What is the purpose of prefix-lists in inbound filtering?

• A. To enforce explicit allow/deny rules for prefixes.
• B. To encrypt prefixes to protect them in transit.
• C. To determine the MED value for routes.
• D. To carry IRR data in updates.

Answer: (A)

Prefix-lists in inbound filtering are used to enforce explicit allow or deny rules for prefixes received from a neighbor. When a BGP speaker receives routes from a peer and a prefix-list is applied, each incoming prefix is compared against the list. If a matching entry says permit, the route is accepted into the local BGP table; if it says deny, the route is rejected. This provides precise, scalable control over which prefixes we learn, helping to prevent unwanted or misadvertised routes from entering our network and reducing the chance of route leaks or instability. This tool is not about encryption, it doesn’t set or determine MED values, and it doesn’t carry IRR data in updates.