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Consider the network shown below with six routers R1 to R6 connected with links having weights as shown in the following diagram.

Network of six routers R1 to R6 with weighted links: R1-R2 2, R1-R3 6, R2-R3 1, R2-R4 8, R2-R5 1, R3-R4 4, R3-R5 3, R4-R6 3, R5-R6 7

What is the initial routines table of Router R4 ?

  • A.\(R_{1}\) 10, \(R_{2}\) 8, \(R_{3}\) 4, \(R_{4}\) 0, \(R_{5}\) ∞, \(R_{6}\) 3
  • B.\(R_{1}\) ∞, \(R_{2}\) 8, \(R_{3}\) 4, \(R_{4}\) 0, \(R_{5}\) 7, \(R_{6}\) 3
  • C.\(R_{1}\) 10, \(R_{2}\) 8, \(R_{3}\) 4, \(R_{4}\) 10, \(R_{5}\) 7, \(R_{6}\) 3
  • D.\(R_{1}\) ∞, \(R_{2}\) 8, \(R_{3}\) 4, \(R_{4}\) 0, \(R_{5}\) ∞, \(R_{6}\) 3

Answer: D

The initial table of R4 is R1 ∞, R2 8, R3 4, R4 0, R5 ∞, R6 3.

In distance vector routing, a router's initial table contains only what it knows directly: a distance of 0 to itself, the link weight to each directly connected neighbour, and infinity to every other router. R4 is linked to R2 (weight 8), R3 (weight 4) and R6 (weight 3).

R1 and R5 are not neighbours of R4, so their entries are infinite at the start. Option A gives R1 the value 10 and option B gives R5 7, which are distances learnt only after routing information is exchanged. Option C also sets R4's distance to itself to 10 instead of 0.

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