Cisco IOS Route Filtering: Distribution Lists, Prefix Lists, Route-Maps
Working reference for the three Cisco IOS route-filtering tools. Distribution lists with ACL or prefix-list reference for inbound/outbound filtering at the routing process boundary. Prefix lists with ge/le qualifiers for prefix-length range matching, common patterns (default route only, any prefix, host routes, /24-/28 of a /8). Route-maps with sequenced clauses, the implicit final deny and the explicit permit-anything-else fallthrough, the menu of match conditions (ip address, next-hop, route-source, interface, metric, tag, as-path, community) and set actions (next-hop, metric, local-preference, community, as-path prepend, tag, weight). Where route-maps plug in: BGP neighbors, redistribution, PBR. Route tags as the glue between protocols to prevent redistribution loops. Pitfalls: missing fallthrough, prefix-list ge/le defaults, OSPF flood-can't-be-filtered, redistribution loops without tags.