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.
