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24-3
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 24 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.
Egress CoS Queues
The switch supports four CoS queues for each egress port. For each queue, you can specify these types
of scheduling:
• Strict priority scheduling
Strict priority scheduling is based on the priority of queues. Packets in the high-priority queue
always transmit first, and packets in the low-priority queue do not transmit until all the 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 sends corresponds to the relative
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
Default QoS Configuration
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.