Where are restrictions on routing behavior enforced?

• A. On the Next Hop
• B. On the AS path
• C. On the Origin
• D. On the Communities

Answer: (B)

Restrictions on routing behavior are enforced on the AS_PATH attribute. This is the sequence of autonomous systems a route has traversed, and operators use it to shape policy: they can accept or reject routes based on which ASes appear in the path, on how long the path is, or on specific patterns. This path-based filtering is a direct way to implement safeguards like avoiding certain transit providers, preventing routing through prohibited networks, or enforcing business relationships between ASes.

The next hop matters for reachability—whether the next hop is reachable and how to reach it—but it doesn’t encode policy about which paths are acceptable. The Origin attribute tells you where the route came from, not how to enforce routing behavior. Communities can be used to mark routes for policy actions, but the actual enforcement of path-based restrictions most directly uses the AS_PATH information.