41-25
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
78-13315-02
Chapter 41 Configuring QoS
Understanding How QoS Works
Overview
QoS schedules traffic through the transmit queues based on CoS values and uses CoS-value-based
transmit-queue drop thresholds to avoid congestion in traffic transmitted from Ethernet ports.
Note
Ethernet egress port scheduling and congestion avoidance uses Layer 2 CoS values. Ethernet egress
port marking writes Layer 2 CoS values and, for 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)
—Two standard queues with two thresholds each
•
tx-(1p2q2t)
—One strict-priority queue and two standard queues with two thresholds each
•
tx-(1p3q1t)
—One strict-priority queue and three standard queues with one threshold each
All ports have a low-priority and a high-priority standard transmit queue.
1p3q1t
ports have a
medium-priority standard transmit queue.
1p2q2t
and
1p3q1t
ports have a strict-priority transmit queue
in addition to the standard queues.
On
2q2t
ports, the default QoS configuration allocates a minimum of 80 percent of the total transmit
queue size to the low-priority standard queue and a minimum of 20 percent to the high-priority standard
queue.
On
1p2q2t
and
1p3q1t
ports, the switch services traffic in the strict-priority queue before servicing the
standard queues. When the switch is servicing a standard queue, after transmitting a packet, it checks
for traffic in the strict-priority queue. If the switch detects traffic in the strict-priority queue, it suspends
its service of the standard queue and completes service of all traffic in the strict-priority queue before
returning to the standard queue.
On
1p2q2t
ports, the default QoS configuration allocates a minimum of 70 percent of the total transmit
queue size to the low-priority standard queue, a minimum of 15 percent to the high-priority standard
queue, and a minimum of 15 percent to the strict-priority queue.
On
1p3q1t
ports, the transmit queue size is not configurable and is allocated equally among all queues.
Scheduling and Congestion Avoidance
These sections describe scheduling and congestion avoidance:
•
2q2t Ports, page 41-26
•
1p2q2t Ports, page 41-26
•
1p3q1t Ports, page 41-26
Note
The explanations in these sections use default values. You can configure many of the parameters (for
more information, see the
“Configuring QoS” section on page 41-30
). All ports of the same type use
the same drop-threshold configuration.
