Allied Telesis AT-S62 - Manual (Page 124)

Chapter 7: MAC Address Table
124 Section I: Basic Operations
MAC Address Overview
The AT-8500 Series switch has a MAC address table with a storage
capacity of 8,000 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 is
not already in the table. The result is a table that contains the MAC
addresses of all the devices 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 other network packets.
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,
excluding the port where the packet was received. If the ports have been
grouped into virtual LANs, the switch floods the packet only to those ports
that belong to the same VLAN from where the packet originated. 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 where 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.
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.
Dynamic MAC addresses are not stored indefinitely in the MAC address
table. The switch deletes a dynamic MAC address from the table if it does
not receive any frames from the node after a specified period of time. The
switch assumes that the node with that MAC address is no longer active
and that its MAC address can be purged from the table. This prevents the