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Example of Distance Vector Routing 1 - Georgia Tech - Network Implementation

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  1. Let's suppose that we have a three node network with the costs on the edges as shown. Initially, each node has a single distance
  2. vector representing the shortest path cost to each other incident node in the graph. For example, the
  3. distance between x and x is obviously zero. And the shortest known distance between x and
  4. y, from x's perspective is one, the direct path. Similarly, the shortest known distance between x and z
  5. to x at the outset is five because all it knows is the direct path. Note that a
  6. shorter path between x and z exists via y, but x simply doesn't know about it yet. Now
  7. in distance vector routing, every node send its vectors to every other adjacent node. And each node then
  8. updates its routing table according to the Bellman-Ford equation. Let's look at what happens when node x learns of y's distance vectors. Well in
  9. this case, the distance from x to z will be computed as the minimum of the sums of all distances to z through any
  10. intermediate node. So the cost between x and y is one, and the distance between y and z as discovered by y's distance vector
  11. is two. Therefore, x can update its shortest cost distance to z as three. Similarly, x
  12. will receive a distance vector from z, five two zero, but of course, when it uses
  13. the Bellman-Ford equation to update its distances, again the distance between z and x will be
  14. updated from five to three. We can repeat this exercise at other nodes, as they receive distance
  15. vectors from other nodes in the topology. And quickly, every node in the network has a complete routing table. Now
  16. when costs decrease, the network converges quickly but one problem is that when failures occurs, bad news can actually travel slowly.

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