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.
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