Packet Modification - Cisco Catalyst 3750 Software Configuration Manual

Chapter 26 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 and non-IP packets, ingress classification involves assigning a QoS label to a packet based
on the DSCP or CoS of the received packet. However, the packet is not modified at this stage; only
an indication of the assigned DSCP or CoS value is carried along. The reason for this is that QoS
classification and forwarding lookups occur in parallel, and it is possible that the packet is forwarded
with its original DSCP to the CPU where it is again processed through software.
The ES ports classify traffic on egress, and this classification can be used for queuing or for marking
the CoS, DSCP, IP precedence, or MPLS EXP bits. Any packet modifications that result from
ingress classification are applied before the packet reaches the egress classification stage. For
example, if the switch receives traffic with a CoS value of 2 and an ingress action resets the CoS to
4, the packet will have a CoS of 4 (instead of a CoS of 2 and an indicator that the CoS should be set
to 4) when it moves to the egress classification stage.
During ingress 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.
During egress policing on the ES ports, marking actions can set the CoS, DSCP, IP precedence, or
the MPLS EXP bits. Any markings performed by an ingress policer are applied before the packet
reaches the egress classification stage.
Depending on the QoS label assigned to a frame and the mutation chosen, the DSCP and CoS values
of the frame are rewritten. If you do not configure the mutation map and if you configure the port to
trust the DSCP of the inbound frame, the DSCP value in the frame is not changed, but the CoS is
rewritten according to the DSCP-to-CoS map. If you configure the port to trust the CoS of the
inbound frame and it is an IP packet, the CoS value in the frame is not changed, but the DSCP might
be changed according to the CoS-to-DSCP map.
The input mutation causes the DSCP to be rewritten depending on the new value of DSCP chosen.
The set action in a policy map also causes the DSCP to be rewritten.
This information applies to both standard and ES ports. On the ES ports, the switch also applies trust
policies to 802.1Q tunneling frames at egress.
Catalyst 3750 Metro Switch Software Configuration Guide
Configuring Standard QoS
26-41