AT-8500 Series Fast Ethernet Switch Installation Guide
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A Few Basics about Ethernet Switching
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 in your network has a MAC address and each
MAC address is unique. The address is assigned to a device by the
device’s manufacturer. For example, the network interface cards that
you install in your computers have a unique MAC address assigned to
them by the adapter manufacturers.
An AT-8500 Series Fast Ethernet Switch has a MAC address table capable
of storing up to 8,000 MAC addresses. 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 packet 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.
If the ports have been grouped into virtual LANs, the switch floods the
packet only to those ports which belong to the same VLAN as the port
on which the packet was received. This prevents packets from being
forwarded into inappropriate LAN segments, increasing network
security. When the destination an end node responds, the switch adds
its MAC address and port number to the 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. Because 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.
