Essay, Research Paper: Bridges

Bridges are used to interconnect LANs using one or more of the IEEE 802.x standards, that is Ethernet or Token Ring LAN environments and also others. Bridges are devices with two or more ports, which connect LANs at the Data Link layer of the OSI Model. The purpose of bridges is to allow nodes connected to different LANs to communicate as if they were located on the same LAN. In contrast to repeaters, that use the Physical Layer and allow all traffic to cross LAN segments, bridges are more intelligent and limit the traffic to the section of the network on which it is relevant. In bridging networks, node addresses have no specific relationship to location. For this reason, messages are broadcast out and accepted only by the intended destination node. Bridges learn which addresses are on which network and develop a forwarding table so that subsequent messages can be forwarded to the right network and thus to the right receiving node location. The bridge forwarding table is like a built in database, which the bridge examines the source field of every data packet it sees on each port and builds up entries into the forwarding table of which network node addresses is connected to which port. This means that the bridge is kind of learning routes, and will not have to re-transmit every time a packet is sent if the bridge knows that the destination node address is connected to the same port as the bridge saw the packet on. Forwarding table entries also age, so if a given address has not been heard from in a specified period of time then the address is deleted from the forwarding table.
The bridging method for Ethernet networks is called Transparent Bridging.