What is administrative distance in routing, and how does it affect route selection when multiple routes to a destination are learned from different protocols?

• A. It is the trust value assigned to a route source; lower administrative distance wins.
• B. It is the metric used to compute path cost; higher distance wins.
• C. It is a timer that ages routes; older routes are preferred.
• D. It is the security level of a route; higher distance means more trust.

Answer: (A)

Administrative distance is the trust value assigned to a route source. When a router learns the same destination from multiple routing protocols, it uses this trust value to decide which route to keep. The route from the source with the lower administrative distance is considered more reliable, so it wins over routes from sources with higher distances, even if the internal metrics of those protocols differ. Only if two sources have the same administrative distance does the router look at the protocol’s own metric and other tie-breakers to choose. This is why a route learned from a protocol with a smaller distance is preferred over one learned from a protocol with a larger distance, regardless of the path cost reported by those protocols.