When a switch doesn't know where the destination MAC is, it floods. The frame goes out all ports except the one it came in on. This happens for unknown unicast, broadcast, and multicast frames. Flooding ensures delivery when the switch lacks information. Once the destination replies, the switch learns its location. Too much flooding wastes bandwidth. Watch for MAC table overflow attacks.
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$ curl repovive.com/roadmaps/network-design/physical-and-data-link-fundamentals/switch-flooding
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