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Software Configuration Guide—Release 12.2(25)SG
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Chapter27 Configuring Quality of Service
Configuring Auto-QoS
The internal IP DSCP is used to determine the transmit queue to which the packet is enqueued on the
transmission interface. See “Configuring Transmit Queues” on page48 for details on how to configure
the DSCP to transmit queues.
The internal IP DSCP is also used to determine the transmit CoS marking if the packet is transmitted
with a IEEE 802.1q or ISL tag on a trunk interface. See “Configuri ng the DSCP-to-CoS Map” on page 53
for details on how to configure the DSCP to CoS mapping.
Configuring Auto-QoS
You can use the auto-QoS feature to simplify the deployment of existing QoS features. Auto-QoS makes
assumptions about the network design, and as a result, the switch can prioritize different traffic flows
and appropriately use the egress queues instead of using the default QoS behavior. (The default is that
QoS is disabled. The switch then offers best-effort service to each packet, regardless of the packet
content or size, and sends it from a single queue.)
When you enable auto-QoS, it automatically classifies traffic based on ingress packet label. The switch
uses the resulting classification to choose the appropriate egress queue.
You use auto-QoS commands to identify ports connected to Cisco IP phones and to identify ports that
receive trusted voice over IP (VoIP) traffic through an uplink. Auto-QoS then performs these functions:
•Detects the presence or absence of IP phones
•Configures QoS classification
•Configures egress queues
These sections describe how to configure auto-QoS on your switch:
•Generated Auto-QoS Configuration, page 27-17
•Effects of Auto-QoS on the Configuration, page 27-18
•Configuration Guidelines, page 27-18
•Enabling Auto-QoS for VoIP, page 27-19

Generated Auto-QoS Configuration

By default, auto-QoS is disabled on all interfaces.
When you enable the auto-QoS feature on the first interface, these automatic actions occur:
•QoS is globally enabled (qos global configuration command).
•DBL is enabled globally (qos dbl global configuration command)
•When you enter the auto qos voip trust interface configuration command, the ingress classification
on the specified interface is set to trust the CoS label received in the packet if the specified interface
is configured as Layer 2 (and is set to trust DSCP if the interface is configured as Layer 3). (See
Table27-2.)
•When you enter the auto qos voip cisco-phone interface configuration command, the trusted
boundary feature is enabled. It uses the Cisco Discovery Protocol ( CDP) to detect the presence or
absence of a Cisco IP phone. When a Cisco IP phone is detected, the ingress classification on the