How MAC Address Learning Functions on a Switch

200-301 By islever Editorial Last updated

Describe switching concepts

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 each frame it receives.

Question

How does MAC learning function?
  1. sends a retransmission request when a new frame is received
  2. enabled by default on all VLANs and interfaces Correct Answer
  3. populates the ARP table with the egress port
  4. protects against denial of service attacks

Key Takeaways

MAC learning is automatic and default-on, which is why 'enabled by default on all VLANs and interfaces' is the textbook description.

A switch learns by inspecting the source MAC of every incoming frame and mapping it to the receiving port/VLAN in the MAC address table, with an aging timer. This dynamic learning is on by default.

Common mistake: Thinking the switch sends retransmission requests (A) — that is TCP, not switching. Or confusing learning with ARP-table population (C) or DoS protection (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.

Why the Other Options Are Wrong

A is wrong: retransmission requests are a transport-layer (TCP) function, not switching. C is wrong: the switch populates the MAC address table (mapping source MAC to ingress port), not the ARP table, and uses the source MAC, not egress. D is wrong: plain MAC learning is not a DoS control.

Community Comment Notes

The vote is B (83) versus A (17). A few ask why not A; the explanation is that retransmission is L4 behavior, while MAC learning is the default-on L2 function described by B.

Official Reference

https://learningnetwork.cisco.com/s/ccna-exam-topics

This link opens the official vendor documentation in a new tab.

Community Votes

B
83%
A
17%

83% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 4 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 4

  1. kalitwol 👍 1
    can someone explain why its A not B
  2. Rolfer 👍 2 Answered B
    B is correct.
  3. d5c03de 👍 2 Answered B
    Wrong it's B
  4. vlkiii36 👍 1 Answered A
    Its A.

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