routemaps

Routemaps are policy mechanisms in computer networks that govern how routes are selected, modified, or redistributed between routing protocols. They act as an intermediate layer between protocol decision processes and the routing table, enabling administrators to implement custom rules based on route attributes, source, or destination.

A routemap consists of sequential statements, each identified by a type such as permit or deny. Within a statement, match clauses specify which routes the rule applies to (for example, based on prefix, protocol, or route tag), and set clauses describe the actions to take (for example, set next-hop, set local preference, set metric, or set tag).

Routemaps are commonly used to control redistribution between protocols, filter routes entering or leaving a device, and implement policy-based routing where traffic is steered according to criteria other than destination address. They can also influence attributes used by routing protocols, such as the next hop, weight, or AS path length.

Implementation specifics vary by vendor. In many platforms routemaps are applied at redistribution points or on interfaces and are evaluated in sequence from the lowest to the highest sequence number. A later deny or permit can override earlier decisions. They can work with BGP, OSPF, EIGRP, or static routes, and are a central tool for traffic engineering and security policies.

Example (conceptual): route-map RM-EXAMPLE permit 10; match ip address prefix-list PL-LOCAL; set local-preference 200; set next-hop 10.0.0.1.

Best practices include clear naming, incremental changes, testing in a lab, and documenting the rationale. Overly broad matches or complex chains can increase troubleshooting complexity. See also policy-based routing, route filtering, and route redistribution.