Allied Telesis AT-CM70S - Manual (Page 58)

Chapter 1: Converteon™ Fast Ethernet and Gigabit Ethernet Line Cards
58
A Few Basics about Media Converters
An Ethernet media converter can interconnect network devices over large
distances by transferring Ethernet traffic between twisted pair port and
fiber optic cabling.
MAC Address
Table
A Converteon™ media converter line card has a MAC address table that
can store up to 2K to 32K MAC addresses, depending on the line card
model, as listed below:
ˆ AT-CM20x Series – 2K MAC Addresses
ˆ AT-CM212x/1 Series – 2K MAC Addresses
ˆ AT-CM70S – 2K MAC Addresses
ˆ AT-CM2K0S – 8K MAC Addresses
ˆ AT-CV10x Series – NO MAC Address Table (non-bridging card)
ˆ AT-CV1KSS – NO MAC Address Table (non-bridging card)
The line card 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 line card 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 line card’s
ports, and the port number where each address was learned.
When the line card 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 line card ports for receiving
and transmitting data.
If the line card 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 line card.
If the ports have been grouped into virtual LANs, the line card 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 line card adds its MAC
address and port number to the table.
If the line card receives a packet with a destination address that is on the
same port on which the packet was received, it discards the packet