Route-map
A flexible "if-this-do-this" tool in routing: a sequence of rules that match traffic or routes on various criteria and then take an action — filter, change attributes, or set a path. The routing world's equivalent of a conditional script.
Structure: numbered statements with match (what to hit — a prefix-list, ACL, BGP community, metric) and set (what to do — set the next hop, change local-preference, add a community, tag the route). The statements are tested in order, the first match applies (permit/deny). Use case: BGP policy (manipulate AS-path, local-pref, MED to steer traffic in/out), policy-based routing (steer traffic on source), route redistribution (filter and tag routes between protocols), and route filtering in general. The most versatile tool in a network engineer's kit — nearly all advanced traffic and route steering goes through route-maps. Related to prefix-list and policy-based routing.