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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 33 Configuring QoS Understanding QoS

Output Policy Maps

Output policy map classification criteria include matching a CoS, a DSCP, an IP precedence, or a QoS
group value. Output policy maps can have any of these actions:
• Queuing (queue-limit)
• Scheduling (bandwidth, priority, and shape average)
Output policy maps do not support matching of access groups. You can use QoS groups as an alternative
by matching the appropriate access group in the input policy map and setting a QoS group. In the output
policy map, you can then match the QoS group. See the “Classification Based on QoS Groups” section
on page 33-10 for more information.
Output policies do not support marking or policing (except in the case of priority with policing). There
is no egress packet marking on the switch (no set command in an output policy).
The class class-default is used in a policy map for any traffic that does not explicitly match any other
class in the policy map. There can be a maximum of four classes in the output policy map (including
class-default) because egress ports have a maximum of four queues.
An output policy map attached to an egress port can match only the packets that have already been
matched by an input policy map attached to the ingress port for the packets. You can attach an output
policy map to any or all ports on the switch. Beginning with Cisco IOS Release 12.2(35)SE, the switch
supports configuration and attachment of a unique output policy map for each port. However, these
output policy maps can contain only three unique configurations of queue limits. These three unique
queue-limit configurations can be included in as many output policy maps as there are ports on the
switch. There are no limitations on the configurations of bandwidth, priority, or shaping.
Beginning with Cisco IOS Release 12.2(44)SE, you can configure the output policy classification
criteria for CPU-generated traffic by using the cpu traffic qos [cos value | dscp value | precedence value
| qos-group value] global configuration command.
Classification
Classification distinguishes one kind of traffic from another by examining the fields in the packet header.
When a packet is received, the switch examines the header and identifies all key packet fields. A packet
can be classified based on an ACL, on the DSCP, the CoS, or the IP precedence value in the packet, or
by the VLAN ID. Figure 33-3 has examples of classification information carried in a Layer 2 or a Layer
3 IP packet header, using six bits from the deprecated IP type of service (ToS) field to carry the
classification information.
• On ports configured as Layer 2 IEEE 802.1Q trunks, all traffic is in IEEE 802.1Q frames except for
traffic in the native VLAN. Layer 2 IEEE 802.1Q frame headers have a 2-byte Tag Control
Information field that carries the CoS value, called the User Priority bits, in the three
most-significant bits, and the VLAN ID value in the 12 least-significant bits. Other frame types
cannot carry Layer 2 CoS values.
Layer 2 CoS values range from 0 to 7.
• Layer 3 IP packets can carry either an IP precedence value or a DSCP value. QoS supports the use
of either value because DSCP values are backward-compatible with IP precedence values.
IP precedence values range from 0 to 7. DSCP values range from 0 to 63.
• Output remarking is based on the Layer 2 or Layer 3 marking type, marking value and packet type.