Which routing protocol has an administrative distance (AD) of 20?

The routing protocol that has an administrative distance (AD) of 20 is eBGP (Exterior Border Gateway Protocol). The administrative distance is a value that routers use to determine the trustworthiness of a routing information source.

In the context of routing protocols, eBGP is used to exchange routing information between different autonomous systems on the internet. This protocol is typically more trusted than others due to its reliability in large-scale networks. The lower the AD value, the more preferred the route is; hence, eBGP having an AD of 20 signifies that it is often selected over routes learned through protocols with higher AD values.

For better understanding, it’s useful to know that iBGP (internal BGP) has an AD of 200, OSPF (Open Shortest Path First) has an AD of 110, and RIP (Routing Information Protocol) has an AD of 120, highlighting why eBGP is favored in networks utilizing multiple routing protocols for interdomain routing.