52-29
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 52 Configuring QoS
Understanding How QoS Works
•
tx-(1p3q8t)
indicates one strict-priority queue and three standard queues, each with eight
configurable WRED-drop thresholds (on the
1p3q8t
ports, each standard queue also has one
nonconfigurable tail-drop threshold).
•
tx-(1p7q8t)
indicates one strict-priority queue and seven standard queues, each with eight
configurable WRED-drop thresholds (on the
1p7q8t
ports, each standard queue also has one
nonconfigurable tail-drop threshold).
For the port types with a strict-priority queue, the switch services the traffic in the strict-priority transmit
queue before servicing the standard queues. When the switch is servicing a standard queue, after
transmitting a packet, it checks for the traffic in the strict-priority queue. If the switch detects the traffic
in the strict-priority queue, it suspends its service of the standard queue and completes the service of all
the traffic in the strict-priority queue before returning to the standard queue.
Scheduling and Congestion Avoidance
QoS implements the CoS-value-based transmit-queue drop thresholds to avoid congestion in the
transmitted traffic. See the
“QoS Default Configuration” section on page 52-30
for the default
CoS-to-threshold mapping.
For some port types, you can configure each standard transmit queue to use both a nonconfigurable
100-percent tail-drop threshold and a configurable WRED-drop threshold (see the
“1p3q1t Transmit
Queues” section on page 52-70
and the
“1p2q1t, 1p3q8t, and 1p7q8t Transmit Queues” section on
page 52-72
). The switch uses the tail-drop thresholds for the traffic carrying the CoS values that are
mapped only to a queue. The switch uses the WRED-drop thresholds for the traffic carrying the CoS
values that are mapped to a queue and a threshold.
Marking
When the traffic is transmitted from the switch, QoS writes the ToS byte into the IP traffic (Layer 3
switching engine only) and the CoS value that was used for scheduling and congestion avoidance into
the ISL or 802.1Q traffic (for more information, see the
“Final Layer 3 Switching Engine CoS and ToS
Values” section on page 52-27
).
QoS Statistics Data Export
QoS statistics data export generates per-port and per-aggregate policer utilization information and
forwards this information in the UDP packets to traffic monitoring, planning, or accounting applications.
You can enable QoS statistics data export per port or per aggregate policer. The statistics data that is
generated per port consists of the counts of the input and output packets and bytes. The aggregate policer
statistics consists of the counts of the allowed packets and the counts of the packets exceeding the
policed rate.
The QoS statistics data collection occurs periodically at a fixed interval, but the interval at which the
data is exported is configurable. QoS statistics collection is enabled by default, and data export is
disabled by default for all the ports and all the aggregate policers that are configured on the
Catalyst 6500 series switch.
