Understanding Network Bridges: Functionality and Operation

This tutorial dives into the fundamentals of networking, covering essential topics such as circuit switching vs. packet switching, the intricacies of the TCP/IP protocol, the functions of ARP and RARP protocols, the significance of IP and MAC addresses, and a detailed look at networking devices like hubs, switches, bridges, routers, gateways, and firewalls.

This page specifically focuses on network bridges, exploring their basics and functionalities.

What is a Network Bridge?

A network bridge operates at Layer 2 (the Data Link Layer) of the OSI model. Its primary function is to connect multiple network segments at this layer.

Unlike hubs or repeaters, which simply broadcast traffic, a bridge intelligently manages network traffic as described below.

Bridges utilize a bridge table (also known as a forwarding database) to efficiently route frames across network segments. Initially, this table is empty. The bridge populates it by observing frames received from the various nodes (computers) connected to the network segments.

When a frame arrives, the bridge consults its table. If the destination address is not found, the bridge broadcasts the frame to all its ports. The destination network node then responds, establishing a route that is then recorded within the table.

How a Bridge Works: Filtering, Forwarding, and Flooding

When a bridge receives a frame, it examines the destination MAC address and determines whether to filter, _flood, or _copy** the frame to the appropriate network segment. Here’s a breakdown of these processes:

1. Filtering: If the destination node resides on the same network segment as the originating node, the bridge prevents the frame from being forwarded to other network segments. This process is known as filtering and reduces unnecessary traffic.

2. Forwarding: If the destination node is located on a different network segment, the bridge forwards the received frame only to that specific segment.

3. Flooding: If the destination address in the received frame is unknown (i.e., not present in the bridge table), the bridge forwards the frame to all network segments except the one from which the frame originated. This process is called flooding. The destination node will then respond and its location added to the bridge table for future use.