Cisco Systems OL-4266-08 manual Understanding Port-Based Queue Types, 42-22

Chapter 42 Configuring PFC QoS

Understanding How PFC QoS Works

	With a PFC3, policing uses the Layer 2 frame size. With a PFC2, policing uses the Layer 3 packet size.
	You specify the bandwidth utilization limit as a committed information rate (CIR). You can also specify
	a higher peak information rate (PIR). Packets that exceed a rate are “out of profile” or “nonconforming.”
	In each policer, 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”). Because out-of-profile packets do
	not retain their original priority, they are not counted as part of the bandwidth consumed by in-profile
	packets.
	If you configure a PIR, the PIR out-of-profile action cannot be less severe than the CIR out-of-profile
	action. For example, if the CIR out-of-profile action is to mark down the traffic, then the PIR
	out-of-profile action cannot be to transmit the traffic.
	For all policers, PFC QoS uses a configurable global table that maps the internal DSCP value to a
	marked-down DSCP value. When markdown occurs, PFC QoS gets the marked-down DSCP value from
	the table. You cannot specify marked-down DSCP values in individual policers.
Note	• Policing with the conform-action transmit keywords supersedes the ingress LAN port trust state
	of matched traffic with trust DSCP or with the trust state defined by a trust policy-map class
	command.
	• By default, the markdown table is configured so that no markdown occurs: the marked-down DSCP
	values are equal to the original DSCP values. To enable markdown, configure the table appropriately
	for your network.
	• When you apply both ingress policing and egress policing to the same traffic, both the input policy
	and the output policy must either mark down traffic or drop traffic. PFC QoS does not support
	ingress markdown with egress drop or ingress drop with egress markdown.

Understanding Port-Based Queue Types

Port-based queue types are determined by the ASICs that control the ports. The following sections describe the queue types, drop thresholds, and buffers that are supported on the Cisco 7600 series router LAN modules:

•Ingress and Egress Buffers and Layer 2 CoS-Based Queues, page 42-22

•Ingress Queue Types, page 42-24

•Egress Queue Types, page 42-25

•Module to Queue Type Mappings, page 42-26

Ingress and Egress Buffers and Layer 2 CoS-Based Queues

The Ethernet LAN module port ASICs have buffers that are divided into a fixed number of queues. When congestion avoidance is enabled, PFC QoS uses the traffic’s Layer 2 CoS value to assign traffic to the queues. The buffers and queues store frames temporarily as they transit the switch. PFC QoS allocates the port ASIC memory as buffers for each queue on each port.

The Cisco 7600 series router LAN modules support the following types of queues:

•Standard queues

•Strict-priority queues

	Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
42-22	OL-4266-08