Routing Loops


Routing Loops are a risk in networks that utilize an older dynamic routing protocol like RIP. A routing loop is a scenario where data, instead of being routed to its correct destination, is sent from router to router endlessly. This scenario can be caused by routers not receiving updated routing information quickly enough, and as a result, forwarding packets incorrectly and propagating routing information to neighbor routers incorrectly. When every router in the system has the correct routing information the network is said to be converged. Therefore, it is desirable to use a routing protocol that can converge a network quickly and prevent routing loops.

Typically distance vector routing protocols like RIPv1, RIPv2 and IGRP, do not converge networks as quickly as link state routing protocols like OSPF and ISIS, with the EIGRP routing protocol being the exception.

Count-to-inifinity is a RIP routing loop scenario whereby the routes in the routing tables keep increasing their hop-count metric. This is caused by incorrect routing information being propagated on the network.

Distance Vector routing protocols have been designed and improved over the years to minimize the possibility of routing loops. RIP uses the following methods and rules to avoid routing loops and count-to-infinity: split horizon, hold down timers, route poisoning, poison reverse, and TTL values.     

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Author: Dan

Dan teaches computer networking and security classes at Central Oregon Community College.

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