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Consider a network with $6$ routers $\textbf{R1}$ to $\textbf{R6}$ connected with links having weights as shown in the following diagram.

All the routers use the distance vector based routing algorithm to update their routing tables. Each router starts with its routing table initialized to contain an entry for each neighbor with the weight of the respective connecting link. After all the routing tables stabilize, how many links in the network will never be used for carrying any data?

  1. $4$
  2. $3$
  3. $2$
  4. $1$
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7 Answers

Best answer
55 votes
55 votes

Answer (C)

Following will be distance vectors of all nodes.

Shortest Distances from $R_1$ to $R_2, R_3, R_4, R_5\ and\ R_6$
$R_1 (5, 3, 12, 12, 16)$
Links used$:R_1-R_3, R_3-R_2, R_2-R_4, R_3-R_5, R_5-R_6$

Shortest Distances from $R_2$ to $R_3, R_4, R_5\ and\ R_6$
$R_2 (2, 7, 8, 12)$
Links used$: R_2-R_3, R_2-R_4, R_4-R_5, R_5-R_6$

Shortest Distances from $R_3$ to $R_4, R_5$ and $R_6$
$R_3(9, 9, 13)$
Links used$:R_3-R_2, R_2-R_4, R_3-R_5, R_5-R_6$

Shortest Distances from $R_4$ to $R_5$ and $R_6$
$R_4 (1, 5)$
Links used$:R_4-R_5, R_5-R_6$

Shortest Distance from $R_5 to R_6$
$R_5$ (4)
Links Used: $R_5-R_6$

If we mark, all the used links one by one, we can see that following links are never used.
$R_1-R_2$
$R_4-R_6$

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29 votes
C is the right answer.. The links $R_1-R_2$  and $R_4-R_6$ will never be used for
data transfer because there are shorter paths available in any case.
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19 votes
Ans __2

Use your intuition and you will gain answer 2 link

These r respectively R1---R2 and R4------R6.

Find shortest path from each node to other node and mark each edge visited.Then u will automatically find out unused edge.
7 votes
7 votes
INORDER TO REACH R1 TO R2 COST(6)WE HAVE A BETTER PATH R1--> R3-->R2 COST(5), SO WE CAN REMOVE THIS LINK.

Similarly, WE HAVE A BETTER PATH FOR R4 TO R6 COST(8) VIA R5 COST(4).

SO ANSWER IS 2.

OPTION C

YOU NEED NOT DRAW ENTIRE ROUTING TABLE AT ALL.
Answer:

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