How MAC Learning Adds Source Addresses to the CAM Table

200-301 By islever Editorial Last updated

Describe switching concepts

Short answer

A MAC learning adds each frame's unknown source MAC address to the CAM (MAC address) table, binding it to the ingress port.

Question

How does MAC learning function?
  1. adds unknown source MAC addresses to the CAM table Correct Answer
  2. protects against denial of service attacks
  3. populates the ARP table with the egress port
  4. sends an ARP request to locate unknown destinations

Key Takeaways

MAC learning is driven by the SOURCE MAC of incoming frames — the switch populates the CAM table so future frames to that MAC can be forwarded precisely.

When a switch receives a frame, it reads the source MAC and records it with the receiving port/VLAN in the MAC address table (CAM table). Unknown source MACs are learned this way; the table ages entries that stop being refreshed.

Common mistake: Confusing MAC learning with ARP learning (C) or DoS protection (B). Learning populates the MAC/CAM table from source MACs; it is not an ARP function.

Expert Analysis

Why the Answer Is Correct

A is correct. MAC learning takes the source MAC of each received frame and adds unknown ones to the CAM (MAC address) table, associating them with the ingress port and VLAN.

Why the Other Options Are Wrong

B is wrong: MAC learning is not a denial-of-service control. C is wrong: the switch populates the MAC address table (using the source MAC and ingress port), not the ARP table, and not with an egress port. D is wrong: a switch does not send ARP for unknown destinations — it floods the frame.

Community Comment Notes

The vote is A (100). Top commenters describe MAC learning as building the CAM table from source MAC addresses of received frames.

Official Reference

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

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

Community Votes

A
100%

100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 9 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 9

  1. NiMAMA 👍 9 Answered A
    MAC learning is a fundamental function in Ethernet switches that allows switches to build and maintain a table of MAC addresses associated with the ports through which they are learned. The correct option describing MAC learning is: A. adds unknown source MAC addresses to the CAM table Explanation: When a switch receives an Ethernet frame, it examines the source MAC address in the frame's header. If the MAC address is not already in the switch's Content Addressable Memory (CAM) table (also known as MAC address table), the switch adds an entry to the CAM table associating the MAC address with the port on which the frame was received. This process is known as MAC learning. Over time, the switch builds a table mapping MAC addresses to the ports through which they are learned, allowing it to forward frames more efficiently by only sending them out the appropriate port based on their destination MAC address.
  2. picho707 👍 4 Answered A
    Who is answering these questions?
  3. TNgen9 👍 2 Answered A
    I can't imagine how you answer B -,-
  4. BTK0311 👍 3
    WTF???
  5. JulesAZ 👍 2 Answered A
    Just to be clear!
Show 4 more comments
  1. ladarius01 👍 2
    Answer is A
  2. squagmire 👍 2 Answered A
    yea its A
  3. f2faf2e 👍 2 Answered A
    it is A
  4. DominikRWeber 👍 1 Answered A
    it's A

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