31 Days Before Your CCNA Exam

Day 18 155Table 18-3Comparing Features of IGPs: RIPv2, OSPF, and EIGRPFeatures RIPv2 OSPF EIGRPMetric Hop count Bandwidth Function ofbandwidth, delaySends periodic updates Yes (30 seconds) No NoFull or partial routing updates Full Partial PartialWhere updates are sent (224.0.0.9) (224.0.0.5, 224.0.0.6) (224.0.0.10)Route considered to 16 hops Depends on MaxAge of LSA, A delay of all 1sunreachablewhich is never incremented past3,600 secondsSupports unequal-cost No No Yesload balancingRouting Loop PreventionWithout preventive measures, distance vector routing protocols could cause severe routing loops inthe network. A routing loop is a condition in which a packet is continuously transmitted within aseries of routers without ever reaching its intended destination network. A routing loop can occurwhen two or more routers have inaccurate routing information to a destination network.A number of mechanisms are available to eliminate routing loops, primarily with distance vectorrouting protocols. These mechanisms include the following:■■■■■■Defining a maximum metric to prevent count to infinity: To eventually stop the incrementingof a metric during a routing loop, “infinity” is defined by setting a maximum metricvalue. For example, RIP defines infinity as 16 hops—an“unreachable” metric. When therouters “count to infinity,” they mark the route as unreachable.Hold-down timers: Used to instruct routers to hold any changes that might affect routes for aspecified period of time. If a route is identified as down or possibly down, any other informationfor that route containing the same status, or worse, is ignored for a predetermined amountof time (the hold-down period) so that the network has time to converge.Split horizon: Used to prevent a routing loop by not allowing advertisements to be sent backthrough the interface they originated from. The split horizon rule stops a router from incrementinga metric and then sending the route back to its source.Route poisoning or poison reverse: Used to mark the route as unreachable in a routingupdate that is sent to other routers. Unreachable is interpreted as a metric that is set to themaximum.Triggered updates: A routing table update that is sent immediately in response to a routingchange. Triggered updates do not wait for update timers to expire. The detecting router immediatelysends an update message to adjacent routers.TTL Field in the IP Header: The purpose of the Time to Live (TTL) field is to avoid a situationin which an undeliverable packet keeps circulating on the network endlessly. With TTL,