User:Asidas/sandbox
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).
========
| Z |
1 /======== \ 5
====== / \=======
| Y |___________| X |
====== 2 ========
- 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:
̣===== | A | ===== \______ | \======= ===== ====== | C | ——— | D | | B |______/======= ===== ======
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[edit]
Compputer Networking: A top-Down Approach, Seventh Edition. Harlow, England: Pearson. 2017. p. 418. {{cite book}}: Cite uses deprecated parameter |authors= (help)
Category:Internet Standards Category:Internet protocols Category:Routing protocols