1. References

Poison Reverse is an implemented algorithm that is often used within Distance vector routing. The use of poison reverse is to solve the count-to-infinity problem (more about the count to infinity problem can be found in distance vector routing).

The basic idea of poison reverse is to make sure that a path does not turn back into the same node if a cost has changed within the network. An example of this would be:

Node Z routes via node Y to destination X. If the cost between Z and Y increases the count to infinity problem will occur and here we implement the use of poison reverse.

As long as Z routes via node Y to get to X, Z will broadcast an infinite cost to the destination X, to the node which Z routes via (Y).

        1 /======== \\ 5  ====== /           \\=======

• the numbers between nodes is the cost of the links.

Following this topology and we assume this distance vector table of Z:

Destination Z Y X

  Z             0    1   3  Y             1    0   2  X             3    2   0

As Z routes via Y to get to X and because of that have the cost 3.

The poison reverse kicks in when we broadcast our distance vector to our neighbors:

The distance tables we broadcast is:

To Y:

[0, 1, ∞]

To X:

[0, 1, 3]

As we see in the distance vector that is broadcast to node Y the end destination X has an infinity value. This solves the count-to-infinity problem since if the link between Y and Z will not bounce between each other and instead directly try another path.

Although poison reverse is not always working. If there's a topology like this:

If the link between C and D would fail node C can still try to go through B to get to the destination. This will cause B to route through A and from there we have an loop we can not solve with poison reverse.^[1]

This can though be completed with an implementation of a distance vector protocol called RIP.

References

• Lăculeţe Gară
• Lăculețe
• Lăculețe Gară
• Lăculețe-Gară
• Lăcustă
• Lăcăuţi River
• Lădeşti
• Lădăuţi
• Lădăuți
• Lăicăi
• Lăk District
• Lăleşti
• Lălești
• Lămăşoaia River
• Lănciţa River (Bărbat)
• Lăngeşti
• Lăngești
• Lăpoșel
• Lăpuş
• Lăpuşani
• Lăpuşata
• Lăpuşel
• Lăpuşel River
• Lăpuş Mountains
• Lăpuşna County