When two routes have the same administrative distance (AD) in a routing system, the decision about which route is preferred is based on the route metric.

The metric is a value that helps the router determine the “cost” associated with using that route. The concept of “cost” can vary depending on the routing protocol used (for example, the hop count in RIP, the link cost in OSPF, or the value of the AS_PATH attribute in BGP), but in general, a lower metric indicates a preferred route over one with a higher metric.

Route Selection Process

1. Administrative Distance (AD): It is the first criterion used to select the best route. The AD is an indicator of the reliability of the route source. The lower the number, the more preferred the route source. If there are multiple routes to the same destination with different AD, the route with the lowest AD is preferred.
2. Metric: When two or more routes to the same destination have the same AD, the metric determines which of these routes is the best. The metric can be based on various factors such as bandwidth, delay, load, reliability, monetary cost, hop count, etc.
3. Other Factors: If the routes have the same AD and identical metrics, the router can use other criteria to select the route, such as load balancing, where traffic is distributed among the routes based on the router's configuration.

Practical example

Suppose a router has two routes to the same destination:

• Route 1: AD of 110 (by OSPF) with a metric of 10.
• Route 2: AD of 110 (also by OSPF) with a metric of 20.

In this case, both routes have the same administrative distance, so the router will look at the metric to decide. Since Route 1 has a lower metric, it will be considered the best route and will be the one the router uses to send packets to that destination.

It is important to remember that the specific details of how the metric is calculated and how it compares to other routes may vary between different routing protocols and network configurations.