Bit of revision from ICND1 in this one, when a router is making the decision of where to send a packet as multiple options fill up its routing table.
1- Next Hop – If one of the routes that is in the routers routing table has a next hop that the router doesn’t know how to get to it invalidates the route and does not use it.
2- The next item the router considers when seleting a route is route specificity. The more specific a route the more likely it is to be chosen as the elected route. This takes precedence to administrative distance.
3- Administrative distance is the number used to decide which route to trust based on how a route found its way into the routing table (assuming the specificity is the same). Below is a graph of some common Route Sources and their associated Administrative Distances. The lower the Administrative Distance the more preferable the route will be.
4- If the specificity and the administrative distance are both the same then the metric characteristic will be used to break the tie. It’s synonymous with hop count – so the router will choose the route that’s the fewest hops away from its destination.
|Route Source||Administrative Distance|
|Directly Connected Interface||0|
|EIGRP Summary Route||5|
2.1.a Router on a stick