AT-FS724L Fast Ethernet Switch Installation Guide
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the port on the switch automatically configures itself as MDI-X. This feature
allows you to use either a crossover cable or a straight-through cable when
connecting a device to a twisted pair port.
MAC Address Table
The heart of an Ethernet switch is the Media Access Control (MAC) address
table. Every device that you attach to an Ethernet network has a MAC address.
This address is assigned to the device by the device’s manufacturer. For
example, each Network Interface Card (NIC) that you install into your network
computers has a MAC address that was assigned to it by the card’s
manufacturer.
A switch’s MAC address table is a list of the MAC addresses of the devices that
are connected to its ports. The switch uses this table to direct data frames to
their appropriate destination end-nodes, and in some cases, to discard frames
that it receives. The switch creates the MAC address table by examining the
frames that it receives on its ports. Each frame is examined for its source
address; that is, the MAC address of the end-node that sent the frame. The
switch checks to determine whether the address is already in its MAC address
table. If it is not, the switch adds the address to the table along with the port
number on which the frame was received. The result is a table that contains a
list of all the MAC addresses of end-nodes that have sent frames to the switch
and the ports on the switch to which the end-nodes are connected.
The switch also checks the destination MAC address of each frame it receives.
The destination address is the MAC address of the end-node to which the frame
is intended. If the address is in the table, the switch directs the frame directly
to the port where the end-node is located. This helps to ensure that end-nodes
will only receive traffic that is intended for them and not have to deal with
traffic intended for other end-nodes.
If the destination address is not in the MAC address table, the switch
broadcasts the frame to all switch ports. When the destination end-node
responds, the switch will be able to match the address to a port so that the next
time a frame is destined to that particular end-node, the switch will be able to
forward the frame to the correct port instead of having to broadcast the frame
to all ports.
In some cases a switch will even discard a frame. If the switch receives a frame
that is destined to an end-node on the same port on which the frame was
received, the switch discards the frame.
The MAC address table in the AT-FS724L switch can store up to 4,000 MAC
addresses. To prevent the table from becoming filled with addresses of end-
nodes that are no longer active, the switch has a MAC address aging timer.
