Allied Telesis AT-S63 - Manual (Page 672)

Chapter 29: MAC Address Table
672 Section IV: Port Security
MAC Address Overview
Each hardware device that you connect to your Ethernet network has a
unique MAC address assigned to it by the device’s manufacturer. For
example, every network interface card (NIC) that you use to connect your
computers to your network has a MAC address assigned to it by the
adapter’s manufacturer.
The AT-9400 Series switch contains a MAC address table with a storage
capacity of 16,000 entries. 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. Because 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.
The type of MAC address described above is referred to as a dynamic
MAC address. Dynamic MAC addresses are addresses that the switch
learns by examining the source MAC addresses of the frames received on
the ports.