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A transparent bridge is a bridge (Network Device) that specifically monitors incoming network traffic to identify media access control (MAC) addresses of the hosts. These bridges are operated in such a way that is transparent to all the network's connected hosts, which is why it is called a "Transparent".

According to IEEE 802.1d specifications,

A system equipped with a transparent bridge must fulfill the three requirements, such as:
  • Frames must be forwarded from one station to another. 
  • The forwarding table is automatically made by the learning process of frame movements in the network. 
  • Loops in the system must be avoided.

Transparent bridges maintain a list of MAC addresses, as do routers, but based on all the received frame’s source data-link MAC addresses. They use dynamic algorithm to accomplish this. The algorithm has five sections, such as:

Forwarding: A transparent bridge must forward correctly the frames to the ports connected to the hosts.

Learning: The bridge can recognize or learn the destination address of a frame by checking the response from the receiving host.

Filtering: After the learning process, the bridge determines the destination address of a frame and makes an entry into the forwarding table contained by the host. This is called filtering.

Flooding: At first, the bridge forwards the frames to every single hosts since it doesn’t know the destination address, this situation is “flooding”.

Avoiding Loops: The bridge must forward the frames in such a way, that it doesn’t make any unnecessary loops among the hosts.
transparent plastic, transparent background
Transparent bridge
Taken from the picture above,

Let us assume there are three hosts, A, B and C, and a bridge with three ports. 

1. 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. 

2. Host A sends a frame to the bridge that is addressed to Host B (Forwarding). 

3. The bridge checks the frame's source address and creates an address and port number entry for Host A in its forwarding table (Learning). 

4. The bridge then examines the frame's destination address, but does not find it in its forwarding table. As a result, the bridge over here sends the frame to all available other ports (2 and 3). This is called Flooding

5. The frame is then received by Host B and Host C, which also check the destination address. Host B identifies the destination address match and sends a response to Host A.

6. On the return path, the bridge adds an address and port number entry for Host B to its forwarding table
(Filtering)

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.

Note: Transparent bridge is often called “Adaptive bridge”, because it learns which node is connected to which port by examining the packets that are transmitted.


Study Sessions


Q1. What is a Transparent Bridge? What is the difference between a bridge and a transparent bridge?

Q2. What is flooding? Why does it occur? And how it can be solved

Q3. Why transparent bridge is often called an "Adaptive Bridge"?

  
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