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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 37 Configuring Quality of Service Configuring QoS on Supervisor Engines II-Plus, II+10GE, IV, V, V-10GE, 4924, 4948, and 4948-10GE
Configuration Guidelines
Before beginning the QoS configuration, you should be aware of this information:
If you have EtherChannel ports configured on your switch, you must configure QoS classification
and policing on the EtherChannel. The transmit queue configuration must be configured on the
individual physical ports that comprise the EtherChannel.
If the IP fragments match the source and destination configured in the ACL used to classify the
traffic for quality of service, but do not match the Layer 4 port numbers in the ACL, they are still
matched with the ACL and may get prioritized. If the desired behavior is to give best-effort service
to IP fragments, the following two ACEs should be added to the ACL used to classify the traffic:
access-list xxx deny udp any any fragments
access-list xxx deny tcp any any fragments
It is not possible to match IP options against configured IP extended ACLs to enforce QoS. These
packets are sent to the CPU and processed by software. IP options are denoted by fields in the IP
header.
Control traffic (such as spanning tree BPDUs and routing update packets) received by the switch are
subject to all ingress QoS processing.
You cannot use set commands in policy maps if IP routing is disabled (enabled by default).
On a dot1q tunnel port, only Layer 2 match criteria can be applied to tagged packets. However, all
match criteria can be applied for untagged packets.
On a trunk port, only Layer 2 match criteria can be applied to packets with multiple 802.1q tags.
Note QoS processes both unicast and multicast traffic.
DCSP-to-Transmit queue map DSCP 0–15 Queue 1
DSCP 16–31 Queue 2
DSCP 32–47 Queue 3
DSCP 48–63 Queue 4
High priority transmit queue Disabled
With QoS disabled
Interface trust state Trust DSCP
With QoS enabled With QoS enabled and all other QoS parameters at default values, QoS sets
IP DSCP to zero and Layer 2 CoS to zero in all traffic transmitted.
Interface trust state Untrusted
Table 37-2 QoS Default Configuration (continued)
Feature Default Value