Routing loop problem

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The routing loop problem is a common problem with various routing algorithms. It is similar to a switching loop but instead occurs at layer 3 rather than layer 2.

[edit] Theory

For example, in the network given below, node A is transmitting data to node C via node B. If the link between nodes B and C goes down and B has not yet informed node A about the breakage, node A transmits the data to node B assuming that the link A-B-C is operational and of lowest cost. Node B knows of the broken link and tries to reach node C via node A, thus sending the original data back to node A. Furthermore, node A receives the data that it originated back from node B and consults its routing table. Node A's routing table will say that it can reach node C via node B (because it still has not been informed of the break) thus sending its data back to node B creating an infinite loop.

[edit] Routing Loops in Networks

Conditions known simply as "routing loops" sometimes occur in networks using old or flawed routing protocols that do not prevent against routing loops. A routing loop can have a catastrophic impact on a network, and in some cases, completely crippling it.

[edit] Prevention

All newer (link state) routing protocols (EIGRP,OSPF,IS-IS) have built in loop prevention; however, the older distance vector protocols (RIP, IGRP) commonly do not implement the newest forms of loop prevention and therefore are sometimes susceptible to routing loops. Split horizon is the most common implementation of routing loop prevention.