Problems in Distance Vector Routing | Distance Vector Routing

Problems in Distance Vector Routing:-

A. Two-Node Loop Instability

B. Three- Node Instability

•       The main problem with DVR is instability, which means the network using this protocol will become unstable.

According to the figure:-

•       At the beginning both nodes A and B know how to reach node X.

•       Suddenly, link between A and X fails, and node a changes its table.

•       If A can send its table to B immediately, everything is fine.

•       Otherwise, if B sends its routing table to A before receiving A’s routing table, the system becomes unstable.

•       Node A receives the update, assuming that B has found a way to reach X, and immediately updates the table.

•       Based on triggering update strategy, A send its new update to B.

•       Now, B thinks, something has changed around A and updates its routing table.

•       Node  A thinks that route to X is through B and B thinks that route to X is through A.

•        In this way, the cost of reaching X gradually increases until it  reaches infinity.

•       Lastly,  both A and B  get to know that X cannot be reached, and the system becomes unstable.

Solution to Two-node Loop Instability:-

1.Defining Infinity

2.  Split Horizon 

3.  Split Horizon and Poison Reverse

Defining Infinity

•       The first obvious solution is to minimize or redefine infinity to a smaller number.

•       The infinity should be –

Max (size of the network in each direction) + 1 , if the distance between each hop is considered as 1.

•        Generally, in most implementations of distance vector routing  protocol infinity is defined as 16.

•        So, the size of the network in each direction should be 15.

•       From  here , we can conclude DVR protocol cannot be used in large networks.

Split Horizon

•       When node B gets A’s table and thinks that optimum route to reach  X is through A, it does not need to advertise this information to A;  as the information has come from  A only.

•       Node A keeps the value of the distance to X  as infinity.

•       Later, when node A sends its routing table to B, node B also corrects its routing table.

•       As a result, the system becomes stable after the first update, i.e. both A and B knows that X is not reachable.

Split Horizon and Poison Reverse

There is problem with split horizon-

•       Distance Vector uses timer and if there are no news about a route , the node deletes the route from its table.

•       In our scenario, when node B eliminates the route to X from its advertisement to X,  node A cannot guess that this is due to split horizon or because B has not received any news about X recently.

•        So to avoid this problem Split Horizon, is combined with another strategy called Poison Reverse.

Node B can still advertise the value  for X , but if the source of information is A, it can replace the distance with infinity as a warning:

“Do not use this value, what I know about this route comes from you.”

B. Three - Node Instability

•       Two node instability is removed by spilt horizon combined with poison reverse.

•       But, if the instability is between three nodes, stability cannot be guaranteed.