Routing protocols are used to exchange information about network routes between devices on a network. One way to determine which routing protocol to use is by considering the administrative distance.

The administrative distance is a measure of the trustworthiness of a routing protocol. It is a value assigned to each routing protocol, a lower value means it is more trustworthy, it will be preferred over a protocol with a higher administrative distance. When a router receives multiple routes to the same destination from different routing protocols, it will choose the route with the lowest administrative distance.

Routing protocols and their administrative distances:

As you can see, directly connected routes have the lowest administrative distance, followed by static routes. This makes sense, as these routes are directly connected to the router and do not rely on any external protocol for information.

EIGRP, OSPF, and IS-IS are considered more trustworthy than BGP and RIP, as they have lower administrative distances. However, it is important to note that the administrative distance is just one factor to consider when choosing a routing protocol. Other factors, such as scalability, support for different network types, and security, may also be important considerations.

In summary, the administrative distance is a measure of the trustworthiness of a routing protocol. It is used by routers to determine which route to use when multiple routes to the same destination are available. Understanding the administrative distances of different routing protocols can help you choose the best one for your network.