Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 44 Configuring Quality of Service
Overview of QoS
Active Queue Management
Active queue management (AQM) is the pro-active approach of informing you about congestion before
a buffer overflow occurs. AQM is done using Dynamic buffer limiting (DBL). DBL tracks the queue
length for each traffic flow in the switch. When the queue length of a flow exceeds its limit, DBL drop
packets.
Sharing Link Bandwidth Among Transmit Queues
The eight transmit queues for a transmit port share the available link bandwidth of that transmit port.
You can set the link bandwidth to be shared differently among the transmit queues using the
bandwidth command in the policy-map class configuration command in class mode.
With this command, you assign the minimum guaranteed bandwidth for each transmit queue.
By default, all queues are scheduled in a round robin manner.
Strict Priority / Low Latency Queueing
You can only configure one transmit queue on a port as strict priority (termed Low Latency Queue, or
LLQ).
LLQ provides strict-priority queuing for a traffic class. It enables delay-sensitive data, such as voice, to
be sent before packets in other queues. The priority queue is serviced first until it is empty or until it falls
under r its shape rate. Only one traffic stream can be destined for the priority queue per class-level policy.
You enable the priority queue for a traffic class with the
priority policy-map class configuration
command in class mode.
Traffic Shaping
Traffic Shaping provides the ability to control the rate of outgoing traffic in order to make sure that the
traffic conforms to the maximum rate of transmission contracted for it. Traffic that meets certain profile
can be shaped to meet the downstream traffic rate requirements to handle any data rate mismatches.
Each transmit queue can be configured to transmit a maximum rate using the shape command in the
policy-map class configuration command in class mode
The configuration allows you to specify the maximum rate of traffic. Any traffic that exceeds the
configured shape rate is queued and transmitted at the configured rate. If the burst of traffic exceeds the
size of the queue, packets are dropped to maintain transmission at the configured shape rate.
Packet Modification
A packet is classified, policed, and queued to provide QoS. Packet modifications can occur during this
process:
• For IP packets, classification involves assigning a DSCP to the packet. However, the packet is not
modified at this stage; only an indication of the assigned DSCP is carried along. The reason for this
is that QoS classification and ACL lookup occur in parallel, and it is possible that the ACL specifies
that the packet should be denied and logged. In this situation, the packet is forwarded with its
original DSCP to the CPU, where it is again processed through ACL software.