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 |
| Static Route | 1 |
| EIGRP Summary Route | 5 |
| External BGP | 20 |
| Internal EIGRP | 90 |
| IGRP | 100 |
| OSPF | 110 |
| RIP | 120 |
| EGP | 140 |
| ODR | 160 |
| External EIGRP | 170 |
| Internal BGP | 200 |
| Unknown/Other | 255 |
2.1.a Router on a stick
2.1.b SVI