Which statement best describes a transit AS?

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

Answer: (B)

A transit AS is a network that sits between other networks and forwards traffic for them, effectively providing reachability from one external AS to another. In practice, this means it carries traffic from customers of one network to destinations reachable through a different network, which is a role commonly filled by ISPs offering internet transit.

This description is the best fit because it captures the purpose of a transit provider: not just serving its own customers, but enabling paths across the Internet by routing traffic between multiple external ASes. It’s different from an AS that only connects to customers, which wouldn’t forward traffic for others, and from an AS that never forwards traffic, which would be a stub rather than a transit. Relying solely on static routes isn’t typical for a transit role, since transit requires dynamic routing (like BGP) to handle diverse and changing paths between many networks.