tailieunhanh - Internetworking with TCP/IP- P35

Internetworking with TCP/IP- P35: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. Routing Information Protocol RIP 299 As Figure shows router Rl has a direct connection to network 7 so there is a route in its table with distance 7 which will be included in its periodic broadcasts. Router R2 has learned the route from R installed the route in its routing table and advertises the route at distance 2. Finally R3 has learned the route from R2 and advertises it at distance 3. Now suppose that R s connection to network 7 fails. 7 will update its routing table immediately to make the distance 76 infinity . In the next broadcast will report the higher cost route. However unless the protocol includes extra mechanisms to prevent it some other router could broadcast its routes before 7 . In particular suppose R2 happens to advertise routes just after R s connection fails. If so RI will receive R2s message and follow the usual distance-vector algorithm it notices that R2 has advertised a route to network 7 at lower cost calculates that it now takes 3 hops to reach network 7 2 for R2 to reach network 7 plus 7 to reach R2 and installs a new route with R2 listed as the next hop. Figure depicts the result. At this point if either 7 or R2 receives a datagram destined for network 7 they will route the datagram back and forth until the datagram s time-to-live counter expires. Subsequent RIP broadcasts by the two routers do not solve the problem quickly. In the next round of routing exchanges Rt broadcasts its routing table entries. When it learns that 7 s route to network 7 has distance 3 R2 calculates a new distance for its route making it 4. In the third round R receives a report from R2 which includes the increased distance and then increases the distance in its table to 5. The two routers continue counting to RIP infinity. Solving The Slow Convergence Problem For the example in Figure it is possible to solve the slow convergence problem by using a technique known as split horizon update. When using split horizon a router does