Match each route-selection concept to its most accurate meaning.

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Longest-prefix match is the fundamental routing principle where the router selects the route with the most specific subnet mask that matches the destination IP address. This means that if multiple routes match, the one with the longest subnet mask (most bits set) is chosen because it precisely targets the destination network. This concept is critical in Cisco routing as it ensures traffic is forwarded along the most exact path available. After longest-prefix match narrows down candidate routes, administrative distance (AD) is used to rank routes learned from different sources or routing protocols. AD is a numeric value that represents the trustworthiness of a route source; lower AD values are preferred. For example, directly connected interfaces have an AD of 0, static routes default to 1, EIGRP routes have 90, and OSPF routes have 110. This ranking ensures that if multiple protocols provide routes to the same destination, the router installs the most reliable one. Metrics come into play when multiple routes to the same destination exist within the same routing protocol. Each protocol uses its own metric calculation method: EIGRP uses bandwidth and delay, OSPF uses cost based on interface bandwidth, and RIP uses hop count. The route with the lowest metric is preferred. Floating static routes are static routes configured with a higher administrative distance than the primary route, so they remain inactive until the primary route fails. This design prevents routing conflicts and provides a reliable backup path, a common practice in Cisco network failover strategies.

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