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Administrative distance (AD) is an arbitrary numerical value assigned to a routing protocol, a static route or a directly-connected route based on its perceived quality of routing. The administrative distance (AD) value is often used by Cisco routers to determine the "best" route that should be used when multiple paths to the same destination exist.〔Cisco Systems (2013), (What is Administrative Distance? ), retrieved 14 September 2013〕 A routing protocol with a lower administrative distance is considered "better" and is given priority over routing protocols with higher administrative distances.〔 The "better" route is selected by the router and is inserted into the router's routing table to be used to route traffic. For example, routes issued by EIGRP are considered more reliable than routes issued by RIP. This is because EIGRP has an administrative distance of 90 while RIP has an administrative distance of 120. The administrative distance (AD) value may be set manually by a network administrator. If a network administrator fails to properly set the administrative distance or configures multiple routing protocols with the same administrative distance, the router will use the default administrative distance. ==Default administrative distances== The following table lists the default administrative distances for various routing protocols used on Cisco routers.〔 Please note that:- * An administrative distance of 255 will cause the router to disbelieve the route entirely and not use it. An administrative distance of 255 will cause the router to remove the route from the RIB and not use it. * Since IOS 12.2, the administrative distance of a static route with an exit interface is 1. Prior to the release of 12.2 it was in fact 0. * Only the interface itself has an administrative distance of 0, since a route cannot be less than 1. * Directly connected routes have an administrative distance of 0. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Administrative distance」の詳細全文を読む スポンサード リンク
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