What is Administrative Distance and how does it influence route selection when multiple protocols provide a route?

• A. A measure of the reliability of the next hop, with higher values preferred.
• B. The maximum time a route can stay in the routing table.
• C. A function of the time since the route was learned.
• D. A trust value assigned to each routing protocol; lower AD is preferred over higher AD.

Answer: (D)

Administrative Distance is a trust value assigned to routing information from each routing protocol. The lower this value, the more trustworthy the router considers that source. When multiple protocols offer a route to the same destination, the router compares their Administrative Distances and installs the route from the source with the smallest distance. This is why a route learned from a more trusted protocol is chosen even if another protocol would report a shorter path by its own metrics. If two sources share the same AD, the router then uses the protocol’s metric to decide and, if needed, other tie-breakers. This concept isn’t about the reliability of the next hop, the lifetime of a route, or how long it’s been learned—it's about how much the router trusts the source of the routing information.