What is Bridge in Computer Network?

In this article on what is Bridge in computer network; we will discuss the definition of a bridge, how it works, its functionality, models, advantages and disadvantages.  

Table of Contents

• What is Bridge in computer network?
• Functions of bridge
• How does a bridge work?
• Types of bridges
• Model of bridges in computer networks
• Advantages of bridge
• Disadvantages of bridge
• Understanding The Difference: Bridge Vs Switch Vs Router

What is Bridge in computer network?

A bridge is a network device that connects multiple subnetworks to create a single network. It provides interconnection with other computer networks that use the same protocol. Through a bridge, multiple LANs can be connected to form a larger and extended LAN. This function of creating a single aggregate network from multiple network segments is called network bridging. It works in the data link layer, which is the second network layer in the OSI model.

Functions of Bridges

Now, let us take a look at the functions of bridges:

1. Store MAC address in the PC that is used in the network for reducing network traffic. 
2. Divide local area networks into multiple segments. 
3. Connects multiple networks to ensure communication between them.
4. It connects LAN segments into a single network. 
5. Recognizes areas where data is to be sent and on which device it will be sent. 
6. Maintains MAC address table to discover new segments.
7. Used in load filtering of network traffic by separating it into segments or packets.

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How does a bridge work?

The bridge works in the following way:

1. Physical Configuration and Initialization: The bridge is connected to multiple network segments via physical network interfaces. Each interface operates in promiscuous mode, capturing all incoming network traffic. Interfaces can support different network types (Ethernet, Wi-Fi, etc.). Bridge hardware contains dedicated memory and processing components for packet handling
2. MAC Address Learning Mechanism: Bridge maintains a dynamic MAC address table (also called forwarding table or CAM table). Initially, the table is empty. As network traffic flows, the bridge learns source MAC addresses from incoming frames. Each MAC address is mapped to its corresponding network interface. The table has limited memory and it can store upto 8-16k MAC addresses. Entries have a time-to-live (TTL) period which is usually 300 seconds.
3. Packet Filtering Process: Each incoming frame is examined at Layer 2 i.e. Data Link Layer. Bridge extracts source and destination MAC addresses. It compares the destination address against MAC address table amd determines appropriate forwarding strategy based on address mapping. It also implements intelligent forwarding to minimize unnecessary network traffic.
4. Forwarding Decision Logic:  In case, the destination MAC is unknown, the bridge floods the packet to all interfaces. If the destination MAC is on the same interface as source, the packet is filtered. In case, the destination MAC is on a different interface, the packet is selectively forwarded. It maintains low-latency transmission with minimal processing overhead.
5. Frame Transmission: It encapsulates the original frame for transmission and adds necessary Layer 2 headers. It calculates and verifies frame checksum as well as manages transmission timing and collision avoidance. It supports full-duplex and half-duplex transmission modes.

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Types of Bridges in Computer Networks

Following are the different types of bridges that are used:

1. Transparent Bridges

Transparent bridges are used to connect multiple network segments with other bridges to make routing decisions. This bridge fetches the MAC addresses of all nodes and associated ports. The transparent bridge then filters incoming frames with destination MAC addresses on the same port. It then forwards incoming frames to the destination MAC through associates port@. These bridges are very popular and operate in a transparent manner to all networks that are connected to hosts. 

2. Source Routing Bridges

Source Routing Bridges decide the route between two hosts. They use the destination MAC address of a frame to direct it through the source routing algorithm. Every station on extended LAN knows about the route over which the frame is sent. These are used in IBM token ring networks. They learn their bridging information from the information that is included in the structure of a packet. Through this method, two similar network segments get connected to the data link layer. This is done in a distributed manner wherever end stations join within the bridging algorithm. 

3. Translational Bridges

Translational bridges are used for changing one type of networking system to another. They can connect different types of networks such as Ethernet and Token ring networks. Translation bridges reorder the source and destination address bits while translating between these frame formats. These bridges can add and remove data based on the direction of transmission. They forward the frame of data link layer among LANs using different types of network protocols. 

Models of Bridges in Computer Networks

There are two main models of bridging: Local bridging and remote bridging. Let us learn more about them.

1. Local bridging

It is created by connecting LAN switches with local cables. This enables free communication between computers connected to virtual hubs and physical LANs on the same Ethernet segment. Local bridging is used when LANs in the same geographical location have to communicate at the link layer.

2. Remote bridging 

It is a model where two bridges are connected through WAN. It is used for bridging LANs when they are in different geographical locations and they have to communicate with each other at the link layer. 

Advantages of Bridges

Bridges are smart network devices that have the following advantages: 

1. Improve the performance of bridges by segmenting large, busy networks into multiple smaller and interconnected networks.
2. Cost-effective because they are simple and inexpensive. 
3. Increases available bandwidth to individual nodes as fewer network nodes share the collision domain.
4. Reduces network congestion by dividing the LAN into multiple smaller segments.

Disadvantages of Bridges

Like every device, bridges in computer networks also have their limitations.

1. Cannot read specific IP addresses.
2. Unable to provide a communication network among networks of different protocols.
3. Cannot limit the capacity of broadcast messages as they transfer all the messages.
4. Unable to handle complicated variable data load.

Understanding the Difference: Bridge Vs Switch Vs Router

The following table provides the difference among three similar types of devices, including:

Conclusion 

Bridges are smart networking devices used as repeaters to extend a network. They have many benefits and disadvantages. Based on your system and network's requirements, you should choose a network device that can fulfil them.