Switch Action on a Frame from an Unknown Source MAC

200-301 By islever Editorial Last updated

Describe switching concepts

Short answer

C The switch associates the unknown source MAC with the ingress port and adds it to the MAC address table (it learns the source).

Question

When a switch receives a frame from an unknown source MAC address, which action does the switch take with the frame?
  1. It sends the frame to ports within the CAM table identified with an unknown source MAC address.
  2. It floods the frame out all interfaces, including the interface it was received on.
  3. It associates the source MAC address with the LAN port on which it was received and saves it to the MAC address table. Correct Answer
  4. It attempts to send the frame back to the source to ensure that the source MAC address is still available for transmissions.

Key Takeaways

Unknown SOURCE MAC -> learn it (add to CAM). Unknown DESTINATION MAC -> flood it. The question tests that distinction.

On receiving a frame, the switch always learns the source MAC by binding it to the receiving port/VLAN in the MAC table. Flooding is the action for an UNKNOWN DESTINATION, not an unknown source. The source is simply recorded.

Common mistake: Picking B (flooding): that is the response to an unknown destination, not an unknown source. Here the source is what is unknown, so the switch learns it.

Expert Analysis

Why the Answer Is Correct

C is correct. When a frame arrives from an unknown source MAC, the switch learns it: it binds that MAC to the ingress port/VLAN and stores it in the MAC address table.

Why the Other Options Are Wrong

A is nonsense (the CAM table maps known sources, not unknown ones). B is the action for an unknown DESTINATION, not an unknown source. D is wrong: the switch does not echo the frame back to the source.

Community Comment Notes

The vote is C (85) versus B (15). Commenters stress reading carefully: it is an unknown SOURCE MAC, so the switch learns it, not floods.

Official Reference

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

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

Community Votes

C
85%
B
15%

85% of anonymous learners picked answer C. 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. [Removed] 👍 4 Answered C
    C is correct unknown source = saving unkown destination = flooding
  2. TmNvrWts 👍 3 Answered C
    Answer is C you can trust me Read careful. unknown SOURCE mac, not destination.
  3. d5c03de 👍 4 Answered C
    It's c fam
  4. Aritra09 👍 2 Answered B
    Correct answer should be B

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