What is a transit AS?

• A. An AS that never forwards traffic
• B. An AS that only connects to customers
• C. An AS that routes traffic from one external AS to another external AS. Transit AS are generally ISPs.
• D. An AS that only uses static routes

Answer: (C)

The important idea here is that a transit Autonomous System serves as an intermediary that forwards traffic between other external ASes. In practice, a transit provider (often an ISP) connects to multiple networks and uses BGP to exchange routes, allowing traffic from one external AS to be carried to another external AS through its network. This enables customers connected to the transit provider to reach destinations beyond their own immediate network.

So, a transit AS is not just connected to customers or restricted to static routing; it actively forwards routes between different networks and provides reachability across different parts of the Internet.