3.3.m Implement and troubleshoot loop prevention mechanisms

Loops occur when routers act on the basis of inaccurate or old information. Link-state protocols like OSPF use reliable flooding mechanisms to ensure that all routers are acting on the basis of the same information. That is what link-state protocols avoid. All routers in a link-state database have the same view of the network.

Distance vector protocols are susceptible to routing loops. Split horizon is one of the features of distance vector routing protocols that prevents them. This feature prevents a router from advertising a route back onto the interface from which it was learned. Route poisoning is another method for preventing routing loops employed by distance vector routing protocols. When a router detects that one of its directly connected routes has failed, it sends the advertisement for that route with an infinite metric (” poisoning the route”). A router that receives the update knows that the route has failed and doesn’t use it anymore. Holddown is also a loop-prevention mechanism employed by distance vector routing protocol. This feature prevents a router from learning new information about a failed route.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Location 2832).  . Kindle Edition.