RoutingPolicy

RoutingPolicy is a framework used by routers and routing daemons to influence how routing information is processed, advertised, and selected. It allows operators to shape traffic paths, control which routes are accepted or exported, and implement policy-based routing decisions beyond basic protocol behavior. Routing policies are implemented through device-specific configuration languages and are evaluated as routes are learned, redistributed, or advertised.

A policy is typically composed of terms. Each term has match conditions (criteria that a route must satisfy) and set actions (modifications or decisions applied to that route). Common match criteria include destination prefix and mask, prefix length, AS path, next hop, route origin, and community attributes. Set actions can adjust local preference or metric, modify the next hop, prepend or modify the AS path, attach or remove communities, or tag a route for further handling. Most policies enforce a default deny rule when no terms match, ensuring explicit control over routing behavior.

Routing policies are used for a variety of purposes, such as traffic engineering, route filtering, and redistribution control. In BGP, they are often employed to influence outbound advertisements, inbound route acceptance, or to shape path selection. They can also govern redistribution between routing protocols, filter routes entering an interface, or apply policy at virtual routing and forwarding instances.

Different network platforms implement routing policies with their own languages and concepts. Examples include Junos policy-options with terms, Cisco IOS route-maps and distribute-lists, and FRRouting/Quagga-style route policies. Regardless of syntax, the core idea remains: define conditions under which routes are matched and specify the actions to apply to those routes.