Cisco WS-X6K-SUP1A-PFC= Software Guide - Page 728

Understanding How QoS Works Chapter 41 Configuring QoS Policing Rules You can create named policing rules that specify bandwidth utilization limits, which you can apply to traffic by including the policing rule name in an ACE (for more information, see the "Creating Policing Rules" section on page 41-34). Policing uses a token bucket scheme. As packets arrive, the packet size in bytes is added to the bucket level. Every 0.25 milliseconds, a value equal to the token rate is subtracted from the bucket level. You specify the bandwidth utilization limits as an average rate and a maximum burst size. Packets that exceed these limits are "out of profile." Traffic is in profile as long as it flows in at an average rate and never bursts beyond the burst size. In each policing rule, you specify if out-of-profile packets are to be dropped or to have a new DSCP value applied to them (applying a new DSCP value is called "markdown"). Since out-of-profile packets do not retain their original priority, they are not counted as part of the bandwidth consumed by in-profile packets. For all policing rules, QoS uses a configurable table that maps received DSCP values to marked-down DSCP values (for more information, see the "Mapping DSCP Markdown Values" section on page 41-57). When markdown occurs, QoS gets the marked-down DSCP value from the table. You cannot specify a marked-down DSCP value in individual policing rules. Note By default, the markdown table is configured so that no markdown occurs: the marked-down DSCP values are equal to the received DSCP values. To enable markdown, configure the table appropriately for your network. You give each policing rule a unique name when you create it and then use the name to include the policing rule in an ACE. The same policing rule can be used in multiple ACEs. You can create these policing rules: • Microflow-QoS applies the bandwidth limit specified in a microflow policing rule separately to each flow that matches any ACEs that use that particular microflow policing rule. You can create up to 63 microflow policing rules. • Aggregate-QoS applies the bandwidth limits specified in an aggregate policing rule cumulatively to all flows that match any ACEs that use that particular aggregate policing rule. You can create up to 1023 aggregate policing rules. • With a PFC2, you can specify a dual rate aggregate policing rule with a normal rate and an excess rate. - Normal rate-packets exceeding this rate are marked down. - Excess rate-packets exceeding this rate are either marked down or dropped as specified by the drop indication flag. Note The drop indication flag applies to the excess rate policer and cannot be set for the normal rate policer. To achieve the effect of a drop indication flag for the normal rate aggregate policer, set the excess rate equal to the normal rate and set the drop indication flag. Alternatively, you can set the normal rate without specifying an excess rate, which automatically sets the excess rate to the normal rate when the drop indicator flag is on. You can include both a microflow policing rule and an aggregate policing rule in each ACE to police a flow based on both its own bandwidth utilization and on its bandwidth utilization combined with that of other flows. 41-22 Catalyst 6000 Family Software Configuration Guide-Releases 6.3 and 6.4 78-13315-02