Chapter 1: Product Description
18
A Few Ethernet Switching Basics
An Ethernet switch interconnects network devices, such as workstations,
printers, routers, and other Ethernet switches, so that they can
communicate with each other by sending and receiving Ethernet frames.
MAC Address
Table
Every hardware device on your network has a unique MAC address. This
address is assigned to the device by the device’s manufacturer. For
example, when you install a Network Interface Card (NIC) in a computer
so that you can connect it to the network, the NIC already has a MAC
address assigned to it by its manufacturer.
The AT-FS717FC/SC Fast Ethernet switch can contain up to 4K entries in
its MAC address table. The switch uses the table to store the MAC
addresses of the network end-nodes connected to the ports, along with
the port number on which each address was learned.
A switch learns the MAC addresses of the end-nodes by examining the
source address of each packet received on a port. It adds the address and
port on which the packet was received to the MAC table if the address had
not already been entered in the table. The result is a table that contains all
the MAC addresses of the devices that are connected to the switch’s
ports, and the port number where each address was learned.
When the switch receives a packet, it also examines the destination
address and, by referring to its MAC address table, determines the port on
which the destination end-node is connected. It then forwards the packet
to the appropriate port and on to the end-node. This increases network
bandwidth by limiting each frame to the appropriate port when the
intended end-node is located, freeing the other switch ports for receiving
and transmitting data.
If the switch receives a packet with a destination address that is not in the
MAC address table, it floods the packet to all the ports on the switch.
When the destination end-node responds, the switch adds its MAC
address and port number to the table. Packets will now be sent only to the
appropriate ports.
Note
If no packets are received from a learned address for 5 minutes, that
address will be flushed from the MAC address table.
If the switch receives a packet with a destination address that is on the
same port on which the packet was received, it discards the packet
without forwarding it on to any port. Since both the source end-node and
the destination end-node for the packet are located on the same port on
the switch, there is no reason for the switch to forward the packet.