Cisco Systems 4500 Packet Modification, Per-Port Per-VLAN QoS, QoS and Software Processed Packets

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 37 Configuring Quality of Service
About QoS
Traffic Shaping
Traffic shaping provides the ability to control the rate of outgoing traffic in order to make sure that the
traffic conforms to the maximum rate of transmission contracted for it. Traffic that meets certain profile
can be shaped to meet the downstream traffic rate requirements to handle any data rate mismatches.
Each transmit queue can be configured to transmit a maximum rate using the shape command. The
configuration allows you to specify the maximum rate of traffic. Any traffic that exceeds the configured
shape rate is queued and transmitted at the configured rate. If the burst of traffic exceeds the size of the
queue, packets are dropped to maintain transmission at the configured shape rate.
Packet Modification
A packet is classified, policed, and queued to provide QoS. Packet modifications can occur during this
process:
• 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. This is because QoS
classification and ACL lookup occur in parallel; the ACL might specify 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.
• 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 used both
for queueing and scheduling decisions and for writing the CoS priority value in the tag if the packet
is being transmitted on either an ISL or 802.1Q trunk port.
• 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.
Note If you are running Supervisor Engines II-Plus, II-Plus-10GE, IV, V, V-10GE, or using a ME Catalyst
4900, Catalyst 4900, or Catalyst 4900-10GE series switch, you must enter the qos rewrite ip dscp
command to enable update of packets and DSCP/ToS fields based on the switch’s QoS policies.
Per-Port Per-VLAN QoS
Per-port per-VLAN QoS (PVQoS) offers differentiated quality of services to individual VLANs on a
trunk port. It enables service providers to rate-limit individual VLAN-based services on each trunk port
to a business or a residence. In an enterprise voice-over-IP environment, it can be used to rate-limi t voice
VLAN even if an attacker impersonates a Cisco IP phone. A per-port per-VLAN service policy can be
separately applied to either ingress or egress traffic.
QoS and Software Processed Packets
The Catalyst 4500 platform does not apply the QoS marking or policing configuration for any packets
that are forwarded or generated by the Cisco IOS software. This means that any input or output QoS
policy configured on the port or VLAN is not applied to packets if the Cisco IOS is forwarding or
generating packets.