A bridged LAN

Interconnection by bridge
• The no frills bridge : simply transmit all traffic from one LAN segment onto all
the other segments
– Advantages: two stations can be transmitting at the same time. Bridge
will buffer a packet until it can transmit on a LAN
– Disadvantages: total bandwidth still that can be safely utilized is still the
minimum bandwidth of each LAN segment
• Keeping a database of all stations on each LAN segment
– Manually enter addresses in such a database
– Partition addresses into ranges on each LAN
• Eg. LAN 1 has 1-50, LAN 2 has 51-100, LAN 3 has 101-150
– Have the MAC address be hierarchically divided into a LAN address and a
station address (like the IP address)
– None of these solutions are really used
• Better solution: the transparent learning bridge
– Learn on which segment a station resides
– Transmit a packet only onto the correct segment
• bridges learn which hosts can be reached through which interfaces: maintain
filtering tables
– when frame received, bridge “learns” location of sender: incoming LAN
– records sender location in filtering table
• filtering table entry:
– (Node LAN Address, Bridge Interface, Time Stamp)
– stale entries in Filtering Table dropped (TTL can be 60 minutes)
• filtering procedure:
if destination is on LAN on which frame was received
then drop the frame
else { lookup filtering table
if entry found for destination
then forward the frame on interface indicated;
else flood; /* forward on all but the interface on
which the frame arrived*/