A switch has the amazing property (particularly for the already-overworked network administrator) that its table is built automatically, dynamically, and autonomously - without any interference from a network administrator or from a configuration protocol. In other words, switches are self-learning. This capability is accomplished as follows:

1. The switch table is initially empty.

2. For each incoming frame received on an interface, the switch stores in its table (1) the MAC address in the frame's source address field, (2) the interface from which the frame arrived, and (3) the current time. In this way the switch records in its table the LAN segment on which the sending node resides. If every node in the LAN eventually sends a frame, then every node will eventually get recorded in the table.

3. The switch deletes an address in the table if no frames are received with that address as the source address after some period of time (the aging time). In this way, if a PC is replaced by another PC (with a different adapter), the MAC address of the original PC will eventually be purged from the switch table.

Let's study the self-learning property for the uppermost switch in "Link-layer Switches" Figure 1 and its corresponding switch table in "Link-layer Switches" Figure 2. Suppose at time 9:39 a frame with source address 01-12-23-34-45-56 arrives from interface 2. Suppose that this address is not in the switch table. Then the switch adds a new entry to the table, as shown in Figure 1.

Switch learns about the location of an adapter with address

Continuing with this same example, suppose that the aging time for this switch is 60 minutes, and no frames with source address 62-FE-F7-11-89-A3 arrive to the switch between 9:32 and 10:32. Then at time 10:32, the switch removes this address from its table.

Switches are plug-and-play devices because they require no interference from a network administrator or user. A network administrator wanting to install a switch need do nothing more than connect the LAN segments to the switch interfaces. The administrator need not configure the switch tables at the time of installation or when a host is removed from one of the LAN segments. Switches are also full-duplex, meaning for any link connecting a node to the switch, the node and the switch can transmit at the same time without collisions.


network administrator, self-learning, switch table

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