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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 37 Configuring Quality of Service About QoS
The following table provides the default DSCP-to-transmit queue mapping:
For example, if you are sending two streams, one with a DSCP of 16 and other with a value of 0, they
will transmit from different queues. Even though an aggressive flow in txQueue 2 (packets with DSCP
of 16) can saturate the link, packets with a DSCP of 0 are not blocked by the aggressive flow, because
they transmit from txQueue 1. Even without DBL, packets whose DSCP value places them in txQueue
1, 3, or 4 are not dropped because of the aggressive flow.
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 two 1000BASE-X ports on the WS-X4232-GB-RJ module
The first two 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 Fa st 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 is queued and transmitted at the configure d rate. If the
burst of traffic, exceeds the size of the queue, packets are dropped to maintain transmission at the
configured shape rate.
DSCP txQueue
0-15 1
16-31 2
32-48 3
49-63 4