RoutingPolicies

RoutingPolicies are a set of rules and procedures used to influence the selection and dissemination of routes within a network. They are applied to routing information as it is sourced, propagated, or used to forward traffic, rather than being part of the basic path computation algorithms of a routing protocol. RoutingPolicies enable operators to express business or technical goals, such as routing to preferred customers, avoiding congested links, or limiting exposure to certain prefixes.

RoutingPolicies are defined by match conditions and actions. Match conditions specify attributes of routes, such as prefix length, origin, AS path, next-hop, or community attributes. Actions modify route attributes (e.g., local preference, MED, next hop), filter routes (permit or deny), or control redistribution between protocols. Policies are applied in a defined order, and the first matching rule often determines the outcome.

Common constructs include route-maps or policy statements in routers, route-redistribution filters, prefix-lists, and BGP communities. In software-defined networking, RoutingPolicies can be expressed as intents or flow-based rules that drive the controller's forwarding decisions.

Use cases include: steering egress traffic toward preferred links, preferring customers or certain prefixes, dampening or filtering unstable routes, and shaping inbound or outbound traffic. Proper design requires testing to prevent unintended routing changes or loops, and documentation to ensure consistency across devices and teams.

RoutingPolicies are complementary to routing protocols; they do not replace them but augment their behavior by applying organizational or operational constraints to routing decisions.