Networking 101: Understanding the Data Link Layer

“Ethernet switches, as they’re called now, began life as a
“bridge.” Traditional bridges would read all Ethernet frames, and
then forward them out every port, except the ones they came in on.
They had the ability to allow redundancy via STP, and they also
began learning which MAC addresses were on which port. At this
point, a bridge then became a learning device, which means they
would store a table of all MAC addresses seen on a port. When a
frame needed to be sent, the bridge could look up the destination
MAC address in the bridge table, and know which port is should be
sent out.”

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