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• Main idea: A bridge should
easily connect any set of LAN’s together and make
the connection transparent to the
stations.
• No maintenance, software
upgrade and routing table upload should be necessary.
• The bridge listens to both
network at all times.
• Any frame received is buffered.
• Next the bridge should be able
to decide if the frame was addressed to a station in
the same network. If not, it
should select the proper LAN and broadcast the frame
there.
• Backward Learning:
• The bridge keeps a table
containing hashed (address, network) entry pairs.
• The bridge accepts any frame,
if the destination address is in the table then the frame is forwarded to the
proper network, otherwise the frame is broadcast onto all networks
(except the one its coming from).
• For each incoming frame the bridge
also read the source address and updates the hash table by
inserting the source address and the network id into the tables.
• Entries in the table can live
for a certain time, and if there is no packet
traffic from or to that an
address the entry is removed from the table.
• Maintain a forwarding database
or cache of station MAC addresses and the bridge port that the stations are on
• Promiscuously listen to packets
arriving on any port
• For each packet arriving at the
bridge:
• Store the stations source
address and arriving port in the cache (if an entry
already exists for an address
update if different)
– determine if the destination
address is in the cache
• If entry then forward only on
the appropriate port unless the port is the same as the arrival port
• If no such entry then forward
packet on all segments except the one the packet was received on.
– Age each entry in the cache and delete after an
appropriate time
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