Active and programmable network technologies strive to support completely new forms of data-path processing capabilities inside the network. This in conjunction with the ability to dynamically deploy such active services at strategic locations inside the network enables totally new types of applications. In this paper we exploit these network-side programming capabilities to realise a new active network application that dynamically evaluates network link costs based on in-line traffic measurements. The performance experienced by the data packets (e.g. delays, jitter and packet loss) along network or virtual links is used to compute link costs based on multiple cost metrics. The results are published by means of a routing metric broker, which enables available routing protocols to calculate different sets of routes for different QoS metrics – as for example suggested for ToS-based routing [RFC1583].