What is the administrative distance (AD) of an unusable route?

The administrative distance (AD) of an unusable route is 255. This value is significant in routing because it indicates that a route is not reachable or should not be used for routing decisions. When a routing protocol assigns an administrative distance of 255 to a route, it effectively marks it as unreachable or invalid, meaning that the router will ignore this route when making routing decisions.

In the context of routing protocols, administrative distance acts as a technique for determining which route to use when more than one route to a destination exists. A higher AD like 255 indicates that the route is completely unreliable, while a lower AD signifies more reliable routes. Because 255 is the highest possible value for administrative distances, it clarifies that the route should never be considered in the routing process.

In contrast, other values represent valid routes that can be used by the router. For example, the value of 0 typically indicates a directly connected route, while 1 represents a static route. A value of 120 is commonly associated with the EIGRP routing protocol, marking it as a viable route but not as reliable as directly connected or static routes.