Explain distance vector routing with example.

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In distance-vector routing (DVR), each router is required to inform the topology changes to its neighboring routers periodically. Historically it is known as the old ARPNET routing algorithm or Bellman-Ford algorithm.

How the DVR Protocol Works

  • In DVR, each router maintains a routing table. It contains only one entry for each router. It contains two parts − a preferred outgoing line to use for that destination and an estimate of time (delay). Tables are updated by exchanging the information with the neighbor’s nodes.
  • Each router knows the delay in reaching its neighbors (Ex − sending echo requests).
  • Routers periodically exchange routing tables with each of their neighbors.
  • It compares the delay in its local table with the delay in the neighbor’s table and the cost of reaching that neighbor.
  • If the path via the neighbor has a lower cost, then the router updates its local table to forward packets to the neighbor.

Example − Distance Vector Router Protocol:

In the network shown below, there are three routers, A, B, and C, with the following weights − AB =2, BC =3 and CA =5.

Step 1 − In this DVR network, each router shares its routing table with every neighbor. For example, A will share its routing table with neighbors B and C, and neighbors B and C will share their routing table with A.

- Hamro CSIT

Form A A B C
A 0 2 3
B
C

 

Form B A B C
A
B 2 0 1
C

 

Form C A B C
A
B
C 3 1 0

Step 2 − If the path via a neighbor has a lower cost, then the router updates its local table to forward packets to the neighbor. In this table, the router updates the lower cost for A and C by updating the new weight from 4 to 3 in router A and from 4 to 3 in router C.

Form A A B C
A 0 2 3
B
C

 

Form B A B C
A
B 2 0 1
C

 

Form C A B C
A
B
C 3 1 0

Step 3 − The final updated routing table with lower cost distance vector routing protocol for all routers A, B, and C is given below −

Router A:

Form A A B C
A 0 2 3
B 2 0 1
C 3 1 0

Router B:

Form B A B C
A 0 2 3
B 2 0 1
C 3 1 0

Router C:

Form C A B C
A 0 2 3
B 2 0 1
C 3 1 0
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