Lec-12: Bridges In Computer Networks | Physical and data link layer device

Summary

This video explains the role of bridges in computer networks, highlighting how they connect two different LANs, such as token ring and Ethernet networks. The script discusses the two types of bridges—static and dynamic—explaining their functions in forwarding and filtering data based on MAC addresses. It also covers the concept of collision domains, the store-and-forward technique used by bridges, and the creation of spanning trees to avoid packet loops. The video is particularly useful for students preparing for UGC NET, PSU exams, and IT officer exams, though it is less relevant for GATE.

Takeaways

• 😀 Bridges are used to connect two different LANs, enabling communication between devices with different topologies.
• 😀 Unlike hubs, which can connect two LANs, bridges specifically connect two different LANs, such as token ring and Ethernet networks.
• 😀 Bridges operate at the physical and data link layers, which allows them to read and forward packets based on MAC addresses.
• 😀 Forwarding involves sending packets to the appropriate destination, while filtering prevents unnecessary traffic by blocking packets aimed at devices on the same side of the bridge.
• 😀 Static bridges require manual configuration of MAC addresses and port connections by a network administrator.
• 😀 Dynamic (transparent) bridges learn MAC addresses dynamically, building and updating a forwarding table without manual intervention.
• 😀 Dynamic bridges initially broadcast packets to discover the source and destination MAC addresses, updating their tables as they learn the network's layout.
• 😀 The store and forward method used by bridges prevents collisions by storing packets temporarily in memory before forwarding them.
• 😀 Unlike hubs and repeaters, which can experience multiple collisions, bridges significantly reduce the likelihood of collisions by using buffering and processing.
• 😀 The Spanning Tree Protocol (STP) helps bridges avoid network loops, ensuring packets are forwarded along the most efficient path without getting stuck in loops.
• 😀 Bridges are particularly relevant for exams such as UGC NET, PSU, and IT Officer exams in the banking sector, where knowledge of these concepts is often tested.

Q & A

What is the primary purpose of a bridge in computer networks?

The primary purpose of a bridge in computer networks is to connect two different LANs (Local Area Networks), allowing communication between devices in different networks with different topologies, such as token ring and Ethernet.

How does a bridge forward packets in a network?

A bridge forwards packets by checking the MAC (Media Access Control) addresses in the data link layer. It determines if the packet needs to be sent ahead or filtered by examining whether the destination MAC address is on the same side or the opposite side of the bridge.

What is the key difference between a bridge and a hub?

While both bridges and hubs connect devices in a network, a hub broadcasts packets to all devices, regardless of whether they are needed. In contrast, a bridge checks MAC addresses and selectively forwards packets, which helps in reducing unnecessary traffic and filtering packets.

What is meant by filtering in the context of bridges?

Filtering refers to the process where the bridge checks if a packet needs to be forwarded or blocked. If the destination MAC address is on the same side of the bridge, the packet is filtered, meaning it is not forwarded to the other side, thus preventing unnecessary network traffic.

What are the two types of bridges discussed in the script?

The two types of bridges are static bridges and dynamic (or transparent) bridges. Static bridges require the network administrator to manually configure MAC addresses and ports, while dynamic bridges learn the MAC addresses and port associations automatically over time.

How does a static bridge maintain its MAC address table?

A static bridge requires the network administrator to manually input the MAC addresses and their corresponding ports into the bridge's MAC address table. This means if a device moves or its MAC address changes, the administrator must update the table manually.

What is the disadvantage of using static bridges?

The main disadvantage of static bridges is the need for manual updates. If a device changes its MAC address or moves to a different port, the network administrator must manually update the table, which can be cumbersome and error-prone.

How do dynamic bridges differ from static bridges in terms of learning MAC addresses?

Dynamic bridges learn MAC addresses automatically. When a packet is received, the bridge updates its MAC address table based on the source and destination addresses and the ports used. This allows the bridge to forward or filter packets intelligently without requiring manual configuration.

What is the 'store and forward' technique used by bridges?

The 'store and forward' technique involves the bridge storing incoming packets in its buffer, processing them, and then forwarding them to the appropriate destination. This method helps prevent collisions and ensures that packets are properly processed before transmission.

Why do bridges help prevent collisions in a network?

Bridges prevent collisions by using the 'store and forward' technique. Since the bridge processes packets in its buffer before forwarding them, multiple packets can be stored and processed sequentially, reducing the likelihood of collisions that would occur if they were sent simultaneously.

What role does the Spanning Tree Protocol play in bridge operations?

The Spanning Tree Protocol (STP) is used by bridges to prevent loops in the network. Since bridges may have multiple paths for forwarding packets, STP ensures that only one path is active at a time, eliminating the risk of packets circulating in an endless loop.