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A transparent bridge is a common type of bridge that observes incoming network traffic to identify media access control (MAC) addresses. These bridges operate in a way that is transparent to all the network's connected hosts.A transparent bridge records MAC addresses in a table that is much like a routing table and evaluates that information whenever a packet is routed toward its location. A transparent bridge may also combine several different bridges to better inspect incoming traffic. Transparent bridges are implemented primarily in Ethernet networks.
Transparent bridges maintain a list of MAC addresses, as do routers, based on all the received frames' source data-link MAC addresses. These tables are used for address look-up while forwarding a frame.
Transparent bridges save and maintain the source-route addresses of incoming frames by listening to all the connected bridges and hosts. They use a transparent bridging algorithm to a accomplish this. The algorithm has five parts:
For example, consider three hosts, A, B and C, and a bridge with three ports. Host A is connected to Bridge Port 1, Host B is connected to Bridge Port 2 and Host C is connected to Bridge Port 3. Host A sends a frame to the bridge that is addressed to Host B. The bridge checks the frame's source address and creates an address and port number entry for Host A in its forwarding table. The bridge then examines the frame's destination address, but does not find it in its forwarding table. As a result, the bridge sends the frame to all the other ports (2 and 3). This is called flooding. The frame is then received by Host B and Host C, which also check the destination address. Host B recognizes a destination address match and sends a response to Host A.
On the return path, the bridge adds an address and port number entry for Host B to its forwarding table. The bridge already has Host A's address in its forwarding table so it forwards the response only to Port 1. In this way, none of the Port 3 hosts are burdened with response requirements. Through this process, two-way communication between Host A and Host B is facilitated without the need for further flooding.