Network Bridges: Definition, Operation, and Role

What is a Network Bridge?

A network bridge is a Layer 2 (Data Link layer) device that connects and segments different network segments within a LAN. Its primary function is to filter and forward traffic based on MAC addresses and to divide large collision domains into smaller, more efficient ones.

Example: John’s company has two departments—Accounts and HR—each with its own group of computers. A bridge connects both segments, allowing necessary traffic to cross while keeping local traffic isolated.

Role in Dividing Collision Domains and Extending LANs

Each port on a bridge creates a separate collision domain, reducing congestion and improving network efficiency.
Bridges extend LANs by connecting multiple segments to communicate as if part of a single network.

Types of Bridges

Bridge Type	Description	Typical Use Case
Transparent Bridges	Most common type, operates invisibly to devices, learns and forwards frames without modification.	Ethernet networks requiring simple segmentation.
Source Routing Bridges	Used mainly in Token Ring networks, rely on source device to define the frame path.	Token Ring legacy networks.
Translational Bridges	Connect two different types of networks (e.g., Ethernet to Token Ring).	Legacy network integration across different media.

Bridge Operation

Frame Forwarding Based on MAC Addresses

Bridges inspect the MAC address of incoming frames to decide whether to forward or filter them. If the destination MAC is on another segment, the frame is forwarded; if local, it is filtered.

Example: When John in Accounts sends a file to HR, the bridge consults its MAC table. If it knows HR’s MAC is on the other segment, it forwards the frame; otherwise, it filters it out.

Learning MAC Addresses and Building Forwarding Tables

Bridges dynamically build MAC address tables by observing the source MAC address of incoming frames and mapping them to ports.

Bridges vs. Switches

Aspect	Bridge	Switch
Number of Ports	Typically 2–4	Multiple (24, 48, or more)
Frame Processing	Software-based (slower)	Hardware/ASIC-based (faster)
Scalability	Limited	High
Functionality	Basic segmentation	Advanced features like VLANs, security, QoS
Usage Today	Rare, mostly legacy	Common and essential for modern LANs

Spanning Tree Protocol (STP) and Bridges

Role in Preventing Loops

In bridged networks, redundant paths can cause loops, leading to broadcast storms. The Spanning Tree Protocol (IEEE 802.1D) enables bridges to negotiate a loop-free topology by blocking redundant links.

Bridge Protocol Data Units (BPDUs)

Bridges exchange BPDUs to share network topology information, elect a root bridge, and determine which ports should forward or block traffic.

Example: John adds a redundant cable between Accounts and HR segments. STP ensures only one path is active to prevent broadcast storms.

Bridging Domains and VLANs

Traditional bridges do not support VLANs natively. Some modern bridging devices can forward VLAN-tagged traffic or act as VLAN trunks in special scenarios. However, VLANs are primarily implemented using switches.

Advantages and Limitations of Bridges

Advantages	Limitations
Reduces collision domains, improving network efficiency. Extends LANs without requiring routing. Operates transparently—no device configuration needed.	Slower software-based processing compared to switches. Limited scalability with few ports. Lacks advanced features like security and VLAN support.

Configuration and Management of Bridges

Bridges can be managed by configuring bridge groups and interface settings, especially in legacy or Linux-based environments.

Commands such as show bridge can display forwarding tables and status.

Monitoring MAC address tables helps ensure correct learning and forwarding behavior.

Use Cases for Bridges

Legacy network integration: Connecting older network segments with modern LANs.
LAN extension without routers: Simple expansion where routing is not required.
Traffic isolation: Reducing collisions and broadcast traffic in small environments.

Example: John uses a bridge to connect his office’s wired LAN to a wireless segment, enabling seamless communication without the complexity of routing.

Key Points & Tips for the Exam

Bridges operate at OSI Layer 2 and forward traffic based on MAC addresses.
Each port on a bridge is a separate collision domain.
Know the differences between bridges and switches: switches have more ports, hardware acceleration, and advanced features.
Bridges help eliminate collisions but do not prevent broadcast traffic.
Understand the importance of STP to prevent loops in bridged networks.
Bridges are mostly legacy technology; switches have largely replaced their function.
Translational bridges connect different network media types (e.g., Ethernet to Token Ring).
Useful in small, low-traffic, or special integration scenarios.

Network Bridge Quiz

1. At which OSI layer does a network bridge operate?

A. Layer 1 (Physical) B. Layer 3 (Network) C. Layer 2 (Data Link) D. Layer 4 (Transport)

Correct answer is C. Bridges operate at Layer 2, using MAC addresses to filter and forward traffic.

2. What is the primary function of a bridge in a network?

A. Connect and segment different LAN segments, filtering traffic by MAC address B. Route packets between different IP networks C. Convert analog signals to digital D. Encrypt data at the application layer

Correct answer is A. Bridges connect LAN segments and reduce collision domains by filtering based on MAC addresses.

3. How does a bridge decide whether to forward or filter a frame?

A. Based on the destination IP address B. By checking if the destination MAC address is on a different segment C. By packet size D. Randomly

Correct answer is B. Bridges forward frames only if the destination MAC is on a different segment; otherwise, they filter it.

4. Which type of bridge is most common and operates invisibly to devices?

A. Source routing bridge B. Translational bridge C. Wireless bridge D. Transparent bridge

Correct answer is D. Transparent bridges are common and operate without requiring configuration on devices.

5. What does the Spanning Tree Protocol (STP) do in a bridged network?

A. Prevents loops by blocking redundant paths B. Routes packets between VLANs C. Increases broadcast traffic D. Encrypts network traffic

Correct answer is A. STP blocks redundant paths to prevent broadcast storms and loops.

6. How do bridges build their MAC address forwarding tables?

A. Manually configured by administrators B. Using IP address tables C. Dynamically by observing source MAC addresses on incoming frames D. By scanning network broadcasts

Correct answer is C. Bridges learn MAC addresses dynamically by inspecting the source address of received frames.

7. What is a limitation of software-based bridges compared to modern switches?

A. They support VLANs extensively B. They are slower and have limited scalability C. They provide hardware-based filtering D. They handle Layer 3 routing

Correct answer is B. Software-based bridges are slower and less scalable than hardware-based switches.

8. What is a translational bridge used for?

A. Connecting two identical Ethernet segments B. Encrypting LAN traffic C. Routing between VLANs D. Connecting two different types of networks, e.g., Ethernet and Token Ring

Correct answer is D. Translational bridges connect different network media types or protocols.

9. How many collision domains does each port on a bridge create?

A. One per port B. One for the entire bridge C. None D. Equal to the number of VLANs

Correct answer is A. Each bridge port represents a separate collision domain, reducing collisions.

10. Which statement is true comparing bridges and switches?

A. Bridges have dozens of ports; switches usually have 2-4 B. Bridges are faster because they use hardware ASICs C. Switches are multiport, hardware-based, and more scalable than bridges D. Switches cannot segment collision domains

Correct answer is C. Switches offer higher port density and faster, hardware-based forwarding.

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