14-3
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
—
Release 8.1
78-15486-01
Chapter 14 Configuring QoS
Understanding How QoS Works
•
Marking is the application of QoS labels to traffic.
•
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 and transmit queues. 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. A transmit queue is a queue on the egress port where outgoing frames
are stored before transmission. With multiple transmit queues, traffic with higher-priority CoS
values can be placed in a reserved transmit queue.
•
Policing is the process by which the switch limits the bandwidth consumed by a flow of traffic.
Policing can mark or drop traffic.
Understanding Classification and Marking at the Ingress Port
ISL or 802.1Q frames are not classified or marked at the ingress port; the existing CoS value is honored.
When an 802.1Q frame enters the switch through a supported ingress port, QoS accepts the User Priority
bits as the CoS value.
QoS classifies and marks all other frame types that enter the switch with the default CoS value
configured for the entire switch. You cannot mark traffic on a per-port basis.
Note
The Catalyst 4500 series, 2948G, and 2980G switches support frame classification and marking only on
unclassified frames entering the switch.
Understanding Scheduling
There are two user-configurable transmit queues and one non-user-configurable transmit queue drop
threshold for each port. You can specify such ports using the 2q1t keyword in QoS-related commands.
QoS uses the transmit queues to schedule transmission of network traffic from the switch through egress
ports. By default, all traffic is assigned to queue 1 and threshold 1 when QoS is enabled. All traffic that
is destined for a transmit queue, regardless of classification, is subject to tail drop when the queue is full
(that is, frames at the end of the queue are dropped).
Caution
When you disable QoS, the switch assigns unicast traffic to queue 1 and broadcast, multicast, and
unknown traffic to queue 2. If you enable QoS but do not modify the CoS-to-transmit queue mappings,
switch performance could be affected because all traffic is assigned to queue 1. If you enable QoS, we
recommend that you modify the CoS-to-transmit queue mappings.
Note
To configure the CoS values that are mapped to each transmit queue, see the
“Mapping CoS Values to
Transmit Queues and Drop Thresholds” section on page 14-6
.
