Consider a computer network using the distance vector routing algorithm in its…
2021
Consider a computer network using the distance vector routing algorithm in its network layer. The partial topology of the network is shown below.
The objective is to find the shortest-cost path from the router R to routers P and Q. Assume that R does not initially know the shortest routes to P and Q. Assume that R has three neighbouring routers denoted as X, Y and Z. During one iteration, R measures its distance to its neighbours X, Y, and Z as 3, 2 and 5, respectively. Router R gets routing vectors from its neighbours that indicate that the distance to router P from routers X, Y and Z are 7, 6 and 5, respectively. The routing vector also indicates that the distance to router Q from routers X, Y and Z are 4, 6 and 8 respectively. Which of the following statement(s) is/are correct with respect to the new routing table o R, after updation during this iteration?
- A.
The distance from R to P will be stored as 10.
- B.
The distance from R to Q will be stored as 7.
- C.
The next hop router for a packet from R to P is Y.
- D.
The next hop router for a packet from R to Q is Z.
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Correct answer: B, C
Key insight: a router updates its distance to a destination by taking the minimum of (cost to neighbour + neighbour's advertised cost to that destination).
Compute costs from R to P via neighbours:
via X: 3 + 7 = 10
via Y: 2 + 6 = 8
via Z: 5 + 5 = 10
Choose the minimum: 8 via Y. So R should store distance 8 to P with next hop Y.
Compute costs from R to Q via neighbours:
via X: 3 + 4 = 7
via Y: 2 + 6 = 8
via Z: 5 + 8 = 13
Choose the minimum: 7 via X. So R should store distance 7 to Q with next hop X.
Final routing entries (after this iteration): R -> P: distance 8, next hop Y; R -> Q: distance 7, next hop X.
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