Packet Modification - Cisco WS-C3550-12G Software Configuration Manual

Chapter 20 Configuring QoS

Packet Modification

A packet is classified, policed, and queued to provide QoS. Packet modifications can occur during this
process:
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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). Once again, the DSCP in the packet is not
modified, but an indication of the marked-down value is carried along. 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.
Catalyst 3550 Multilayer Switch Software Configuration Guide
Understanding QoS
20-17