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MAC Address Learning
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All Ethernet nodes are identified by their MAC addresses. An Ethernet switch examines each Ethernet frame for its destination MAC address and sends it to the appropriate port. This mapping of destination MAC address to the switch port, where it needs to be forwarded to, is maintained in a table known as the MAC address table.
Switches have mechanisms for learning the MAC addresses of the nodes in a network. Whenever the switch receives a frame to be forwarded, it checks whether the destination MAC address is in the table, and if it is in the table, the frame is sent out to that particular port. In this way, only frames with unknown MAC addresses are broadcast.
Every switch has a limitation to the number of MAC addresses that can be stored in the table. A large MAC address table size ensures that the node's MAC address tables don't get filled up during peak-traffic. There is also a provision for aging of learned entries. At times, Ethernet nodes (which are identified by their MAC addresses) can shift from one port to another. In this case, aging is required to ensure that the MAC address table reflects the current topology. The aging time is configurable in accordance to 802.1d.
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