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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter27 Configuring Quality of Service
Overview of QoS
Sharing Link Bandwidth Among Transmit Queues
The four 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 bandwidth
command in interface transmit queue configuration 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.
For systems using Supervisor Engine II-Plus, Supervisor Engine II-Plus TS, Supervisor Engine III, and
Supervisor Engine IV, bandwidth can be configured on these ports only:
•Uplink ports on supervisor engines
•Ports on the WS-X4306-GB GBIC module
•Ports on the WS-X4506-GB-T CSFP module
•The 2 1000BASE-X ports on the WS-X4232-GB-RJ module
•The first 2 ports on the WS-X4418-GB module
•The two 1000BASE-X ports on the WS-X4412-2GB-TX module
For systems using Supervisor Engine V, bandwidth can be configured on all ports (10/100 Fast Ethernet,
10/100/1000BASE-T, and 1000BASE-X).
Strict Priority / Low Latency Queueing
You can configure transmit queue 3 on each port with higher priority using the priority high tx-queue
configuration command in the interface configuration mode. When transmit queue 3 is configured with
higher priority, packets in transmit queue 3 are scheduled ahead of packets in other queues.
When transmit queue 3 is configured at a higher priority, the packets are scheduled for transmission
before the other transmit queues only if it has not met the allocated bandwidth sharing configuration.
Any traffic that exceeds the configured shape rate will be queued and transmitted at the configured rate.
If the burst of traffic, exceeds the size of the queue, packets will be dropped to maintain transmission at
the configured shape rate.
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. The
configuration allows you to specify the maximum rate of traffic. Any traffic that exceeds the configured
shape rate will be queued and transmitted at the configured rate. If the burst of traffic exceeds the size
of the queue, packets will be dropped to maintain transmission at the configured shape rate.