What is the maximum possible AD for any route type?

The maximum possible administrative distance (AD) for any route type is 255. Administrative distance is a feature used in networking to rate the trustworthiness of a routing information source. A lower AD indicates a more reliable routing source.

In this context, 255 serves as a maximum value and is generally used to indicate an unreachable route. When a routing protocol has an AD of 255, it indicates that the router should not use that route for any routing decisions. This effectively renders it as unusable.

The other values you might encounter, such as 200, 170, and 120, pertain to specific routing protocols. For example, OSPF typically has an AD of 110 and EIGRP has a default AD of 20 for internal routes. However, they all fall below the maximum threshold of 255. Thus, the choice of 255 is correct as it represents the upper limit of administrative distances for routes in a Cisco routing environment.