Bridges are used to interconnect LAN segment in cheap and easily configured manner. It can also device the networks to isolate traffic or problems. For example, if the amount of traffic between two computers are more, slowing down the entire operation, a bridge can isolate quality of those computers.
Bridging, is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown devices. Once a device has been located, its location is recorded in a table where the MAC address is stored so as to preclude the need for further broadcasting. The utility of bridging is limited by its dependence on flooding, and is thus only used in local area networks.
A network bridge connects multiple network segments at the (data link layer) Layer 2 of the OSI model. In Ethernet networks, the term bridge formally means a device that behaves according to the IEEE 802.1D standard. A bridge and switch are very much alike; a switch being a bridge with numerous ports. Switch or Layer 2 switch is often used interchangeably with bridge.
Bridges are similar to repeaters or network hubs, devices that connect network segments at the (physical layer) Layer 1 of the OSI model. However, with bridging, traffic from one network is managed rather than simply rebroadcast to adjacent network segments. Bridges are more complex than hubs or repeaters. Bridges can analyze incoming data packets to determine if the bridge is able to send the given packet to another segment of the network.
Bridges can be used to :
- Expand the distance of the segment
- Reduce network traffic resulting from an excessive number of attached computers. A bridge can handle overload by dividing it into two separate networks, reducing the amount of traffic on each segment and improve its performance.
- Connect unlike network segment such as Ethernet and token ring, bridge can be used to forward data between them.
- Listens to all traffic
- Checks the source and destination MAC address in the packet
- Builds up a MAC table when information is available
- Once it knows where the nodes are located, it forwards traffic; if the destination is not listed in the routing table, the bridge forwards to all segments and if the destination address is found the it forwards to that segment.
Advantages of Network Bridges :
- Self-configuring
- Simple bridges are inexpensive
- Isolate collision domain
- Reduce the size of collision domain by micro-segmentation in non-switched networks
- Transparent to protocols above the MAC layer
- Allows the introduction of management/performance information and access control
- LANs interconnected are separate, and physical constraints such as number of stations, repeaters and segment length don't apply
- Helps minimize bandwidth usage
- Does not limit the scope of broadcasts [broadcast domain cannot be controlled]
- Does not scale to extremely large networks
- Buffering and processing introduces delays
- Bridges are more expensive than repeaters or hubs
- A complex network topology can pose a problem for transparent bridges. For example, multiple paths between transparent bridges and LANs can result in bridge loops. The spanning tree protocol helps to reduce problems with complex topologies.
- Local Bridge : are used where the network is being locally segmented. Ex., in same building or same floor.
- Remote Bridge : are used in pairs and where the network is remotely segmented. The segments are physically apart, or in different buildings. Remote bridges can have different communication media in between.
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