Egress Policing And Marking - Cisco Catalyst 3750 Software Configuration Manual

Metro switch

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Understanding Hierarchical QoS

Egress Policing and Marking

Egress traffic policing controls the maximum rate of traffic sent on a port. A policer defines the

bandwidth limitations of the traffic and the action to take if the limits are exceeded. It is often configured

on ports at the edge of a network to limit traffic into or out of the network. In most policing

configurations, traffic that falls within the rate parameters is sent. Traffic that exceeds the parameters is

considered to be out of profile or nonconforming and is dropped or sent with a different priority.

You can configure a two-rate traffic policer within a policy map at the class level, at the VLAN level,

and at the physical level by using the police cir or the police cir percent policy-map class configuration

command. At the physical level of the hierarchy, you can police only the class-default class in an egress

policy attached to an ES port.

You can configure a two-rate traffic policer to limit the transmission rate of a traffic class and mark

actions (conform, exceed, and violate) for each packet. Within the conform, exceed, and violate

categories, you decide packet treatments. In the most common configurations, you configure packets that

conform to be sent, packets that exceed to be sent with a decreased priority, and packets that violate to

be dropped. You can decrease the priority of the CoS, the DSCP, the IP precedence, or the MPLS EXP

bits in the packet.

The two-rate policer manages the maximum rate of traffic through a token-bucket algorithm. The

algorithm uses the configured committed information rate (CIR) and the peak information rate (PIR) rate

values to control the maximum rate of traffic allowed on a port at a given moment in time. The algorithm

is affected by all traffic leaving the port, and it manages network bandwidth when several large packets

are sent in the same traffic stream.

A token bucket is provided for the CIR and the PIR as shown in

Figure 26-10 Egress Two-Rate Policing and Marking Flowchart

Packet of size B

Catalyst 3750 Metro Switch Software Configuration Guide

size of B larger

than number of tokens

available in PIR

Conform burst

size of B larger

than number of tokens

available in CIR

Configuring QoS