Bridging vs Routing

Question:
What are the differences between bridging and routing?

Answer:

A bridge is a device that connects two or more network segments. A routeris also a device that connects two or more network segments, so what is thedifference?

When a bridge is installed between two networks, it gathers the packets fromone network and repeats them at the other and vice-versa. This way, nodeson one network can talk to nodes on the other. Bridges also have suchfeatures as broadcast filtering to allow for greater efficiency and reducetraffic. Therefore, in its most simple form, a bridge is a two-port networkdevice that connects two network segments. Some advanced implementation ofbridging can monitor traffic and determine which nodes are on which segmentand later use this information while forwarding packets. A main factor tonote is the fact that bridging takes place in the Data-link Layer of theOSI reference model.

Routers, on the other hand, can be classified as packet exchanges orswitches. They deal with packets of information. Most importantly, routersoperate in the Network Layer of the OSI model. They interconnect networksegments, LANs and WANs. In their advanced forms, they can provide variousschemes of filtering, path control and traffic control functions.

Routers can handle one or more protocols such as TCP/IP, IPX, etc. Byoperating in the Network Layer, router allow networks to be segmented bycreating uniquely addressable networks. Each segment can have its ownnetwork number and this information used during packet addressing/delivery.

When a packet arrives at a router, its checksum information is verified toensure accuracy. The physical and data-link layer (layers below the networklayer) information is stripped. The network layer information is thendeciphered. If the packet is meant for the router that received it, therest of the information is processed. On the other hand, if the packet wasaddressed to another destination on the same network, the router simplyforwards the packet. Filtering controls are implemented at this point ifimplemented and packet might be discarded if the situation demands. If thedestination address belongs to a remote network, the router consults itsrouting table before forwarding the packet. The router maintains a routingtable internally to keep a picture of the network and the various paths ineffect. Sometime, the packet itself might contain source routinginformation and if this is the case, it is utilized by the router duringdelivery or forwarding. If the destination cannot be found or the hop countis exceeded (such a feature is sometimes used in protocols such as TCP/IP),the packet is discarded and an error message is sent back to the source ofthe packet.

When two or more network segments are connected by bridges and a node sendsout a packet, a typical bridge would repeat the packet on all the segmentson the segments that are connected to it. Contrast this with routing whichis discussed below. A router would only copy the packet to the network thathosts the intended destination rather than send packets on all links.

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