Which statement about BGP prefix filtering strategies is true?

• A. Prefix-lists are used to enforce explicit allow/deny rules for prefixes.
• B. Prefix-lists carry ROA data in the BGP update.
• C. Prefix-lists auto-generate BGP Communities.
• D. Prefix-lists determine the MED value for routes.

Answer: (A)

Prefix filtering with BGP relies on prefix-lists to enforce explicit allow/deny rules for prefixes. A prefix-list defines which prefixes (and optionally which prefix lengths) are permitted or denied, and when you apply that list to a BGP session—often through a route-map—you control which routes are accepted into your BGP table or which routes you advertise to a neighbor. This explicit allow/deny mechanism is the core purpose of prefix-lists in BGP policy.

ROA data isn’t carried in BGP updates. ROA information comes from RPKI validation and is used to verify that a route’s origin is legitimate; it isn’t transmitted as part of BGP update attributes. That’s why a statement claiming prefix-lists carry ROA data isn’t correct.

Prefix-lists don’t auto-generate BGP Communities. Communities are a separate BGP attribute that can be assigned or modified by policy (often via route-maps), but not produced merely by a prefix-list.

Prefix-lists don’t determine the MED value. MED is a BGP path attribute and can be set or altered by policy to influence path selection, typically via route-maps, not by the prefix-list itself.

So, the statement that prefix-lists are used to enforce explicit allow/deny rules for prefixes is the accurate description.