Cisco Systems 2940 Configuring QoS

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Catalyst 2940 Switch Software Configuration Guide

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Chapter24 Configuring QoS Configuring QoS

Each port on the switch has a single receive queue buffer (the ingress port) for incoming traffic. When

an untagged frame arrives, it is assigned the value of the port as its port default priority. You assign this

value by using the CLI or CMS. A tagged frame continues to use its assigned CoS value when it passes

through the ingress port.

The switch supports four CoS queues for each egress port. For each queue, you can specify the se t ypes

of scheduling:

•Strict priority scheduling

Strict priority scheduling is based on the priority of que ues. Packets i n the h igh-p rior ity qu eu e

always transmit first, and packets in the low-priority queue do not transmit until all th e high-priority

queues become empty.

The default scheduling method is strict priority.

•Weighted round-robin (WRR) scheduling

WRR scheduling requires you to specify a number that indicates the importance (weight) of the

queue relative to the other CoS queues. WRR scheduling prevents the low-priority queues from

being completely neglected during periods of high-priority traffic. The WRR scheduler sends some

packets from each queue in turn. The number of packets it send s cor res ponds t o the rela tive

importance of the queue. For example, if one queue has a weight of 3 and another has a weight of 4,

three packets are sent from the first queue for every four that are sent from the second queue. By

using this scheduling, low-priority queues have the opportunity to send packets even though the

high-priority queues are not empty.

Configuring QoS

Before configuring QoS, you must have a thorough understanding of these items:

•The types of applications used and the traffic patterns on your network.

•Traffic characteristics and needs of your network. Is the traffic bursty? Do you need to reserve

bandwidth for voice and video streams?

•Bandwidth requirements and speed of the network.

•Location of congestion points in the network.

This section describes how to configure QoS on your switch:

•Default QoS Configuration, page 24-3

•Configuring Classification Using Port Trust States, page 24-4

•Configuring the Egress Queues, page 24-8

This is the default QoS configuration:

•The default port CoS value is 0.

•The default port CoS value is assigned to all incoming untagged packets. The CoS value of each

tagged packet remains unaltered.