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SESK: Stuff Everyone Should Know · sesk.info/tech-switch-learns-source-mac

How does an Ethernet switch dynamically build its address table?

It learns the source MAC address of received frames.

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  1. From each received frame's source MAC ✔
  2. From each frame's destination MAC
  3. From routers' IP routing tables

Why: When a frame arrives, a switch associates its source MAC address with the incoming port. Later, that learned table helps the switch send frames toward the port where a destination was seen.

Huh, didn't know that: If a destination MAC is unknown, a switch can flood the frame within the relevant LAN.

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The switch remembered every source because it had a good address book.
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References

Answer: From each received frame's source MAC. When a frame arrives, a switch associates its source MAC address with the incoming port. Later, that learned table helps the switch send frames toward the port where a destination was seen.

  1. NVIDIA Cumulus Linux: Ethernet Bridging - VLANs docs.nvidia.com/networking-ethernet-software/cumulus-linux-518/Layer-2/Ethernet-Bridging-VLANs
    The switch learns the MAC address for a frame when the frame enters the bridge through an interface and records the MAC address in the bridge table.
    Checked 2026-10-11.

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