Content

| 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. 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 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. Instead, routers use administrative distance to choose between routes to the same network offered by different routing protocols.

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 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. 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.

| Interactive Media Activity Drag and Drop: Administrative Distance Upon completion of this activity, the student will be able to identify the administrative distances for all entries in a routing table.

| Web Links What Is Administrative Distance? http://www.cisco.com/warp/public/105/ admin_distance.html