October 26, 2022
Bridge is a network device, which operates at the data link layer of the OSI model. It is designed to combine segments (subnet) of a computer network of different topologies and architectures.

Bridges are very useful for filtering the data load of traffic that divides it into segments or packets. They are used to reduce the load of traffic on the local network or another network.

How it works

Bridge is a repeater that has the feature that it filters content, it reads the MAC addresses of both source and destination. It used to connect two local networks working on the same protocol. It has one input port and one output port, which makes the bridge a two-port device.

When a frame (packet) is received from the network, the bridge checks the destination MAC address in its header. If it belongs to that subnet, transmits the frame on to the segment. If the frame does not belong to that subnet, the bridge does nothing.

Bridges are actually passive devices, as there is no interaction between the bridges and the bridge paths.

Types of bridges

Transparent bridges basically use two operations: forwarding and learning.

These bridges are devices where the stations are completely unaware of the bridge’s existence, i.e. whether the bridge has been added to the network or removed, so these stations will not need to be reconfigured at all. When the bridge process occurs, it creates a bridge table with itself where it stores the MAC addresses of many different terminals. Using this, the next time, this table helps the bridges send a data packet to the specified location. But when a particular address does not match the contents of this connection table, the data packet is forwarded to every terminal connected to the local network except for the one computer from which it is sent.

Translating bridges perform the conversion from one data link layer protocol to another.

This method has the advantage of less overhead, since there is no need to transmit two data link layer headers, the stations of the other network become available. But translating bridges introduce additional delay when converting frame formats, as well as a new calculation of the frame checksum. Another important disadvantage of translating bridges is that they have a significant limitation – maximum frame size of the networks connected by such a bridge must be the same.

Source routing bridges. In these types of bridges, the routing process is performed by the source station, and the frame itself determines the path it should follow.

The host can detect the frame if it needs to, so only a special frame (detection frame) should be sent. Which is distributed across the network along all possible paths associated with the destination. These bridges are mostly used in Token Ring networks.

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