Packet Modification - Cisco Catalyst 2960 series Configuration Manual

The egress queue default settings are suitable for most situations. You should change them only when you
Note
have a thorough understanding of the egress queues and if these settings do not meet your QoS solution.
The switch supports 4 egress queues by default, although there is an option to enable a total of 8 egress
Note
queues. Use the mls qos srr-queue output queues 8 global configuration command to enable all 8 egress
queues. Once 8 egress queues are enabled, you are able to configure thresholds, buffers, bandwidth share
weights, and bandwidth shape weights for all 8 queues. The 8 egress queue configuration is only supported
on a standalone switch.
Related Topics
Configuring Egress Queue Characteristics, on page 618
SRR Shaping and Sharing, on page 555

Packet Modification

A packet is classified, policed, and queued to provide QoS. The following packet modifications can occur
during the process to provide QoS:
• For IP and non-IP packets, 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.
• 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.
• 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 a table map and if you configure the port to trust the
DSCP of the incoming 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 incoming 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.
Standard QoS Default Configuration
Standard QoS is disabled by default.
There is no concept of trusted or untrusted ports because the packets are not modified. The CoS, DSCP, and
IP precedence values in the packet are not changed.
Traffic is switched in pass-through mode. The packets are switched without any rewrites and classified as
best effort without any policing.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
Information About QoS
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