Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
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 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
Drops the frame immediately
Dropping is not the standard behavior for unknown unicast on a normal switch.
B
Best answer
Floods the frame out all ports in the same VLAN except the incoming port
Correct. Unknown unicast is flooded within the VLAN.
C
Distractor review
Sends the frame to the default gateway first
A Layer 2 switch does not send unknown unicast to the default gateway for forwarding decisions.
D
Distractor review
Converts the frame to a broadcast packet
The switch does not rewrite the packet as a broadcast; it floods the existing unicast frame.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that a switch drops a unicast frame if the destination MAC address is unknown or that it sends the frame to the default gateway for forwarding. This mistake arises from confusing Layer 2 switching behavior with Layer 3 routing. In reality, a Layer 2 switch floods unknown unicast frames within the VLAN to discover the destination device. Believing the frame is dropped or sent to the default gateway leads to incorrect answers and misunderstanding of basic switching operations.
Technical deep dive
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.
Related practice questions
Related 200-301 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
CCNA subnetting practice questions
Practise IPv4 subnetting, CIDR, masks, host ranges and subnet selection.
CCNA OSPF practice questions
Practise OSPF neighbours, router IDs, metrics, areas and routing-table interpretation.
CCNA VLAN practice questions
Practise VLANs, access ports, trunks, allowed VLANs and switching scenarios.
CCNA STP practice questions
Practise spanning tree, root bridge election, port roles and STP troubleshooting.
CCNA EtherChannel practice questions
Practise LACP, PAgP, port-channel behaviour and bundle requirements.
CCNA ACL practice questions
Practise standard and extended ACLs, permit/deny logic and traffic filtering.
CCNA NAT practice questions
Practise static NAT, dynamic NAT, PAT and inside/outside address translation.
CCNA DHCP practice questions
Practise DHCP scopes, relay, leases and troubleshooting.
CCNA show ip route practice questions
Practise routing-table output, longest-prefix match, AD and route selection.
CCNA show interfaces trunk practice questions
Practise trunk verification and VLAN forwarding across switches.
CCNA wireless security practice questions
Practise WLAN security, authentication and wireless architecture concepts.
CCNA IPv6 practice questions
Practise IPv6 addressing, routes, neighbour discovery and common IPv6 exam traps.
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?
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?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
Discussion
Loading comments…
Sign in to join the discussion.