What differentiates BGP from IGPs?

• A. BGP uses a single metric to determine the best path.
• B. IGPs focus on routing within one AS.
• C. BGP is policy-free and always selects the shortest path.
• D. BGP is policy-based and uses multiple attributes to select the best path.

Answer: (D)

The main idea here is how path selection is made. BGP chooses the best path based on operator-defined policies and a set of attributes, not just a single numeric cost. Inside an IGP, routing is typically determined by one metric (such as cost or hop count) to optimize routing within an Autonomous System. BGP, on the other hand, exchanges reachability between ASes and relies on multiple attributes to influence the decision: AS_PATH length, local preference, multi-exit discriminator (MED), origin, next-hop, and others. Operators apply policies—via route-maps, prefix-lists, and similar tools—to steer traffic and control which path is preferred, even if that path isn’t the shortest. That combination of policy-driven decisions and multiple attributes is what differentiates BGP from IGPs. The other statements don’t fit because BGP is not limited to a single metric, IGPs don’t define path selection by policy in the same way as BGP, and BGP is not policy-free nor bound to always choose the shortest path.