are times when it is necessary to break up a large LAN into
smaller, more easily managed segments. This decreases the amount
of traffic on a single LAN and can extend the geographical area
past what a single LAN can support. The devices that are used
to connect network segments together include bridges, switches,
routers, and gateways. Switches and bridges operate at the Data
Link layer of the OSI model. The function of the bridge is to
make intelligent decisions about whether or not to pass signals
on to the next segment of a network.
When a bridge receives a frame on the network, the destination
MAC address is looked up in the bridge table to determine whether
to filter, flood, or copy the frame onto another segment. This
decision process occurs as follows:
• Flooding: If the destination address is unknown to the bridge,
the bridge forwards the frame to all segments except the one
on which it was received. This process is known as flooding.
• Forwarding: If the destination device is on a different segment,
the bridge connects the two segments to send the frame to the
appropriate segment. This process is called forwarding.
• Filtering: If the destination device is on the same segment
as the frame, the bridge blocks the frame from going on to other
segments. This process is known as filtering.
placed strategically, a bridge can greatly improve network performance.