Computer Network | Routing Protocols Set 1 (Distance Vector Routing)



A distance-vector routing (DVR) protocol requires that a router inform its neighbors of topology changes periodically. Historically known as the old ARPANET routing algorithm (or known as Bellman-Ford algorithm).

Bellman Ford Basics – Each router maintains a Distance Vector table containing the distance between itself and ALL possible destination nodes. Distances,based on a chosen metric, are computed using information from the neighbors’ distance vectors.

Information kept by DV router -
  • Each router has an ID
  • Associated with each link connected to a router, there is a link cost (static or dynamic).
  • Intermediate hops
Distance Vector Table Initialization -
  • Distance to itself = 0
  • Distance to ALL other routers = infinity number.

Distance Vector Algorithm –

  1. A router transmits its distance vector to each of its neighbors in a routing packet.
  2. Each router receives and saves the most recently received distance vector from each of its neighbors.
  3. A router recalculates its distance vector when:
    • It receives a distance vector from a neighbor containing different information than before.
    • It discovers that a link to a neighbor has gone down.

The DV calculation is based on minimizing the cost to each destination

Dx(y) = Estimate of least cost from x to y 
C(x,v) =  Node x knows cost to each neighbor v
Dx   =  [Dx(y): y ∈ N ] = Node x maintains distance vector
Node x also maintains its neighbors' distance vectors
– For each neighbor v, x maintains Dv = [Dv(y): y ∈ N ]

Note –



  • From time-to-time, each node sends its own distance vector estimate to neighbors.
  • When a node x receives new DV estimate from any neighbor v, it saves v’s distance vector and it updates its own DV using B-F equation:
    Dx(y) = min { C(x,v) + Dv(y)} for each node y ∈ N
    

Example – Consider 3-routers X, Y and Z as shown in figure. Each router have their routing table. Every routing table will contain distance to the destination nodes.
r1
Consider router X , X will share it routing table to neighbors and neighbors will share it routing table to it to X and distance from node X to destination will be calculated using bellmen- ford equation.

 Dx(y) = min { C(x,v) + Dv(y)} for each node y ∈ N

As we can see that distance will be less going from X to Z when Y is intermediate node(hop) so it will be update in routing table X.
dvr2
Similarly for Z also –
dvr3

Finally the routing table for all –
dvr4


Advantages of Distance Vector routing –