41-3
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
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Chapter 41 Configuring QoS
Understanding How QoS Works
•
Scheduling
is the assignment of traffic to a queue. QoS assigns traffic based on CoS values.
•
Congestion avoidance
is the process by which QoS reserves ingress and egress port capacity for
traffic with high-priority CoS values. QoS implements congestion avoidance with CoS value-based
drop thresholds. A drop threshold is the percentage of buffer utilization at which traffic with a
specified CoS value is dropped, leaving the buffer available for traffic with higher-priority CoS
values.
•
Policing
is the process by which the switch limits the bandwidth consumed by a flow of traffic.
Policing can mark or drop traffic.
•
Except where specifically differentiated,
Layer 3 switching engine
refers to either:
–
Supervisor Engine 2 with Layer 3 Switching Engine II (Policy Feature Card 2 or PFC2)
–
Supervisor Engine 1 with Layer 3 Switching Engine WS-F6K-PFC (Policy Feature Card
or PFC)
Flowcharts
Figure 41-1
shows how traffic flows through the QoS features;
Figure 41-2
through
Figure 41-7
show
more details of the traffic flow through QoS features.
Figure 41-1 Traffic Flow Through QoS Features
Note
Traffic that is Layer 3 switched does not go through the Multilayer Switch Feature Card (MSFC or
MSFC2) and retains the CoS value assigned by the Layer 3 switching engine.
Note
Enter the
show port capabilities
command to see the queue structure of a port (for more information,
see the
“Receive Queues” section on page 41-11
and the
“Transmit Queues” section on page 41-25
).
Transmit
frame
Ethernet
egress
port
L3 Switching Engine*
or
L2 Switching Engine
Multilayer Switch
Feature Card (MSFC)
FlexWAN Module
interfaces
*PFC or PFC2
Transmit
cell
ATM-LANE
egress
port
41866
CoS = 0 for all traffic
(not configurable)
Frame enters
switch
Ethernet
ingress
port
Cell enters
switch
ATM-LANE
ingress
port
CoS = 0 for all traffic
(not configurable)