Transparent Bridge

What Does Transparent Bridge Mean?

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.


Techopedia Explains Transparent Bridge

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:

  • Learning
  • Flooding
  • Filtering
  • Forwarding
  • Avoiding loops

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.


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Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.