What does a metric of infinity indicate in routing protocols?

In routing protocols, a metric of infinity signifies that the route is unreachable. When a route's metric is set to infinity, it essentially means that there is no viable path to the destination; the network considers it inaccessible. This concept is often employed in distance-vector routing protocols like RIP, where a route that exceeds a certain distance (commonly 16 hops) is deemed unreachable and assigned an infinite metric.

This mechanism helps routers make informed decisions about the best paths to forward packets. If a route to a destination has an infinite metric, the router will no longer include this route in its routing table, preventing it from attempting to direct traffic to an islanded or non-existent endpoint.

In contrast, other metrics, such as those indicating stability or optimality, refer to the quality or efficiency of a route but do not indicate unreachability. Additionally, a full routing table pertains to different limitations within the router's memory or configuration and does not directly relate to a route's metric.