A Layer 2 switch floods a frame with an unknown destination MAC address out all ports in the same VLAN except the port where the frame entered. That behavior is not a fault; it is normal Ethernet switching. The switch learns from the source MAC address of incoming frames, not from the unknown destination. If the destination MAC is already in the MAC address table, the frame is forwarded only out the associated port. If it is missing, the switch must treat the traffic as unknown unicast and flood within the VLAN so the destination has a chance to receive it. The switch does not drop the frame just because the destination is unknown, and it does not send it only to the CPU for learning. Cisco CCNA v1.1 Network Access covers switching fundamentals, including MAC learning, forwarding, filtering, and flooding. The decisive detail is the VLAN boundary: flooding is limited to the same VLAN and excludes the ingress port.
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