Packet Modification - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 29 Configuring QoS

Packet Modification

A packet is classified, policed, and queued for QoS. Packet modifications can occur during this process:
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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
contents or size, and sends it from a single queue.)
When you enable auto-QoS, it automatically classifies traffic based on the traffic type and 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:
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These sections describe how to configure auto-QoS on your switch:
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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. However, route
lookup is performed based on classified DSCPs.
For non-IP packets, classification involves assigning an internal DSCP to the packet, but because
there is no DSCP in the non-IP packet, no overwrite occurs. Instead, the internal DSCP is translated
to the CoS and is used both for queueing and scheduling decisions and for writing the CoS priority
value in the tag if the packet is being sent on either an ISL or 802.1Q trunk port. Because the CoS
priority is written in the tag, Catalyst 3500 series XL switches that use the 802.1P priority can
interoperate with the QoS implementation on the Catalyst 3550 switches.
During policing, IP and non-IP packets can have another DSCP assigned to them (if they are out of
profile and the policer specifies a markdown DSCP). For IP packets, the packet modification occurs
at a later stage; for non-IP packets the DSCP is converted to CoS and used for queueing and
scheduling decisions.
Detects the presence or absence of IP phones
Configures QoS classification
Configures egress queues
Generated Auto-QoS Configuration, page 29-18
Effects of Auto-QoS on the Configuration, page 29-20
Configuration Guidelines, page 29-20
Enabling Auto-QoS for VoIP, page 29-21
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring Auto-QoS
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