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Figure 8. Egress DSCP Mutation

Egress DSCP mutation provides a way to change the DSCP value in the outgoing packet. An egress DSCP map table is used to tell the switch what

DSCP value to write in the outbound packet based on the packet's internal DSCP value. Egress DSCP mutation relies on the presence of a PFC3A,

PFC3B, or PFC3BXL to support this feature.

Maps: Ingress CoS Mutation

Support for this feature is present on some of the newer Cisco Catalyst 6500 line cards. Specifically, it is supported on the CEF720 series 48 port

GETX and SFP line card, the CEF720 series 4 port 10GE line card and the CEF720 series 24 port SFP line card. Each of these line cards in turn

requires the presence of a Supervisor Engine 720 to work in the chassis.

When a port on one of these line cards is configured as an IEEE 802.1Q trunk port, ingress CoS mutation can be configured. What this feature

allows the user to do is to change the incoming CoS value to another CoS value. A map is provided that lists what the incoming CoS value will be

changed to. The administrator can change the map table to suit local policy requirements.

Ingress and Egress QoS: Policing

The PFC on the supervisor engine implements policing in a number of forms. The following sections provide an insight into the different policing

actions available on the PFC.

Policing: Aggregate Policing

An aggregate policer is a term used to define a rate-limiting policy that applies to all traffic on a given port or VLAN that matches a set of

classification criteria. This type of policer can be applied to traffic traveling in either direction—that is, for inbound traffic or outbound traffic.

The aggregate policer can be applied to a single port or to a VLAN containing multiple ports. The PFC allows up to 1023 aggregate policers to

be defined and active in the system at any one time.

Policing: Microflow Policing

The microflow policer differs slightly from an aggregate in that it applies a rate-limiting policy to each discrete flow. The question then becomes:

What is a flow, and how is it defined by the Cisco Catalyst 6500? A flow is defined as a unidirectional flow of data that is uniquely identified by

primary fields in the packet's IP and TCP/User Datagram Protocol (UDP) headers. Microflow policing, by default, identifies a unique flow by its

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