7.3 Route Redistribution
7.3.3
Administrative distance
If a boundary router is running multiple IP routing protocols, then it may be possible that the router will learn about the same network from more than one routing protocol. For example, RTZ may learn about the 10.0.0.0 network from both RIP and IGRP, see Figure . Which route will RTZ install in its routing table?

A router looks at the metric value to determine the best route. However, in this case, the router would have to compare RIP's simple metric, hop count, with IGRP's composite metric, derived from bandwidth, delay, reliability, load, and MTU. As noted in Chapter 3, Routing Overview, there is no way to precisely compare what are, in effect, apples and oranges. In Figure , IGRP's metric of 10576 cannot be accurately measured against RIP's metric of 3 for the same route. Instead, routers use administrative distance to choose between routes to the same network offered by different routing protocols.

A routing protocol's administrative distance rates its trustworthiness as a source of routing information. Administrative distance is an integer from 0 to 255. The lowest administrative distance has the highest trust rating. An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored. An administrative distance of zero is reserved for directly connected interfaces and will always be preferred.

Specifying administrative distance values enables the Cisco IOS software to discriminate between sources of routing information. If two routes have the same network number (and possibly subnet information), the IOS software always picks the route whose routing protocol has the lowest administrative distance. Although you can not easily compare apples with oranges, we can instruct the router to always choose oranges over apples. Figure shows the default administrative distances for some routing information sources.

The IGRP route will be preferred, or "trusted," over the RIP route to the same network, by virtue of a lower administrative distance (IGRP's 100 vs. RIP's 120). Of course, there may be times that you may actually want the router to believe RIP over IGRP for some reason. Fortunately, the Cisco IOS allows you to manually configure administrative distance, as discussed in the next section.