frame, it consults this table to determine the correct outgoing port to forward the frame. If the destination MAC address is already known, the switch forwards the frame only to the associated port, optimizing network efficiency and reducing unnecessary traffic.
When the switch encounters a unicast frame with a destination MAC address not present in its MAC address table, it treats this as an unknown unicast. The switch then floods the frame out all ports within the same VLAN except the port on which the frame was received. This flooding ensures that the frame reaches the intended recipient even though the switch has not yet learned the destination port. Once the destination device responds, the switch learns its MAC address and updates the table accordingly.
A common exam trap is misunderstanding the behavior of unknown unicast frames. Some candidates incorrectly believe the switch drops the frame or sends it to the default gateway, which is incorrect because a Layer 2 switch operates solely based on MAC addresses and VLANs. The flooding behavior is essential for initial MAC address learning and maintaining network connectivity, especially in dynamic environments where devices frequently join or leave the network.
KKey Concepts to Remember
A Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions.
When a switch receives a unicast frame with a known destination MAC address, it forwards the frame only to the associated port to reduce unnecessary traffic.
If the destination MAC address is unknown, the switch floods the unicast frame out all ports in the same VLAN except the incoming port to locate the destination device.
Flooding unknown unicast frames allows the switch to learn new MAC addresses dynamically as devices respond to the frames.
A Layer 2 switch does not send unknown unicast frames to the default gateway because it operates at the data link layer, not the network layer.
Switches do not convert unicast frames into broadcast frames; they forward the original frame to all relevant ports during flooding.
The MAC address table is updated only when the switch receives frames from devices, enabling efficient future forwarding.
Flooding unknown unicast frames prevents frame loss and ensures connectivity in networks with dynamic device presence.
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.
Key takeaway
A Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review a Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions., then practise related 200-301 questions on the same topic to reinforce the concept.
Switching and Network Access — This question tests Switching and Network Access — A Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions..
What is the correct answer to this question?
The correct answer is: Floods the frame out all ports in the same VLAN except the incoming port — An unknown unicast frame is flooded within the VLAN because the switch does not yet know which port leads to the destination MAC. The frame is not sent back out the receiving port.
What should I do if I get this 200-301 question wrong?
Review a Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
A Layer 2 switch maintains a MAC address table that maps MAC addresses to specific switch ports for forwarding decisions.
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