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A network contains $6$ routers P1 to P6 interconnected with the following link costs:

 

 

Routers exchange information using a distance vector algorithm.

Once routing stabilises, some links do not appear in the forwarding tables.

All such unused link costs are now changed to $1$, and the routing algorithm is executed again until stabilisation.

After reconvergence, how many links still remain unused?

2 Answers

3 3 votes

P1-P2:

direct 4 vs via P1-P3-P2 $=6+2=8 \rightarrow$ use direct.

P1-P3:

direct $6 \rightarrow$ used.

P2-P3:

cost $2 \rightarrow$ used.

P2-P4:

direct 9 vs via P2-P3-P5-P4 $=2+8+2=12 \rightarrow$ direct 9 is used.

P4-P5:

cost $2 \rightarrow$ used.

P5-P6:

cost $3 \rightarrow$ used.

P4-P6:

direct 5 vs via P4-P5-P6 = $2+3=5$, equal

DV will choose fewer hops → direct

→ so P4-P6 is used.

→ No Initial unused links

Every edge is optimal.

Hence no edge cost will be changed.

Final answer: 0

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