Cisco 7609-S Software Configuration Guide - Page 622

Understanding How PFC QoS Works Chapter 41 Configuring PFC QoS Classification and Marking on the MSFC PFC QoS sends IP traffic to the MSFC with the final internal DSCP values. CoS is equal to IP precedence in all traffic sent from the MSFC to egress ports. Figure 41-9 Marking with PFC2 or PFC3 and MSFC2, MSFC2A, or MSFC3 From PFC Multilayer Switch Feature Card (MSFC) marking IP traffic Yes from PFC? No Write ToS byte into packet Route traffic 144800 CoS = IP precedence for all traffic (not configurable) To egress port Note Traffic that is Layer 3 switched on the PFC does not go through the MSFC and retains the CoS value assigned by the PFC. Policers These sections describe policers: • Overview of Policers, page 41-20 • Aggregate Policers, page 41-21 • Microflow Policers, page 41-22 Overview of Policers Policing allows you to rate limit incoming and outgoing traffic so that it adheres to the traffic forwarding rules defined by the QoS configuration. Sometimes these configured rules for how traffic should be forwarded through the system are referred to as a contract. If the traffic does not adhere to this contract, it is marked down to a lower DSCP value or dropped. Policing does not buffer out-of-profile packets. As a result, policing does not affect transmission delay. In contrast, traffic shaping works by buffering out-of-profile traffic, which moderates the traffic bursts. (PFC QoS does not support shaping.) 41-20 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX OL-4266-08