Network bridge basics

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Network Bridge

This tutorial covers everything one like to know about networking basics including circuit switching
vs packet switching, TCP/IP protocol fields, ARP/RARP protocol fields,
what is IP address ,what is MAC address, networking devices which include hub, switch,
bridge, router, gateway and firewall.

This page describes network bridge basics and functions of network bridge.

The network bridge works at layer 2 i.e. data link layer of OSI model.
It connects multiple network segments at this layer.
Bridge does not simply broadcast traffic from one network segment to the other
unlike hub/repeater but also manages the traffic as mentioned below.

network bridge

Bridges use bridge table to send frames across network segments.
The bridge table is initially empty. It is filled by bridge as it starts receiving frames from nodes(computers) attached in the network segments.
Bridge table is also called forwarding database.

If address entry is not found in the table for the frame received;the frame is broadcast to all the ports of the bridge;the destination network node responds and hence route is created.
In general, if bridge receives the frame from the node on the network, it will check the table and find out destination MAC address.
Based on this MAC address bridge will decide wthether to filter, flood or copy the frame onto corresponding network segment.
This process of bridge is decribed below.

1. If the destination node is on the same side of the network segment as the frame arrives from, the bridge blocks the frame from going to the other network segments. This is called filtering.
2. If the destination node is on a different network segment then the bridge forward the received frame to the appropriate network segment.
3. If the destination address in the frame received is unknown, bridge forward it to all the network segments except the source address. This process is called flooding.