Data Communications Networking Devices - 4th Ed.pdf

5.2 ROUTERS ____________________________________________________________________ 565Table operationSimilar to bridges, routers make forwarding decisions using tables. Unlike a bridgethat may employ a simple table look-up procedure to determine if a destinationaddress is on a particular network, a router may employ a much more sophisticatedforwarding decision criterion. For example, a router may be con®gured to analyzeseveral paths based upon an algorithm and to dynamically select a path based uponthe results of the algorithm.Advantages of useThe use of routers provides a number of signi®cant advantages in comparison tothe use of bridges. To illustrate those advantages we will examine the use of routersshown in Figure 5.16 in which four corporate of®ces containing seven local areanetworks are interconnected through the use of four routers. In this examplenetworks Aand B are located in a building in Los Angeles, networks C and D arelocated in New York, network E is located in Washington, DC, and networks F andG are located in Miami.Multiple path transmission and routing controlSuppose a station on network Ain Los Angeles requires transmission to a stationon network G in Miami. Initially, router R1 might use the path R1±R4 to transmitdata between networks. If the path should fail or if an excessive amount of traf®c¯ows between Los Angeles and Miami using that path, router R1 can seek toestablish other paths, such as R1±R3±R4 or even R1±R2±R3±R4. In fact, manyrouters will consider each packet as a separate entity, routing the packet to itsdestination over the best available path at the time of transmission. Although thiscould conceivably result in packets arriving at R4 out of sequence, routers have theability to resequence packets into their original order prior to passing data onto thedestination network.Flow controlAs data ¯ows through multiple paths towards its destination it becomes possible fora link to become congested. For example, data from a station on network C andnetwork E routed to network G might build up to the point where the path R3±R4becomes congested. To eliminate the possibility of packet loss, routers will use ¯owcontrol. That is, they will inhibit transmission onto a link as well as notify otherrouters to inhibit data ¯ow until there is an available level of bandwidth for traf®c.Frame fragmentationAs previously mentioned, bridges cannot break a frame into a series of frames whentransmission occurs between networks with different frame sizes. This situationrequires workstations to be con®gured to use the smallest maximum frame size of