What Is a LAN Bridge/Network Bridge?

A LAN bridge, also known as a network bridge, is a networking device used to connect and manage multiple local area networks (LANs) within the same or different network segments.

A LAN bridge, also referred to as a network bridge, is a critical networking device designed to connect and manage multiple local area networks (LANs) by operating at the data link layer, or Layer 2, of the OSI model.

Unlike routers that operate at the network layer and determine paths for data based on IP addresses, a LAN bridge relies on MAC addresses to decide where data packets should be forwarded. This allows the bridge to effectively filter traffic, ensuring that data is only sent to the relevant segment of the network.

By linking separate LAN segments, a bridge creates a seamless network environment, allowing devices on different segments to communicate as if they were on the same LAN. This not only enhances network efficiency but also helps reduce traffic congestion, as it prevents unnecessary data from circulating across the entire network.

Additionally, by extending the reach of a LAN, a bridge helps to segment larger networks into smaller, more manageable parts, thus improving overall network performance and scalability.

How Does a LAN Bridge Work?

A LAN bridge connects two or more network segments at the data link layer (Layer 2) of the OSI model, allowing them to function as a single, unified network. The bridge has a built-in MAC address table that it uses to filter and forward data packets between the connected segments. When a data packet arrives at one of the bridge’s ports, the bridge examines the packet's destination MAC address.

If the destination MAC address is within the same network segment from which the packet originated, the bridge filters the packet, preventing it from being forwarded to other segments, thus reducing unnecessary traffic. If the destination MAC address belongs to a device on a different segment, the bridge forwards the packet to the appropriate segment.

The bridge continually learns and updates its MAC address table by observing the source MAC addresses of incoming packets. Over time, this learning process allows the bridge to build an accurate map of the network, optimizing the forwarding of data packets. This process helps maintain efficient communication across network segments while minimizing congestion and collisions, contributing to network stability and performance.

LAN Bridge Types

There are three primary types of LAN bridges, each serving a specific purpose in network management: transparent bridges, source routing bridges, and translational bridges.

Transparent Bridge

A transparent bridge is the most common type of LAN bridge. It operates by learning the MAC addresses of devices on the network as it forwards data packets between network segments. The term "transparent" refers to the fact that devices on the network are unaware of the bridge's presence—it operates seamlessly in the background. The bridge builds a MAC address table by observing incoming frames and recording the source addresses. When a frame is received, the bridge uses this table to decide whether to forward the frame to another segment or filter it out, reducing unnecessary traffic. Transparent bridges are ideal for extending LANs or segmenting them to improve performance.

Source Routing Bridge

Source routing bridges are typically used in token ring networks. Unlike transparent bridges, which rely on MAC addresses and a dynamically built forwarding table, source routing bridges require the sending device to specify the route that a data packet should take through the network. This information is included in the frame header, enabling the bridge to forward the packet based on the pre-determined path. While this approach can provide more control over data routing, it requires more complex configuration and is less common in modern Ethernet networks.

Translational Bridge

A translational bridge is used to connect two different types of network architectures, such as Ethernet and token ring networks. Since these networks use different frame formats and protocols, the translational bridge performs the necessary conversions to allow communication between them. This includes translating between different MAC address formats, frame sizes, and network protocols. Translational bridges are particularly useful in environments where legacy systems need to communicate with modern network infrastructures, ensuring interoperability between diverse network technologies.

LAN Bridge Models

LAN bridges can be implemented in various models depending on the network architecture and requirements. The primary LAN bridge models include the following:

Local bridge. A local bridge connects multiple LAN segments within the same physical location or close proximity. This model is typically used to divide a large LAN into smaller, more manageable segments, reducing congestion and improving performance. Local bridges operate by filtering and forwarding data packets between segments based on MAC addresses, allowing devices on different segments to communicate as if they were on the same network.
Remote bridge. A remote bridge connects LAN segments that are geographically separated, often across wide-area networks (WANs) or different locations within an organization. Remote bridges extend the reach of a LAN by forwarding data between distant segments, enabling seamless communication across remote sites. They typically use point-to-point connections, such as leased lines or VPNs, to bridge the gap between networks. Remote bridges are essential for organizations with multiple offices or branches that need to maintain a unified network.
Wireless bridge. A wireless bridge connects LAN segments using wireless technology instead of traditional wired connections. This model is particularly useful in scenarios where physical cabling is impractical or impossible, such as connecting buildings in a campus environment or linking network segments in a temporary setup. Wireless bridges use radio frequency signals to transmit data between segments, providing flexibility in network design. Depending on the network requirements, they operate in either point-to-point or point-to-multipoint configurations.
Layer 2 bridge. A Layer 2 bridge, also known as a data link layer bridge, operates solely at the data link layer (Layer 2) of the OSI model. It forwards frames based on MAC addresses without involving any network layer (Layer 3) information, such as IP addresses. Layer 2 bridges are the most common type of bridge and are used to create a seamless network environment by connecting multiple LAN segments into a single broadcast domain. This model simplifies network management and improves efficiency by reducing broadcast traffic.
Layer 3 bridge. A Layer 3 bridge combines the functionality of a traditional bridge with that of a router. While it primarily operates at the data link layer, it also has the capability to route data based on IP addresses (Layer 3). This hybrid model allows the bridge to perform both bridging and routing functions, making it suitable for complex network environments where segmentation and routing are necessary. Layer 3 bridges are often used in larger networks where advanced traffic management and segmentation are required.

LAN Bridge Advantages and Disadvantages

Understanding the advantages and disadvantages of using a LAN bridge is crucial for determining its suitability in a network environment. This section explores the key advantages and disadvantages of implementing a LAN bridge.

Advantages

Here are some key advantages of using a LAN bridge in a network:

Network segmentation. LAN bridges help segment a large network into smaller, more manageable sections. By dividing the network into segments, bridges reduce overall traffic, limit collisions, and improve performance. Segmentation also allows for better control over traffic flow, ensuring that data is only sent to relevant parts of the network.
Traffic filtering. A LAN bridge filters network traffic based on MAC addresses, ensuring that only necessary data is forwarded between segments. This reduces unnecessary traffic on the network, leading to more efficient data transmission and reduced congestion, which is particularly beneficial in busy network environments.
Extended network reach. Bridges extend the reach of a LAN by connecting multiple segments, even across different physical locations. This allows for the creation of larger, unified networks that can span multiple buildings or sites without requiring all devices to be on the same physical LAN.
Improved network performance. LAN bridges can enhance network performance by reducing unnecessary traffic and limiting broadcast domains. They help to maintain optimal communication speeds between devices by ensuring that only relevant data is transmitted.
Ease of deployment. LAN bridges are relatively easy to deploy and configure compared to more complex networking devices like routers. They can be integrated into existing networks without significant changes to the network architecture, making them a cost-effective solution for improving network performance.
Compatibility. LAN bridges are compatible with different types of network segments, allowing them to connect various network technologies, such as Ethernet and Token Ring. This makes bridges versatile tools for integrating diverse network environments.

Disadvantages

LAN bridges, while beneficial in many scenarios, have certain disadvantages that impact network performance and management. Here are some key disadvantages:

Inefficiency in large broadcast domains. In large networks with extensive broadcast domains, LAN bridges can exacerbate inefficiencies. Since they do not segment broadcast domains, all devices within the domain receive broadcast messages, leading to unnecessary traffic. This can reduce overall network performance, particularly in environments with many devices or high levels of broadcast traffic.
Increased latency. As a LAN bridge processes and forwards data packets between network segments, it can introduce additional latency, especially in larger networks with multiple bridges. This delay occurs because the bridge must examine the destination MAC address and determine the correct segment to which to forward the packet. In time-sensitive applications, this added latency can be detrimental to performance.
Limited scalability. LAN bridges are typically effective in smaller or moderately sized networks, but they become less efficient as the network grows. As more devices and segments are added, the bridge's MAC address table becomes increasingly complex, leading to slower processing times and potential bottlenecks. In large networks, this limitation hinders overall network performance and scalability.
Broadcast traffic propagation. LAN bridges forward broadcast traffic to all connected segments, which leads to unnecessary traffic on the network. In environments with heavy broadcast traffic, this causes congestion and reduces network efficiency. Unlike routers, which contain broadcast domains, bridges extend them, potentially amplifying the negative effects of excessive broadcast traffic.
Complexity in troubleshooting. Networks that rely heavily on LAN bridges can become complex, making troubleshooting more challenging. Identifying the source of a problem is difficult when multiple bridges are involved, as data can traverse several segments before reaching its destination. This complexity increases the time and effort required to diagnose and resolve network issues.
Security vulnerabilities. Because LAN bridges operate at Layer 2, they do not provide the same level of security features as routers, which operate at Layer 3. Bridges do not inspect or filter traffic based on IP addresses, making them more susceptible to certain types of attacks, such as MAC address spoofing. Without additional security measures, a bridged network can be vulnerable to intrusions and unauthorized access.