Cisco WS-C4003 Software Guide - Page 225

Understanding Classification and Marking at the Ingress Port, Understanding Scheduling

Page 225 highlights

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 4000 family, 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 a single non-user-configurable transmit queue drop threshold for each port. Ports of this type are specified 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 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 QoS is disabled, unicast traffic is assigned to queue 1 and broadcast, multicast, and unknown traffic is assigned 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 mapped to each transmit queue, see the "Mapping CoS Values to Transmit Queues and Drop Thresholds" section on page 14-6. 78-12647-02 Software Configuration Guide-Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4 14-3

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14-3
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
78-12647-02
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
4000 family, 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 a single non-user-configurable transmit queue drop
threshold for each port. Ports of this type are specified 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
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 QoS is disabled, unicast traffic is assigned to queue 1 and broadcast, multicast, and unknown
traffic is assigned 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 mapped to each transmit queue, see the
“Mapping CoS Values to Transmit
Queues and Drop Thresholds” section on page 14-6
.