Allied Telesis AT-S63 - Manual (Page 94)

Chapter 5: MAC Address Table
94 Section I: Basic Operations
Overview
The AT-9400 Switch has a MAC address table with a storage capacity of
16,000 entries. The table stores the MAC addresses of the network nodes
connected to its ports and the port numbers where the addresses were
learned.
A switch learns the MAC addresses of the end nodes by examining the
source addresses of the packets received on a port. It adds to the MAC
table the addresses along with the ports on which the packets were
received. The result is a table of all the MAC addresses of the devices that
are connected to a switch’s ports.
When a 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
destination end node is located, freeing the other switch ports for receiving
and transmitting other packets.
If a switch receives a packet with a destination address that is not in the
MAC address table, it floods the packet to all of its ports, excluding the
port where the packet was received. If the ports have been grouped into
virtual LANs, a 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 to inappropriate LAN segments and increases
network security. When the destination node responds, a switch adds its
MAC address and port number to its MAC address table.
If a 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.
Dynamic MAC addresses are not stored indefinitely in the MAC address
table. A 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. A
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
MAC address table from becoming filled with addresses of nodes that are