Cisco Systems A9014CFD manual Weighted Random Early Detection Wred, Amount of bandwidth, 24-46

Chapter 24 Configuring QoS

Configuring Quality of Service (QoS)

g.Use the service-policycommand to apply the service policy to the interface.

Router(config-if)# service-policy output policy1

Weighted Random Early Detection (WRED)

Random Early Detection (RED) is a congestion avoidance mechanism that takes advantage of the congestion control mechanism of TCP. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. WRED drops packets selectively based on IP discard-class. Discard-class is assigned to packets at the ingress, as they enter the network. WRED is useful on any output interface where you expect to have congestion. However, WRED is usually used in the core routers of a network, rather than at the edge. WRED uses discard-class to determine how it treats different types of traffic.

When a packet arrives, the following events occur:

1.The average queue size is calculated.

2.If the average is less than the minimum queue threshold, the arriving packet is queued.

3.If the average is between the minimum queue threshold for that type of traffic and the maximum threshold for the interface, the packet is either dropped or queued, depending on the packet drop probability for that type of traffic.

4.If the average queue size is greater than the maximum threshold, the packet is dropped.

Note Cisco ASR 901 supports configuration of random-detect thresholds only in number-of-packets.

Complete the following steps to configure WRED:

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode
Step 2
	policy-map	Define a new policy map and enter policy map configuration mode.
	Example
	Router(config)# policy-map policy1
	Router(config-pmap)#
Step 3
	class	Specify a traffic class to which the policy applies. This command enters
	Example	policy-map class configuration mode, which allows you to define the
		treatment for the traffic class.
	Router(config-pmap)# class class1
	Router(config-pmap-c)#
Step 4
	bandwidth	Specify the bandwidth allocated for a traffic class attached to the policy
	Example	map. You can define the percentage of bandwidth, or an absolute
		amount of bandwidth.
	Router(config-pmap-c)# bandwidth
	percent 50	This step is optional.

	Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
24-46	OL-23826-09