52-28
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 52 Configuring QoS
Understanding How QoS Works
Classification and Marking on a Supervisor Engine 1 with a Layer 2
Switching Engine
On a Supervisor Engine 1 with a Layer 2 Switching Engine, QoS can classify the traffic that is addressed
to the specified MAC address/VLAN pairs to be marked with a configured CoS value (for more
information, see the “QoS Terminology” section on page 52-2 and the “Mapping a CoS Value to a Host
Destination MAC Address/VLAN Pair” section on page 52-61).
Note Classification and marking on a Supervisor Engine 1 with a Layer 2 Switching Engine uses the Layer 2
CoS values. Classification and marking on a Supervisor Engine 1 with a Layer 2 Switching Engine does
not use or set the Layer 3 IP precedence or DSCP values.
Ethernet Egress Port Scheduling, Congestion Avoidance, and Marking
These sections describe Ethernet egress port scheduling, congestion avoidance, and marking:
• Overview, page 52-28
• Transmit Queues, page 52-28
• Scheduling and Congestion Avoidance, page 52-29
• Marking, page 52-29
Overview
QoS schedules the traffic through the transmit queues based on the CoS values and uses the
CoS-value-based transmit-queue drop thresholds to avoid congestion in the traffic that is transmitted
from the Ethernet ports.
Note Ethernet egress port scheduling and congestion avoidance uses the Layer 2 CoS values. Ethernet egress
port marking writes the Layer 2 CoS values, and for the IP traffic, the Layer 3 ToS byte.
Transmit Queues
Enter the show port capabilities command to see the queue structure of a port. The command displays
one of the following:
• tx-(2q2t) indicates two standard queues, each with two configurable tail-drop thresholds.
• tx-(1p2q1t) indicates one strict-priority queue and two standard queues, each with one configurable
WRED-drop threshold (on the 1p2q1t ports, each standard queue also has one nonconfigurable
tail-drop threshold).
• tx-(1p2q2t) indicates one strict-priority queue and two standard queues, each with two configurable
WRED-drop thresholds.
• tx-(1p3q1t) indicates one strict-priority queue and three standard queues, each with one
configurable WRED-drop threshold (on the 1p3q1t ports, each standard queue also has one
nonconfigurable tail-drop threshold).
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