Internet Technologies Tutorials 2 a) What is so-called Shortest Path Routing? Take the following abstract model of a network as an example, list the steps to find the shortest path from A to
D. Can you find the shortest path from A to all the other nodes? b) Consider the network shown in Figure 2, and assume that each node initially knows the costs to each of its neighbours. Consider the distance vector algorithm and show the distance table entries at node E. Show how the algorithm works by computing a table.
A
2
2
1
E
1
Figure 2
1
5
0
B
1
5
D
1.
What is so called Broadcast Routing? How many approaches can be used for
Broadcast Routing? What are they? Can you tell the steps to use reverse path forwarding to broadcast packet from A to all the other hosts in the following subnet?

Internet Technologies Tutorials 2 c) Consider the network fragment shown in Figure 1 below. x has only two attached neighbours, w and y . w has a minimum-cost path to destination u ( not shown) of 5, and y has a minimum-cost path to u of 6. The complete paths from w and y to u (and between w and y ) are not shown. All link costs in the network have strictly positive integer values.
(15marks)
Figure 1
1.
Give x
’s distance vector for destinations w, y , and u .
2.
Give link-cost change for either c(x,w) or c(x,y) such that x will inform its neighbours of a new minimum-cost path to u as a result of executing the distance vector algorithm
3.
Give the link-cost cost change for either c(x,w) or c(x,y) such that x will not inform its neighbours of a new minimum-cost path to u as a result of executing the distance vector algorithm.
Answer: i).
D x
(y) = 4, D x
(w) = 1, D x
(u) = 6 ii).
First consider what happens if c(x,y) changes. If c(x,y) becomes larger or smaller (as long as c(x,y) > 0 ), the least cost path from x to u will still have cost 6 and pass through w . Thus a change in c(x,y) will not cause x to inform its neighbors of any changes. Now consider if c(x,w) changes.

Internet Technologies Tutorials 2
If c(x,w) =
 
5, then the least-cost path to u continues to pass through w and its cost changes to 5 +

; x will inform its neighbors of this new cost. If c(x,y) =

> 5 , then the least cost path now passes through y and has cost 10; again x will inform its neighbors of this new cost. iii).
Any change in link cost c(x,y) will not cause x to inform its neighbors of a new minimum-cost path to u .