User`s guide

Section II: Local and Telnet Management
162
MAC Address Overview
Every hardware device that you connect to your network has a unique
MAC address associated with it. A MAC address is assigned to a device by
the device’s manufacturer. For example, every network interface card
that you use to connect your computers to your network has a MAC
address assigned to it by the adapter’s manufacturer.
The AT-8000 Series switch contains a 4 kilobyte MAC address table. The
switch uses the table to store the MAC addresses of the network nodes
connected to its ports, along with the port number on which each
address was learned.
The 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 has 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
where the destination 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.
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 onto inappropriate LAN segments and increases network
security. When the destination 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. Since both the source node and the
destination node for the packet are located on the same port on the
switch, there is no reason for the switch to forward the packet. This too
increases network performance by preventing frames from being
forwarded unnecessarily to other network devices.