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Dated: Jul. 05, 2012

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Something About Bridges

LAN Bridges were used to separate and isolate multiple collision domains from each other in large broadcast domains. Bridges had two or more LAN ports to which other central devices, such as network Hubs, were connected via LAN cables. Unlike network Hubs that used to forward data packets to every PC that was connected to the ports, Bridges acted more like LAN Switches to identify destination ports and transferred the data packets only if the packets were destined to reach the segments that were connected to their other ends. Bridges used to calculate and identify the destination segments on the basis of physical (MAC) addresses of the central devices that were connected to their LAN ports.

Working of Bridges

LAN BridgeThe scenario discussed below can be quite helpful in understanding the working of LAN Bridges.

A Bridge B1 has two LAN ports namely L1 and L2. L1 is connected to a Hub H1 which is further connected to two PCs P1 and P2. Similarly L2 is connected to another Hub H2 and H2 is further connected to two other PCs namely P3 and P4. In this scenario if a packet is transferred from P1 and is destined to reach P2 (that belongs to the same segment), because of the default nature of Hubs, the data packet would be forwarded to every port that is connected to H1 including L1. Since L1 is the LAN port of the Bridge, as soon as the packet reaches L1, the Bridge verifies the destination MAC address of the packet and as in this example the packet is destined to reach P2, B1 would not forward the packet from L1 to L2. In this way the LAN segment connected to L2 would remain unaffected in case network congestions occur in the LAN segment connected to L1.

Another possible situation may be, if P1 initiates packet transfer that is destined to reach P4, as the default nature of Hubs, H1 would broadcast the packet to every node connected to it, including L1. At B1’s part, the Bridge verifies the destination MAC address of the packet and since the packet is destined to reach P4 this time, which resides at the segment that is connected to L2, B1 would forward the packet from L1 to L2.

Application of LAN Bridges

LAN Bridges were earlier used when the Hubs were mostly used as central devices and LAN Switches either did not exist or were very expensive to purchase. In such scenarios, in order to isolate collision domains to reduce network congestions, administrators used to install Bridges.

LAN Bridges have become obsolete nowadays and are no more used in any network setup. The reason why this happened is that the Hubs are no longer available in the market, and administrators use LAN Switches as central devices which never allow collisions to occur and therefore no collision domains exist these days.

Since Bridges identify data packets on the basis of MAC addresses, they work on Data Link Layer which is the 2nd layer of OSI reference model.

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