Cisco 3845 Software Guide - Page 501

Chapter 27 Configuring QoS Understanding QoS EoMPLS and QoS EoMPLS supports QoS by using three experimental bits in a label to determine the priority of packets. To support QoS between label edge routers (LERs), you set the experimental bits in both the virtual connection and the tunnel labels. EoMPLS QoS classification occurs on ingress, and you can match on Layer 3 parameters (such as IP or DSCP), and Layer 2 parameters (CoS). See the "Configuring MPLS and EoMPLS QoS" section on page 27-53 for more information about EoMPLS and QoS. Policing After a packet is classified and assigned a QoS label, you can use policing, as shown in Figure 27-5, to regulate the class of traffic. The policing function limits the amount of bandwidth available to a specific traffic flow or prevents a traffic type from using excessive bandwidth and system resources. A policer identifies a packet as in or out of profile by comparing the rate of the inbound traffic to the configuration profile of the policer and traffic class. Packets that exceed the permitted average rate or burst rate are out of profile or nonconforming. These packets are dropped or modified (marked for further processing), depending on the policer configuration. All traffic, whether it is bridged or routed, is subjected to a configured policer. As a result, packets might be dropped or might have the DSCP or CoS fields modified when they are policed and marked. Note Input hierarchical service policies are applied to a traffic stream before any other services act on that traffic. For example, an input hierarchical service policy applied to traffic could change the traffic rate from above a storm-control threshold to below the threshold, preventing storm control from acting on the traffic stream. Figure 27-5 Policing of Classified Packets Receive Classify Packets that conform to the committed information rate (CIR) Queuing, scheduling, and shaping Packets that exceed the CIR 141153 An exceed-action at this point results in dropped or reclassified packets. Drop You can attach a policy map with a policer only in an input service policy. The switch supports 1-rate, 2-color ingress policing and 2-rate, 3-color policing. For 1-rate, 2-color policing, you use the police policy-map class configuration command to define the policer, the committed rate limitations of the traffic, committed burst size limitations of the traffic, and the action to take for a class of traffic that is below the limits (conform-action) and above the limits (exceed-action). If you do not specify burst size (bc), the system calculates an appropriate burst size value. The calculated value is appropriate for most applications. OL-23400-01 Cisco ME 3800X and 3600X Switch Software Configuration Guide 27-13