BROCADE IP PRIMER

Chapter 10: Routing BasicsAdministrative DistanceNow, here's a question. You can run several different dynamic routing protocolson the same router and you still have your connected and static routes.When there are duplicate routes for the same network (say, one that's staticand one that's learned through a dynamic routing protocol), how does therouter choose which one is better?This problem was tackled a while ago. The answer is that each type of route inthe routing table has an administrative distance. The administrative distanceis just a number. It's an eight-bit number, so it can be any number from 0 to255. Think of the administrative distance number as a cost to use the route.As in money, the higher the cost, the lower the desirability. The lower the cost,the better.What determines the cost, the administrative distance? Refer to this table fordetails:Route TypeAdministrative DistanceDirectly Connected 0Static Route 1External BGP 20OSPF 110RIP 120Internal BGP (iBGP) 200You would do well to memorize that table. It never hurts to know them off thetop of your head, especially if you're mixing dynamic routing protocols in yourinfrastructure. There are more routing protocols than what are listed in thetable, but these are the protocols we'll be talking about. Notice that the directlyconnected routes have an administrative distance of 0. This means that nomatter what static routes you may have configured, or what routes your routermay have learned from a dynamic routing protocol, the router will alwayschoose the connected route (if it has one).What if your router has a route that it learned from RIP, but you've configured astatic route for the same network? The router will choose the static route. Ithas a lower administrative distance.What if you router has a route that it learned through RIP, but it also learnedthe same route through OSPF? The router will choose the OSPF route (110 is asmaller number than 120).Now, the router won't forget all these duplicate routes. The router will make adecision as to which route it will use, but it will still remember that it has otheroptions. Why? Failures happen. Often. If the decided route were to fail for230 Brocade IP Primer