How MAC Address Learning Works on a Switch
Short answer
B Dynamic MAC learning is enabled by default on all VLANs and interfaces; the switch records the source MAC and ingress port of every frame it receives.
Question
Key Takeaways
MAC learning is automatic and default-on, which is why "enabled by default on all VLANs and interfaces" is the textbook description rather than a feature you normally enable.
A switch learns by inspecting the source MAC of every incoming frame and mapping it to the receiving port and VLAN in the MAC address table, with an aging timer. This dynamic learning is on by default and needs no configuration.
Common mistake: Thinking the switch "overwrites" entries destructively (option A) — it refreshes and ages them. Also do not confuse MAC learning with port security (C) or CDP neighbor discovery (D).
Expert Analysis
Why the Answer Is Correct
B is correct. Dynamic MAC address learning runs by default on every VLAN and interface; the switch silently records the source MAC and ingress port of each frame into the MAC address table, refreshing the aging timer on repeats.
Why the Other Options Are Wrong
A is misleading: the switch does not destructively "overwrite" a known MAC — it ages and refreshes entries. C is wrong: plain MAC learning is not a DoS protection mechanism (that is port security). D is wrong: forwarding to a neighbor via CDP describes a discovery protocol, not the learning function.
Community Comment Notes
The community pick is B with 88 votes, and several commenters note this is a repeated question. One shows a sample 'show mac address-table' output confirming multiple MACs can map to a single port.
Official Reference
https://learningnetwork.cisco.com/s/ccna-exam-topics
This link opens the official vendor documentation in a new tab.
Community Votes
100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 6 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 6
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lmmujsi 👍 1 Answered BYes, that’s correct. A single port on a network switch can be associated with multiple learned MAC addresses. This is common in scenarios where the port is connected to a switch or hub that has multiple devices behind it. The switch’s MAC address table will have multiple entries for that port, each with a different MAC address corresponding to the devices connected through that intermediary device. This allows the switch to correctly forward frames to any of those devices based on the destination MAC address in the frame. It’s an efficient way to handle traffic in a network with multiple devices.
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lmmujsi 👍 1 Answered BWhy A is not correct: Switch# show mac address-table Vlan Mac Address Type Ports ---- ----------- ---- ----- 100 00a1.b2c3.d4e5 DYNAMIC Gi0/1 100 00a2.b3c4.d5e6 DYNAMIC Gi0/1 200 00b1.c2d3.e4f5 DYNAMIC Gi0/1 200 00b2.c3d4.e5f6 DYNAMIC Gi0/1
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lmmujsi 👍 1 Answered BB. enabled by default on all VLANs and interfaces
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Aritra09 👍 1 Answered BB is correct, this is a repeated question
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MalikMak 👍 1 Answered BCorrect Answer is B
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vlkiii36 👍 1 Answered Aits A.
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