Service provider edge or core routers are not the best traffic-limiting platforms. While they definitely have the capabilities to perform numerous Quality of Service (QoS) actions, complex QoS actions reduce their performance.
P2P traffic is harder to limit than any other traffic because the P2P programs usually use a number of cloaking mechanisms to evade detection (and traffic limits). Furthermore, the P2P programs use a large number of parallel TCP sessions, making the usual QoS algorithms that relied on the cooperative behavior of end users mostly useless.
The best way to deal with P2P traffic is to limit the traffic per user (meaning per IP address). Most high-speed routers don't have that functionality; you have to use dedicated platforms (for example, Cisco's Service Control Engine) to implement per-user traffic shaping or policing.
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