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

segment

– 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*/

• }