Bellman Ford Distance Vector Algorithm3/12/2021
The distance is measured in hops and the vector is the direction to which a datagram is sent over a given set of network nodes.EIGRP uses Diffusing Update Algorithm (Advanced Distance-Vector Routing Protocol), and both IS-IS and OSPF use Dijkstras Algorithm (Link-State Routing Protocol).Have the confidence that you will pass on your first attempt.
A B C 0 1 2 A B C 0 3 4 A B C 0 5 6 F igure 5.10: Bad news travel slowly when nodes use the Bellman-F ord algorithm. T o nd the shortest paths from node A to all the other nodes, Dijkstra s algorithm computes recursively the set P( k ) of the k -c losest nodes to node A (shown with black nodes). T o nd P( 2 ), it adds the c losest node to the set P( 1 ), here B, and wr ites its distance to A, here 1. T o nd P( 3 ), the algorithm adds the c losest node to P( 2 ), here E, and writes its distance to A, here 2. The algorithm continues in this way, breaking ties according to a deterministic rule, here favoring the lexicographic order, thus adding C before F. This is a one-pass algorithm whose number of steps is the number of nodes. After running the algorithm, each node remembers the next step along the shortest path to each node and stores that information in a routing table. ![]() Accordingly, when A gets a packet destined to E,i t sends it along link AB. Node B then forwards the pac ket to E. BEL LMAN-FOR D AND DIST ANCE VECTOR Figure 5.9 shows the operations of a distance vector algor ithm. The routers regular ly send to their neighbors their current estimate of the shortest distance to the other routers. Initially, only node D knows that its distance to D is equal to zero, and it sends that estimate to its neighbors B and C. Router B adds the length 1 of the link BD to the estimate 0 it got from D and concludes that its estimated distance to D is 1. Similar ly, C estimates the distance to D to be equal to 3 0 3. Node A then compar es the distances 2 1 of going to D via node B and 1 3 that corresponds to going to C rst. Node A conc ludes that the estimated shor test distance to D is 3. F or instance, the routing table at router A is the same as using the link state algorithm and is shown in T able 5.1. Bad N ews T ravel Slowly One difculty with this algorithm is that it may take a long time to converge to new shortest paths when a link fails, as the example below shows. The top of F igure 5.10 shows three nodes A, B, C (on the right) attached with links with metric 1. The Bellman-F ord algorithm has converged to the estimates of the lengths of the shortest paths to destination C. The middle part of the gure sho ws the next iteration of the Bellman-F ord algorithm. Node A advertises its estimate 2 of the shortest distance to node C and node B r ealizes that the length of the link BC is now innite. Bellman Ford Distance Vector Algorithm Plus The DistanceN ode B c alculates its new estimate of the shortest distance to C as 1 2 3, which is the length of the link from B to A plus the distance 2 that A advertises from A to C. Upon receiving that estimate 3 from B, A updates its estimate t o134. T h e bottom par t of the gure shows the next iteration of the algorithm. Bellman Ford Distance Vector Algorithm How To Handle SuchW e explain how to handle such slow convergence in the next section.
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