The Cisco administrative distance is a value assigned to all routing protocols to define how trustworthy the information is.
If a Cisco router has multiple routes in its routing table for the same network, it looks to see which route has the lowest cost/metric value. The problem with using cost values is that different routing protocols have different formulas for calculating what the relative cost should be for a route. RIP for example, uses hop count for its cost value. So if RIP detects that there is more than one path to get to the same destination it counts how many routers are along each path and the amount of routers will be equal to the cost value. It will then choose the lowest cost for the best route. However a routing protocol such as EIGRP uses a much more complex formula which is far more accurate but results in a cost value that is incredibly high. In a case where a router has RIP and EIGRP enabled, RIP might report a cost of 5 while EIGRP might report a cost in the tens of thousands and the router would choose the route with the lowest cost value (RIP). This is not a great solution as RIP is not nearly as reliable as EIGRP and only counts hops, whereas EIGRP would look at values such as the bandwidth and delay on the lines. Based on this I would rather my Cisco router choose the route from EIGRP.
In order to fix this problem, Cisco introduced a unique administrative distance value for every routing protocol. This is used before the cost is looked at and allows the router to decide who it’s going to use for routing decisions. As an example, RIP has an administrative distance of 120 and EIGRP has an administrative distance of 90. The router would receive information from both of the routing protocols and where RIP and EIGRP have an entry for the same network, the router will only consider the EIGRP entries and will only add those values to the routing table. Of course if EIGRP has multiple routes for the same network, all of those will be included in the routing table and the router will then use cost to determine which of those is the best route. If you were to look at the routing table by typing “show ip route” you would only see EIGRP entries and no RIP entries.
In human terms, if I had a headache and spoke to a doctor and a friend about what I should take to get rid of it, and my friends tells me to take ‘Drug X’ which costs $1 and the doctor tells me to take ‘Drug Y’ which costs $10, I would trust the doctor as he is qualified in medicine and his information is more trustworthy.
Below are the Cisco administrative distance values for each type of route
|Enhanced Interior Gateway Routing Protocol (EIGRP) summary route||5|
|External Border Gateway Protocol (BGP)||20|
|Intermediate System-to-Intermediate System (IS-IS)||115|
|Routing Information Protocol (RIP)||120|
|Exterior Gateway Protocol (EGP)||140|
|On Demand Routing (ODR)||160|