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8.3 | ![]() |
Route Redistribution | |
8.3.2 | ![]() |
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.
![]() A router looks at the metric value to determine the best route. However,
in this case, the router would have to compare the simple metric of RIP,
its hop
count, with the composite metric of IGRP, this being derived from bandwidth, delay,
reliability, load, and MTU. As noted in Module 3, Routing Overview, the
metrics are not based on the same factors, so it would not be a reliable comparision.
The metric of 10576 from IGRP cannot be accurately
measured against the metric of three (3) from RIP for the same route.
The administrative distance of a routing protocol provides a rate of 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. Figure
The IGRP route will be preferred, or trusted, over the RIP route to the same network. This is because of the lower administrative distance for IGRP, which is 100, as opposed to RIP, which is 120. There may be a time that having the router believe RIP over IGRP is needed. Fortunately, the Cisco IOS allows the administrative distance to be manually configured, as discussed in the next section.
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